text: from __future__ import unicode_literals
  
  import collections
  import copy
  import datetime
  import decimal
  import functools
  import inspect
  import re
  import uuid
  from collections import OrderedDict
  
  from django.conf import settings
  from django.core.exceptions import ObjectDoesNotExist
  from django.core.exceptions import ValidationError as DjangoValidationError
  from django.core.validators import (
      EmailValidator, RegexValidator, URLValidator, ip_address_validators
  )
  from django.forms import FilePathField as DjangoFilePathField
  from django.forms import ImageField as DjangoImageField
  from django.utils import six, timezone
  from django.utils.dateparse import (
      parse_date, parse_datetime, parse_duration, parse_time
  )
  from django.utils.duration import duration_string
  from django.utils.encoding import is_protected_type, smart_text
  from django.utils.formats import localize_input, sanitize_separators
  from django.utils.functional import lazy
  from django.utils.ipv6 import clean_ipv6_address
  from django.utils.timezone import utc
  from django.utils.translation import ugettext_lazy as _
  from pytz.exceptions import InvalidTimeError
  
  from rest_framework import ISO_8601
  from rest_framework.compat import (
      MaxLengthValidator, MaxValueValidator, MinLengthValidator,
      MinValueValidator, ProhibitNullCharactersValidator, unicode_repr,
      unicode_to_repr
  )
  from rest_framework.exceptions import ErrorDetail, ValidationError
  from rest_framework.settings import api_settings
  from rest_framework.utils import html, humanize_datetime, json, representation
  
  
  class empty:
      """
      This class is used to represent no data being provided for a given input
      or output value.
  
      It is required because `None` may be a valid input or output value.
      """
      pass
  
  
  if six.PY3:
      def is_simple_callable(obj):
          """
          True if the object is a callable that takes no arguments.
          """
          if not (inspect.isfunction(obj) or inspect.ismethod(obj) or isinstance(obj, functools.partial)):
              return False
  
          sig = inspect.signature(obj)
          params = sig.parameters.values()
          return all(
              param.kind == param.VAR_POSITIONAL or
              param.kind == param.VAR_KEYWORD or
              param.default != param.empty
              for param in params
          )
  
  else:
      def is_simple_callable(obj):
          function = inspect.isfunction(obj)
          method = inspect.ismethod(obj)
  
          if not (function or method):
              return False
  
          if method:
              is_unbound = obj.im_self is None
  
          args, _, _, defaults = inspect.getargspec(obj)
  
          len_args = len(args) if function or is_unbound else len(args) - 1
          len_defaults = len(defaults) if defaults else 0
          return len_args <= len_defaults
  
  
  def get_attribute(instance, attrs):
      """
      Similar to Python's built in `getattr(instance, attr)`,
      but takes a list of nested attributes, instead of a single attribute.
  
      Also accepts either attribute lookup on objects or dictionary lookups.
      """
      for attr in attrs:
          try:
              if isinstance(instance, collections.Mapping):
                  instance = instance[attr]
              else:
                  instance = getattr(instance, attr)
          except ObjectDoesNotExist:
              return None
          if is_simple_callable(instance):
              try:
                  instance = instance()
              except (AttributeError, KeyError) as exc:
                  # If we raised an Attribute or KeyError here it'd get treated
                  # as an omitted field in `Field.get_attribute()`. Instead we
                  # raise a ValueError to ensure the exception is not masked.
                  raise ValueError('Exception raised in callable attribute "{0}"; original exception was: {1}'.format(attr, exc))
  
      return instance
  
  
  def set_value(dictionary, keys, value):
      """
      Similar to Python's built in `dictionary[key] = value`,
      but takes a list of nested keys instead of a single key.
  
      set_value({'a': 1}, [], {'b': 2}) -> {'a': 1, 'b': 2}
      set_value({'a': 1}, ['x'], 2) -> {'a': 1, 'x': 2}
      set_value({'a': 1}, ['x', 'y'], 2) -> {'a': 1, 'x': {'y': 2}}
      """
      if not keys:
          dictionary.update(value)
          return
  
      for key in keys[:-1]:
          if key not in dictionary:
              dictionary[key] = {}
          dictionary = dictionary[key]
  
      dictionary[keys[-1]] = value
  
  
  def to_choices_dict(choices):
      """
      Convert choices into key/value dicts.
  
      to_choices_dict([1]) -> {1: 1}
      to_choices_dict([(1, '1st'), (2, '2nd')]) -> {1: '1st', 2: '2nd'}
      to_choices_dict([('Group', ((1, '1st'), 2))]) -> {'Group': {1: '1st', 2: '2'}}
      """
      # Allow single, paired or grouped choices style:
      # choices = [1, 2, 3]
      # choices = [(1, 'First'), (2, 'Second'), (3, 'Third')]
      # choices = [('Category', ((1, 'First'), (2, 'Second'))), (3, 'Third')]
      ret = OrderedDict()
      for choice in choices:
          if not isinstance(choice, (list, tuple)):
              # single choice
              ret[choice] = choice
          else:
              key, value = choice
              if isinstance(value, (list, tuple)):
                  # grouped choices (category, sub choices)
                  ret[key] = to_choices_dict(value)
              else:
                  # paired choice (key, display value)
                  ret[key] = value
      return ret
  
  
  def flatten_choices_dict(choices):
      """
      Convert a group choices dict into a flat dict of choices.
  
      flatten_choices_dict({1: '1st', 2: '2nd'}) -> {1: '1st', 2: '2nd'}
      flatten_choices_dict({'Group': {1: '1st', 2: '2nd'}}) -> {1: '1st', 2: '2nd'}
      """
      ret = OrderedDict()
      for key, value in choices.items():
          if isinstance(value, dict):
              # grouped choices (category, sub choices)
              for sub_key, sub_value in value.items():
                  ret[sub_key] = sub_value
          else:
              # choice (key, display value)
              ret[key] = value
      return ret
  
  
  def iter_options(grouped_choices, cutoff=None, cutoff_text=None):
      """
      Helper function for options and option groups in templates.
      """
      class StartOptionGroup(object):
          start_option_group = True
          end_option_group = False
  
          def __init__(self, label):
              self.label = label
  
      class EndOptionGroup(object):
          start_option_group = False
          end_option_group = True
  
      class Option(object):
          start_option_group = False
          end_option_group = False
  
          def __init__(self, value, display_text, disabled=False):
              self.value = value
              self.display_text = display_text
              self.disabled = disabled
  
      count = 0
  
      for key, value in grouped_choices.items():
          if cutoff and count >= cutoff:
              break
  
          if isinstance(value, dict):
              yield StartOptionGroup(label=key)
              for sub_key, sub_value in value.items():
                  if cutoff and count >= cutoff:
                      break
                  yield Option(value=sub_key, display_text=sub_value)
                  count += 1
              yield EndOptionGroup()
          else:
              yield Option(value=key, display_text=value)
              count += 1
  
      if cutoff and count >= cutoff and cutoff_text:
          cutoff_text = cutoff_text.format(count=cutoff)
          yield Option(value='n/a', display_text=cutoff_text, disabled=True)
  
  
  def get_error_detail(exc_info):
      """
      Given a Django ValidationError, return a list of ErrorDetail,
      with the `code` populated.
      """
      code = getattr(exc_info, 'code', None) or 'invalid'
  
      try:
          error_dict = exc_info.error_dict
      except AttributeError:
          return [
              ErrorDetail(error.message % (error.params or ()),
                          code=error.code if error.code else code)
              for error in exc_info.error_list]
      return {
          k: [
              ErrorDetail(error.message % (error.params or ()),
                          code=error.code if error.code else code)
              for error in errors
          ] for k, errors in error_dict.items()
      }
  
  
  class CreateOnlyDefault(object):
      """
      This class may be used to provide default values that are only used
      for create operations, but that do not return any value for update
      operations.
      """
      def __init__(self, default):
          self.default = default
  
      def set_context(self, serializer_field):
          self.is_update = serializer_field.parent.instance is not None
          if callable(self.default) and hasattr(self.default, 'set_context') and not self.is_update:
              self.default.set_context(serializer_field)
  
      def __call__(self):
          if self.is_update:
              raise SkipField()
          if callable(self.default):
              return self.default()
          return self.default
  
      def __repr__(self):
          return unicode_to_repr(
              '%s(%s)' % (self.__class__.__name__, unicode_repr(self.default))
          )
  
  
  class CurrentUserDefault(object):
      def set_context(self, serializer_field):
          self.user = serializer_field.context['request'].user
  
      def __call__(self):
          return self.user
  
      def __repr__(self):
          return unicode_to_repr('%s()' % self.__class__.__name__)
  
  
  class SkipField(Exception):
      pass
  
  
  REGEX_TYPE = type(re.compile(''))
  
  NOT_READ_ONLY_WRITE_ONLY = 'May not set both `read_only` and `write_only`'
  NOT_READ_ONLY_REQUIRED = 'May not set both `read_only` and `required`'
  NOT_REQUIRED_DEFAULT = 'May not set both `required` and `default`'
  USE_READONLYFIELD = 'Field(read_only=True) should be ReadOnlyField'
  MISSING_ERROR_MESSAGE = (
      'ValidationError raised by `{class_name}`, but error key `{key}` does '
      'not exist in the `error_messages` dictionary.'
  )
  
  
  class Field(object):
      _creation_counter = 0
  
      default_error_messages = {
          'required': _('This field is required.'),
          'null': _('This field may not be null.')
      }
      default_validators = []
      default_empty_html = empty
      initial = None
  
      def __init__(self, read_only=False, write_only=False,
                   required=None, default=empty, initial=empty, source=None,
                   label=None, help_text=None, style=None,
                   error_messages=None, validators=None, allow_null=False):
          self._creation_counter = Field._creation_counter
          Field._creation_counter += 1
  
          # If `required` is unset, then use `True` unless a default is provided.
          if required is None:
              required = default is empty and not read_only
  
          # Some combinations of keyword arguments do not make sense.
          assert not (read_only and write_only), NOT_READ_ONLY_WRITE_ONLY
          assert not (read_only and required), NOT_READ_ONLY_REQUIRED
          assert not (required and default is not empty), NOT_REQUIRED_DEFAULT
          assert not (read_only and self.__class__ == Field), USE_READONLYFIELD
  
          self.read_only = read_only
          self.write_only = write_only
          self.required = required
          self.default = default
          self.source = source
          self.initial = self.initial if (initial is empty) else initial
          self.label = label
          self.help_text = help_text
          self.style = {} if style is None else style
          self.allow_null = allow_null
  
          if self.default_empty_html is not empty:
              if default is not empty:
                  self.default_empty_html = default
  
          if validators is not None:
              self.validators = validators[:]
  
          # These are set up by `.bind()` when the field is added to a serializer.
          self.field_name = None
          self.parent = None
  
          # Collect default error message from self and parent classes
          messages = {}
          for cls in reversed(self.__class__.__mro__):
              messages.update(getattr(cls, 'default_error_messages', {}))
          messages.update(error_messages or {})
          self.error_messages = messages
  
      def bind(self, field_name, parent):
          """
          Initializes the field name and parent for the field instance.
          Called when a field is added to the parent serializer instance.
          """
  
          # In order to enforce a consistent style, we error if a redundant
          # 'source' argument has been used. For example:
          # my_field = serializer.CharField(source='my_field')
          assert self.source != field_name, (
              "It is redundant to specify `source='%s'` on field '%s' in "
              "serializer '%s', because it is the same as the field name. "
              "Remove the `source` keyword argument." %
              (field_name, self.__class__.__name__, parent.__class__.__name__)
          )
  
          self.field_name = field_name
          self.parent = parent
  
          # `self.label` should default to being based on the field name.
          if self.label is None:
              self.label = field_name.replace('_', ' ').capitalize()
  
          # self.source should default to being the same as the field name.
          if self.source is None:
              self.source = field_name
  
          # self.source_attrs is a list of attributes that need to be looked up
          # when serializing the instance, or populating the validated data.
          if self.source == '*':
              self.source_attrs = []
          else:
              self.source_attrs = self.source.split('.')
  
      # .validators is a lazily loaded property, that gets its default
      # value from `get_validators`.
      @property
      def validators(self):
          if not hasattr(self, '_validators'):
              self._validators = self.get_validators()
          return self._validators
  
      @validators.setter
      def validators(self, validators):
          self._validators = validators
  
      def get_validators(self):
          return self.default_validators[:]
  
      def get_initial(self):
          """
          Return a value to use when the field is being returned as a primitive
          value, without any object instance.
          """
          if callable(self.initial):
              return self.initial()
          return self.initial
  
      def get_value(self, dictionary):
          """
          Given the *incoming* primitive data, return the value for this field
          that should be validated and transformed to a native value.
          """
          if html.is_html_input(dictionary):
              # HTML forms will represent empty fields as '', and cannot
              # represent None or False values directly.
              if self.field_name not in dictionary:
                  if getattr(self.root, 'partial', False):
                      return empty
                  return self.default_empty_html
              ret = dictionary[self.field_name]
              if ret == '' and self.allow_null:
                  # If the field is blank, and null is a valid value then
                  # determine if we should use null instead.
                  return '' if getattr(self, 'allow_blank', False) else None
              elif ret == '' and not self.required:
                  # If the field is blank, and emptiness is valid then
                  # determine if we should use emptiness instead.
                  return '' if getattr(self, 'allow_blank', False) else empty
              return ret
          return dictionary.get(self.field_name, empty)
  
      def get_attribute(self, instance):
          """
          Given the *outgoing* object instance, return the primitive value
          that should be used for this field.
          """
          try:
              return get_attribute(instance, self.source_attrs)
          except (KeyError, AttributeError) as exc:
              if self.default is not empty:
                  return self.get_default()
              if self.allow_null:
                  return None
              if not self.required:
                  raise SkipField()
              msg = (
                  'Got {exc_type} when attempting to get a value for field '
                  '`{field}` on serializer `{serializer}`.\nThe serializer '
                  'field might be named incorrectly and not match '
                  'any attribute or key on the `{instance}` instance.\n'
                  'Original exception text was: {exc}.'.format(
                      exc_type=type(exc).__name__,
                      field=self.field_name,
                      serializer=self.parent.__class__.__name__,
                      instance=instance.__class__.__name__,
                      exc=exc
                  )
              )
              raise type(exc)(msg)
  
      def get_default(self):
          """
          Return the default value to use when validating data if no input
          is provided for this field.
  
          If a default has not been set for this field then this will simply
          raise `SkipField`, indicating that no value should be set in the
          validated data for this field.
          """
          if self.default is empty or getattr(self.root, 'partial', False):
              # No default, or this is a partial update.
              raise SkipField()
          if callable(self.default):
              if hasattr(self.default, 'set_context'):
                  self.default.set_context(self)
              return self.default()
          return self.default
  
      def validate_empty_values(self, data):
          """
          Validate empty values, and either:
  
          * Raise `ValidationError`, indicating invalid data.
          * Raise `SkipField`, indicating that the field should be ignored.
          * Return (True, data), indicating an empty value that should be
            returned without any further validation being applied.
          * Return (False, data), indicating a non-empty value, that should
            have validation applied as normal.
          """
          if self.read_only:
              return (True, self.get_default())
  
          if data is empty:
              if getattr(self.root, 'partial', False):
                  raise SkipField()
              if self.required:
                  self.fail('required')
              return (True, self.get_default())
  
          if data is None:
              if not self.allow_null:
                  self.fail('null')
              return (True, None)
  
          return (False, data)
  
      def run_validation(self, data=empty):
          """
          Validate a simple representation and return the internal value.
  
          The provided data may be `empty` if no representation was included
          in the input.
  
          May raise `SkipField` if the field should not be included in the
          validated data.
          """
          (is_empty_value, data) = self.validate_empty_values(data)
          if is_empty_value:
              return data
          value = self.to_internal_value(data)
          self.run_validators(value)
          return value
  
      def run_validators(self, value):
          """
          Test the given value against all the validators on the field,
          and either raise a `ValidationError` or simply return.
          """
          errors = []
          for validator in self.validators:
              if hasattr(validator, 'set_context'):
                  validator.set_context(self)
  
              try:
                  validator(value)
              except ValidationError as exc:
                  # If the validation error contains a mapping of fields to
                  # errors then simply raise it immediately rather than
                  # attempting to accumulate a list of errors.
                  if isinstance(exc.detail, dict):
                      raise
                  errors.extend(exc.detail)
              except DjangoValidationError as exc:
                  errors.extend(get_error_detail(exc))
          if errors:
              raise ValidationError(errors)
  
      def to_internal_value(self, data):
          """
          Transform the *incoming* primitive data into a native value.
          """
          raise NotImplementedError(
              '{cls}.to_internal_value() must be implemented.'.format(
                  cls=self.__class__.__name__
              )
          )
  
      def to_representation(self, value):
          """
          Transform the *outgoing* native value into primitive data.
          """
          raise NotImplementedError(
              '{cls}.to_representation() must be implemented for field '
              '{field_name}. If you do not need to support write operations '
              'you probably want to subclass `ReadOnlyField` instead.'.format(
                  cls=self.__class__.__name__,
                  field_name=self.field_name,
              )
          )
  
      def fail(self, key, **kwargs):
          """
          A helper method that simply raises a validation error.
          """
          try:
              msg = self.error_messages[key]
          except KeyError:
              class_name = self.__class__.__name__
              msg = MISSING_ERROR_MESSAGE.format(class_name=class_name, key=key)
              raise AssertionError(msg)
          message_string = msg.format(**kwargs)
          raise ValidationError(message_string, code=key)
  
      @property
      def root(self):
          """
          Returns the top-level serializer for this field.
          """
          root = self
          while root.parent is not None:
              root = root.parent
          return root
  
      @property
      def context(self):
          """
          Returns the context as passed to the root serializer on initialization.
          """
          return getattr(self.root, '_context', {})
  
      def __new__(cls, *args, **kwargs):
          """
          When a field is instantiated, we store the arguments that were used,
          so that we can present a helpful representation of the object.
          """
          instance = super(Field, cls).__new__(cls)
          instance._args = args
          instance._kwargs = kwargs
          return instance
  
      def __deepcopy__(self, memo):
          """
          When cloning fields we instantiate using the arguments it was
          originally created with, rather than copying the complete state.
          """
          # Treat regexes and validators as immutable.
          # See https://github.com/encode/django-rest-framework/issues/1954
          # and https://github.com/encode/django-rest-framework/pull/4489
          args = [
              copy.deepcopy(item) if not isinstance(item, REGEX_TYPE) else item
              for item in self._args
          ]
          kwargs = {
              key: (copy.deepcopy(value) if (key not in ('validators', 'regex')) else value)
              for key, value in self._kwargs.items()
          }
          return self.__class__(*args, **kwargs)
  
      def __repr__(self):
          """
          Fields are represented using their initial calling arguments.
          This allows us to create descriptive representations for serializer
          instances that show all the declared fields on the serializer.
          """
          return unicode_to_repr(representation.field_repr(self))
  
  
  # Boolean types...
  
  class BooleanField(Field):
      default_error_messages = {
          'invalid': _('Must be a valid boolean.')
      }
      default_empty_html = False
      initial = False
      TRUE_VALUES = {
          't', 'T',
          'y', 'Y', 'yes', 'YES',
          'true', 'True', 'TRUE',
          'on', 'On', 'ON',
          '1', 1,
          True
      }
      FALSE_VALUES = {
          'f', 'F',
          'n', 'N', 'no', 'NO',
          'false', 'False', 'FALSE',
          'off', 'Off', 'OFF',
          '0', 0, 0.0,
          False
      }
      NULL_VALUES = {'null', 'Null', 'NULL', '', None}
  
      def to_internal_value(self, data):
          try:
              if data in self.TRUE_VALUES:
                  return True
              elif data in self.FALSE_VALUES:
                  return False
              elif data in self.NULL_VALUES and self.allow_null:
                  return None
          except TypeError:  # Input is an unhashable type
              pass
          self.fail('invalid', input=data)
  
      def to_representation(self, value):
          if value in self.TRUE_VALUES:
              return True
          elif value in self.FALSE_VALUES:
              return False
          if value in self.NULL_VALUES and self.allow_null:
              return None
          return bool(value)
  
  
  class NullBooleanField(Field):
      default_error_messages = {
          'invalid': _('Must be a valid boolean.')
      }
      initial = None
      TRUE_VALUES = {
          't', 'T',
          'y', 'Y', 'yes', 'YES',
          'true', 'True', 'TRUE',
          'on', 'On', 'ON',
          '1', 1,
          True
      }
      FALSE_VALUES = {
          'f', 'F',
          'n', 'N', 'no', 'NO',
          'false', 'False', 'FALSE',
          'off', 'Off', 'OFF',
          '0', 0, 0.0,
          False
      }
      NULL_VALUES = {'null', 'Null', 'NULL', '', None}
  
      def __init__(self, **kwargs):
          assert 'allow_null' not in kwargs, '`allow_null` is not a valid option.'
          kwargs['allow_null'] = True
          super(NullBooleanField, self).__init__(**kwargs)
  
      def to_internal_value(self, data):
          try:
              if data in self.TRUE_VALUES:
                  return True
              elif data in self.FALSE_VALUES:
                  return False
              elif data in self.NULL_VALUES:
                  return None
          except TypeError:  # Input is an unhashable type
              pass
          self.fail('invalid', input=data)
  
      def to_representation(self, value):
          if value in self.NULL_VALUES:
              return None
          if value in self.TRUE_VALUES:
              return True
          elif value in self.FALSE_VALUES:
              return False
          return bool(value)
  
  
  # String types...
  
  class CharField(Field):
      default_error_messages = {
          'invalid': _('Not a valid string.'),
          'blank': _('This field may not be blank.'),
          'max_length': _('Ensure this field has no more than {max_length} characters.'),
          'min_length': _('Ensure this field has at least {min_length} characters.'),
      }
      initial = ''
  
      def __init__(self, **kwargs):
          self.allow_blank = kwargs.pop('allow_blank', False)
          self.trim_whitespace = kwargs.pop('trim_whitespace', True)
          self.max_length = kwargs.pop('max_length', None)
          self.min_length = kwargs.pop('min_length', None)
          super(CharField, self).__init__(**kwargs)
          if self.max_length is not None:
              message = lazy(
                  self.error_messages['max_length'].format,
                  six.text_type)(max_length=self.max_length)
              self.validators.append(
                  MaxLengthValidator(self.max_length, message=message))
          if self.min_length is not None:
              message = lazy(
                  self.error_messages['min_length'].format,
                  six.text_type)(min_length=self.min_length)
              self.validators.append(
                  MinLengthValidator(self.min_length, message=message))
  
          # ProhibitNullCharactersValidator is None on Django < 2.0
          if ProhibitNullCharactersValidator is not None:
              self.validators.append(ProhibitNullCharactersValidator())
  
      def run_validation(self, data=empty):
          # Test for the empty string here so that it does not get validated,
          # and so that subclasses do not need to handle it explicitly
          # inside the `to_internal_value()` method.
          if data == '' or (self.trim_whitespace and six.text_type(data).strip() == ''):
              if not self.allow_blank:
                  self.fail('blank')
              return ''
          return super(CharField, self).run_validation(data)
  
      def to_internal_value(self, data):
          # We're lenient with allowing basic numerics to be coerced into strings,
          # but other types should fail. Eg. unclear if booleans should represent as `true` or `True`,
          # and composites such as lists are likely user error.
          if isinstance(data, bool) or not isinstance(data, six.string_types + six.integer_types + (float,)):
              self.fail('invalid')
          value = six.text_type(data)
          return value.strip() if self.trim_whitespace else value
  
      def to_representation(self, value):
          return six.text_type(value)
  
  
  class EmailField(CharField):
      default_error_messages = {
          'invalid': _('Enter a valid email address.')
      }
  
      def __init__(self, **kwargs):
          super(EmailField, self).__init__(**kwargs)
          validator = EmailValidator(message=self.error_messages['invalid'])
          self.validators.append(validator)
  
  
  class RegexField(CharField):
      default_error_messages = {
          'invalid': _('This value does not match the required pattern.')
      }
  
      def __init__(self, regex, **kwargs):
          super(RegexField, self).__init__(**kwargs)
          validator = RegexValidator(regex, message=self.error_messages['invalid'])
          self.validators.append(validator)
  
  
  class SlugField(CharField):
      default_error_messages = {
          'invalid': _('Enter a valid "slug" consisting of letters, numbers, underscores or hyphens.'),
          'invalid_unicode': _('Enter a valid "slug" consisting of Unicode letters, numbers, underscores, or hyphens.')
      }
  
      def __init__(self, allow_unicode=False, **kwargs):
          super(SlugField, self).__init__(**kwargs)
          self.allow_unicode = allow_unicode
          if self.allow_unicode:
              validator = RegexValidator(re.compile(r'^[-\w]+\Z', re.UNICODE), message=self.error_messages['invalid_unicode'])
          else:
              validator = RegexValidator(re.compile(r'^[-a-zA-Z0-9_]+$'), message=self.error_messages['invalid'])
          self.validators.append(validator)
  
  
  class URLField(CharField):
      default_error_messages = {
          'invalid': _('Enter a valid URL.')
      }
  
      def __init__(self, **kwargs):
          super(URLField, self).__init__(**kwargs)
          validator = URLValidator(message=self.error_messages['invalid'])
          self.validators.append(validator)
  
  
  class UUIDField(Field):
      valid_formats = ('hex_verbose', 'hex', 'int', 'urn')
  
      default_error_messages = {
          'invalid': _('Must be a valid UUID.'),
      }
  
      def __init__(self, **kwargs):
          self.uuid_format = kwargs.pop('format', 'hex_verbose')
          if self.uuid_format not in self.valid_formats:
              raise ValueError(
                  'Invalid format for uuid representation. '
                  'Must be one of "{0}"'.format('", "'.join(self.valid_formats))
              )
          super(UUIDField, self).__init__(**kwargs)
  
      def to_internal_value(self, data):
          if not isinstance(data, uuid.UUID):
              try:
                  if isinstance(data, six.integer_types):
                      return uuid.UUID(int=data)
                  elif isinstance(data, six.string_types):
                      return uuid.UUID(hex=data)
                  else:
                      self.fail('invalid', value=data)
              except (ValueError):
                  self.fail('invalid', value=data)
          return data
  
      def to_representation(self, value):
          if self.uuid_format == 'hex_verbose':
              return str(value)
          else:
              return getattr(value, self.uuid_format)
  
  
  class IPAddressField(CharField):
      """Support both IPAddressField and GenericIPAddressField"""
  
      default_error_messages = {
          'invalid': _('Enter a valid IPv4 or IPv6 address.'),
      }
  
      def __init__(self, protocol='both', **kwargs):
          self.protocol = protocol.lower()
          self.unpack_ipv4 = (self.protocol == 'both')
          super(IPAddressField, self).__init__(**kwargs)
          validators, error_message = ip_address_validators(protocol, self.unpack_ipv4)
          self.validators.extend(validators)
  
      def to_internal_value(self, data):
          if not isinstance(data, six.string_types):
              self.fail('invalid', value=data)
  
          if ':' in data:
              try:
                  if self.protocol in ('both', 'ipv6'):
                      return clean_ipv6_address(data, self.unpack_ipv4)
              except DjangoValidationError:
                  self.fail('invalid', value=data)
  
          return super(IPAddressField, self).to_internal_value(data)
  
  
  # Number types...
  
  class IntegerField(Field):
      default_error_messages = {
          'invalid': _('A valid integer is required.'),
          'max_value': _('Ensure this value is less than or equal to {max_value}.'),
          'min_value': _('Ensure this value is greater than or equal to {min_value}.'),
          'max_string_length': _('String value too large.')
      }
      MAX_STRING_LENGTH = 1000  # Guard against malicious string inputs.
      re_decimal = re.compile(r'\.0*\s*$')  # allow e.g. '1.0' as an int, but not '1.2'
  
      def __init__(self, **kwargs):
          self.max_value = kwargs.pop('max_value', None)
          self.min_value = kwargs.pop('min_value', None)
          super(IntegerField, self).__init__(**kwargs)
          if self.max_value is not None:
              message = lazy(
                  self.error_messages['max_value'].format,
                  six.text_type)(max_value=self.max_value)
              self.validators.append(
                  MaxValueValidator(self.max_value, message=message))
          if self.min_value is not None:
              message = lazy(
                  self.error_messages['min_value'].format,
                  six.text_type)(min_value=self.min_value)
              self.validators.append(
                  MinValueValidator(self.min_value, message=message))
  
      def to_internal_value(self, data):
          if isinstance(data, six.text_type) and len(data) > self.MAX_STRING_LENGTH:
              self.fail('max_string_length')
  
          try:
              data = int(self.re_decimal.sub('', str(data)))
          except (ValueError, TypeError):
              self.fail('invalid')
          return data
  
      def to_representation(self, value):
          return int(value)
  
  
  class FloatField(Field):
      default_error_messages = {
          'invalid': _('A valid number is required.'),
          'max_value': _('Ensure this value is less than or equal to {max_value}.'),
          'min_value': _('Ensure this value is greater than or equal to {min_value}.'),
          'max_string_length': _('String value too large.')
      }
      MAX_STRING_LENGTH = 1000  # Guard against malicious string inputs.
  
      def __init__(self, **kwargs):
          self.max_value = kwargs.pop('max_value', None)
          self.min_value = kwargs.pop('min_value', None)
          super(FloatField, self).__init__(**kwargs)
          if self.max_value is not None:
              message = lazy(
                  self.error_messages['max_value'].format,
                  six.text_type)(max_value=self.max_value)
              self.validators.append(
                  MaxValueValidator(self.max_value, message=message))
          if self.min_value is not None:
              message = lazy(
                  self.error_messages['min_value'].format,
                  six.text_type)(min_value=self.min_value)
              self.validators.append(
                  MinValueValidator(self.min_value, message=message))
  
      def to_internal_value(self, data):
  
          if isinstance(data, six.text_type) and len(data) > self.MAX_STRING_LENGTH:
              self.fail('max_string_length')
  
          try:
              return float(data)
          except (TypeError, ValueError):
              self.fail('invalid')
  
      def to_representation(self, value):
          return float(value)
  
  
  class DecimalField(Field):
      default_error_messages = {
          'invalid': _('A valid number is required.'),
          'max_value': _('Ensure this value is less than or equal to {max_value}.'),
          'min_value': _('Ensure this value is greater than or equal to {min_value}.'),
          'max_digits': _('Ensure that there are no more than {max_digits} digits in total.'),
          'max_decimal_places': _('Ensure that there are no more than {max_decimal_places} decimal places.'),
          'max_whole_digits': _('Ensure that there are no more than {max_whole_digits} digits before the decimal point.'),
          'max_string_length': _('String value too large.')
      }
      MAX_STRING_LENGTH = 1000  # Guard against malicious string inputs.
  
      def __init__(self, max_digits, decimal_places, coerce_to_string=None, max_value=None, min_value=None,
                   localize=False, rounding=None, **kwargs):
          self.max_digits = max_digits
          self.decimal_places = decimal_places
          self.localize = localize
          if coerce_to_string is not None:
              self.coerce_to_string = coerce_to_string
          if self.localize:
              self.coerce_to_string = True
  
          self.max_value = max_value
          self.min_value = min_value
  
          if self.max_digits is not None and self.decimal_places is not None:
              self.max_whole_digits = self.max_digits - self.decimal_places
          else:
              self.max_whole_digits = None
  
          super(DecimalField, self).__init__(**kwargs)
  
          if self.max_value is not None:
              message = lazy(
                  self.error_messages['max_value'].format,
                  six.text_type)(max_value=self.max_value)
              self.validators.append(
                  MaxValueValidator(self.max_value, message=message))
          if self.min_value is not None:
              message = lazy(
                  self.error_messages['min_value'].format,
                  six.text_type)(min_value=self.min_value)
              self.validators.append(
                  MinValueValidator(self.min_value, message=message))
  
          if rounding is not None:
              valid_roundings = [v for k, v in vars(decimal).items() if k.startswith('ROUND_')]
              assert rounding in valid_roundings, (
                  'Invalid rounding option %s. Valid values for rounding are: %s' % (rounding, valid_roundings))
          self.rounding = rounding
  
      def to_internal_value(self, data):
          """
          Validate that the input is a decimal number and return a Decimal
          instance.
          """
  
          data = smart_text(data).strip()
  
          if self.localize:
              data = sanitize_separators(data)
  
          if len(data) > self.MAX_STRING_LENGTH:
              self.fail('max_string_length')
  
          try:
              value = decimal.Decimal(data)
          except decimal.DecimalException:
              self.fail('invalid')
  
          # Check for NaN. It is the only value that isn't equal to itself,
          # so we can use this to identify NaN values.
          if value != value:
              self.fail('invalid')
  
          # Check for infinity and negative infinity.
          if value in (decimal.Decimal('Inf'), decimal.Decimal('-Inf')):
              self.fail('invalid')
  
          return self.quantize(self.validate_precision(value))
  
      def validate_precision(self, value):
          """
          Ensure that there are no more than max_digits in the number, and no
          more than decimal_places digits after the decimal point.
  
          Override this method to disable the precision validation for input
          values or to enhance it in any way you need to.
          """
          sign, digittuple, exponent = value.as_tuple()
  
          if exponent >= 0:
              # 1234500.0
              total_digits = len(digittuple) + exponent
              whole_digits = total_digits
              decimal_places = 0
          elif len(digittuple) > abs(exponent):
              # 123.45
              total_digits = len(digittuple)
              whole_digits = total_digits - abs(exponent)
              decimal_places = abs(exponent)
          else:
              # 0.001234
              total_digits = abs(exponent)
              whole_digits = 0
              decimal_places = total_digits
  
          if self.max_digits is not None and total_digits > self.max_digits:
              self.fail('max_digits', max_digits=self.max_digits)
          if self.decimal_places is not None and decimal_places > self.decimal_places:
              self.fail('max_decimal_places', max_decimal_places=self.decimal_places)
          if self.max_whole_digits is not None and whole_digits > self.max_whole_digits:
              self.fail('max_whole_digits', max_whole_digits=self.max_whole_digits)
  
          return value
  
      def to_representation(self, value):
          coerce_to_string = getattr(self, 'coerce_to_string', api_settings.COERCE_DECIMAL_TO_STRING)
  
          if not isinstance(value, decimal.Decimal):
              value = decimal.Decimal(six.text_type(value).strip())
  
          quantized = self.quantize(value)
  
          if not coerce_to_string:
              return quantized
          if self.localize:
              return localize_input(quantized)
  
          return '{0:f}'.format(quantized)
  
      def quantize(self, value):
          """
          Quantize the decimal value to the configured precision.
          """
          if self.decimal_places is None:
              return value
  
          context = decimal.getcontext().copy()
          if self.max_digits is not None:
              context.prec = self.max_digits
          return value.quantize(
              decimal.Decimal('.1') ** self.decimal_places,
              rounding=self.rounding,
              context=context
          )
  
  
  # Date & time fields...
  
  class DateTimeField(Field):
      default_error_messages = {
          'invalid': _('Datetime has wrong format. Use one of these formats instead: {format}.'),
          'date': _('Expected a datetime but got a date.'),
          'make_aware': _('Invalid datetime for the timezone "{timezone}".'),
          'overflow': _('Datetime value out of range.')
      }
      datetime_parser = datetime.datetime.strptime
  
      def __init__(self, format=empty, input_formats=None, default_timezone=None, *args, **kwargs):
          if format is not empty:
              self.format = format
          if input_formats is not None:
              self.input_formats = input_formats
          if default_timezone is not None:
              self.timezone = default_timezone
          super(DateTimeField, self).__init__(*args, **kwargs)
  
      def enforce_timezone(self, value):
          """
          When `self.default_timezone` is `None`, always return naive datetimes.
          When `self.default_timezone` is not `None`, always return aware datetimes.
          """
          field_timezone = getattr(self, 'timezone', self.default_timezone())
  
          if field_timezone is not None:
              if timezone.is_aware(value):
                  try:
                      return value.astimezone(field_timezone)
                  except OverflowError:
                      self.fail('overflow')
              try:
                  return timezone.make_aware(value, field_timezone)
              except InvalidTimeError:
                  self.fail('make_aware', timezone=field_timezone)
          elif (field_timezone is None) and timezone.is_aware(value):
              return timezone.make_naive(value, utc)
          return value
  
      def default_timezone(self):
          return timezone.get_current_timezone() if settings.USE_TZ else None
  
      def to_internal_value(self, value):
          input_formats = getattr(self, 'input_formats', api_settings.DATETIME_INPUT_FORMATS)
  
          if isinstance(value, datetime.date) and not isinstance(value, datetime.datetime):
              self.fail('date')
  
          if isinstance(value, datetime.datetime):
              return self.enforce_timezone(value)
  
          for input_format in input_formats:
              if input_format.lower() == ISO_8601:
                  try:
                      parsed = parse_datetime(value)
                      if parsed is not None:
                          return self.enforce_timezone(parsed)
                  except (ValueError, TypeError):
                      pass
              else:
                  try:
                      parsed = self.datetime_parser(value, input_format)
                      return self.enforce_timezone(parsed)
                  except (ValueError, TypeError):
                      pass
  
          humanized_format = humanize_datetime.datetime_formats(input_formats)
          self.fail('invalid', format=humanized_format)
  
      def to_representation(self, value):
          if not value:
              return None
  
          output_format = getattr(self, 'format', api_settings.DATETIME_FORMAT)
  
          if output_format is None or isinstance(value, six.string_types):
              return value
  
          value = self.enforce_timezone(value)
  
          if output_format.lower() == ISO_8601:
              value = value.isoformat()
              if value.endswith('+00:00'):
                  value = value[:-6] + 'Z'
              return value
          return value.strftime(output_format)
  
  
  class DateField(Field):
      default_error_messages = {
          'invalid': _('Date has wrong format. Use one of these formats instead: {format}.'),
          'datetime': _('Expected a date but got a datetime.'),
      }
      datetime_parser = datetime.datetime.strptime
  
      def __init__(self, format=empty, input_formats=None, *args, **kwargs):
          if format is not empty:
              self.format = format
          if input_formats is not None:
              self.input_formats = input_formats
          super(DateField, self).__init__(*args, **kwargs)
  
      def to_internal_value(self, value):
          input_formats = getattr(self, 'input_formats', api_settings.DATE_INPUT_FORMATS)
  
          if isinstance(value, datetime.datetime):
              self.fail('datetime')
  
          if isinstance(value, datetime.date):
              return value
  
          for input_format in input_formats:
              if input_format.lower() == ISO_8601:
                  try:
                      parsed = parse_date(value)
                  except (ValueError, TypeError):
                      pass
                  else:
                      if parsed is not None:
                          return parsed
              else:
                  try:
                      parsed = self.datetime_parser(value, input_format)
                  except (ValueError, TypeError):
                      pass
                  else:
                      return parsed.date()
  
          humanized_format = humanize_datetime.date_formats(input_formats)
          self.fail('invalid', format=humanized_format)
  
      def to_representation(self, value):
          if not value:
              return None
  
          output_format = getattr(self, 'format', api_settings.DATE_FORMAT)
  
          if output_format is None or isinstance(value, six.string_types):
              return value
  
          # Applying a `DateField` to a datetime value is almost always
          # not a sensible thing to do, as it means naively dropping
          # any explicit or implicit timezone info.
          assert not isinstance(value, datetime.datetime), (
              'Expected a `date`, but got a `datetime`. Refusing to coerce, '
              'as this may mean losing timezone information. Use a custom '
              'read-only field and deal with timezone issues explicitly.'
          )
  
          if output_format.lower() == ISO_8601:
              return value.isoformat()
  
          return value.strftime(output_format)
  
  
  class TimeField(Field):
      default_error_messages = {
          'invalid': _('Time has wrong format. Use one of these formats instead: {format}.'),
      }
      datetime_parser = datetime.datetime.strptime
  
      def __init__(self, format=empty, input_formats=None, *args, **kwargs):
          if format is not empty:
              self.format = format
          if input_formats is not None:
              self.input_formats = input_formats
          super(TimeField, self).__init__(*args, **kwargs)
  
      def to_internal_value(self, value):
          input_formats = getattr(self, 'input_formats', api_settings.TIME_INPUT_FORMATS)
  
          if isinstance(value, datetime.time):
              return value
  
          for input_format in input_formats:
              if input_format.lower() == ISO_8601:
                  try:
                      parsed = parse_time(value)
                  except (ValueError, TypeError):
                      pass
                  else:
                      if parsed is not None:
                          return parsed
              else:
                  try:
                      parsed = self.datetime_parser(value, input_format)
                  except (ValueError, TypeError):
                      pass
                  else:
                      return parsed.time()
  
          humanized_format = humanize_datetime.time_formats(input_formats)
          self.fail('invalid', format=humanized_format)
  
      def to_representation(self, value):
          if value in (None, ''):
              return None
  
          output_format = getattr(self, 'format', api_settings.TIME_FORMAT)
  
          if output_format is None or isinstance(value, six.string_types):
              return value
  
          # Applying a `TimeField` to a datetime value is almost always
          # not a sensible thing to do, as it means naively dropping
          # any explicit or implicit timezone info.
          assert not isinstance(value, datetime.datetime), (
              'Expected a `time`, but got a `datetime`. Refusing to coerce, '
              'as this may mean losing timezone information. Use a custom '
              'read-only field and deal with timezone issues explicitly.'
          )
  
          if output_format.lower() == ISO_8601:
              return value.isoformat()
          return value.strftime(output_format)
  
  
  class DurationField(Field):
      default_error_messages = {
          'invalid': _('Duration has wrong format. Use one of these formats instead: {format}.'),
          'max_value': _('Ensure this value is less than or equal to {max_value}.'),
          'min_value': _('Ensure this value is greater than or equal to {min_value}.'),
      }
  
      def __init__(self, **kwargs):
          self.max_value = kwargs.pop('max_value', None)
          self.min_value = kwargs.pop('min_value', None)
          super(DurationField, self).__init__(**kwargs)
          if self.max_value is not None:
              message = lazy(
                  self.error_messages['max_value'].format,
                  six.text_type)(max_value=self.max_value)
              self.validators.append(
                  MaxValueValidator(self.max_value, message=message))
          if self.min_value is not None:
              message = lazy(
                  self.error_messages['min_value'].format,
                  six.text_type)(min_value=self.min_value)
              self.validators.append(
                  MinValueValidator(self.min_value, message=message))
  
      def to_internal_value(self, value):
          if isinstance(value, datetime.timedelta):
              return value
          parsed = parse_duration(six.text_type(value))
          if parsed is not None:
              return parsed
          self.fail('invalid', format='[DD] [HH:[MM:]]ss[.uuuuuu]')
  
      def to_representation(self, value):
          return duration_string(value)
  
  
  # Choice types...
  
  class ChoiceField(Field):
      default_error_messages = {
          'invalid_choice': _('"{input}" is not a valid choice.')
      }
      html_cutoff = None
      html_cutoff_text = _('More than {count} items...')
  
      def __init__(self, choices, **kwargs):
          self.choices = choices
          self.html_cutoff = kwargs.pop('html_cutoff', self.html_cutoff)
          self.html_cutoff_text = kwargs.pop('html_cutoff_text', self.html_cutoff_text)
  
          self.allow_blank = kwargs.pop('allow_blank', False)
  
          super(ChoiceField, self).__init__(**kwargs)
  
      def to_internal_value(self, data):
          if data == '' and self.allow_blank:
              return ''
  
          try:
              return self.choice_strings_to_values[six.text_type(data)]
          except KeyError:
              self.fail('invalid_choice', input=data)
  
      def to_representation(self, value):
          if value in ('', None):
              return value
          return self.choice_strings_to_values.get(six.text_type(value), value)
  
      def iter_options(self):
          """
          Helper method for use with templates rendering select widgets.
          """
          return iter_options(
              self.grouped_choices,
              cutoff=self.html_cutoff,
              cutoff_text=self.html_cutoff_text
          )
  
      def _get_choices(self):
          return self._choices
  
      def _set_choices(self, choices):
          self.grouped_choices = to_choices_dict(choices)
          self._choices = flatten_choices_dict(self.grouped_choices)
  
          # Map the string representation of choices to the underlying value.
          # Allows us to deal with eg. integer choices while supporting either
          # integer or string input, but still get the correct datatype out.
          self.choice_strings_to_values = {
              six.text_type(key): key for key in self.choices
          }
  
      choices = property(_get_choices, _set_choices)
  
  
  class MultipleChoiceField(ChoiceField):
      default_error_messages = {
          'invalid_choice': _('"{input}" is not a valid choice.'),
          'not_a_list': _('Expected a list of items but got type "{input_type}".'),
          'empty': _('This selection may not be empty.')
      }
      default_empty_html = []
  
      def __init__(self, *args, **kwargs):
          self.allow_empty = kwargs.pop('allow_empty', True)
          super(MultipleChoiceField, self).__init__(*args, **kwargs)
  
      def get_value(self, dictionary):
          if self.field_name not in dictionary:
              if getattr(self.root, 'partial', False):
                  return empty
          # We override the default field access in order to support
          # lists in HTML forms.
          if html.is_html_input(dictionary):
              return dictionary.getlist(self.field_name)
          return dictionary.get(self.field_name, empty)
  
      def to_internal_value(self, data):
          if isinstance(data, type('')) or not hasattr(data, '__iter__'):
              self.fail('not_a_list', input_type=type(data).__name__)
          if not self.allow_empty and len(data) == 0:
              self.fail('empty')
  
          return {
              super(MultipleChoiceField, self).to_internal_value(item)
              for item in data
          }
  
      def to_representation(self, value):
          return {
              self.choice_strings_to_values.get(six.text_type(item), item) for item in value
          }
  
  
  class FilePathField(ChoiceField):
      default_error_messages = {
          'invalid_choice': _('"{input}" is not a valid path choice.')
      }
  
      def __init__(self, path, match=None, recursive=False, allow_files=True,
                   allow_folders=False, required=None, **kwargs):
          # Defer to Django's FilePathField implementation to get the
          # valid set of choices.
          field = DjangoFilePathField(
              path, match=match, recursive=recursive, allow_files=allow_files,
              allow_folders=allow_folders, required=required
          )
          kwargs['choices'] = field.choices
          super(FilePathField, self).__init__(**kwargs)
  
  
  # File types...
  
  class FileField(Field):
      default_error_messages = {
          'required': _('No file was submitted.'),
          'invalid': _('The submitted data was not a file. Check the encoding type on the form.'),
          'no_name': _('No filename could be determined.'),
          'empty': _('The submitted file is empty.'),
          'max_length': _('Ensure this filename has at most {max_length} characters (it has {length}).'),
      }
  
      def __init__(self, *args, **kwargs):
          self.max_length = kwargs.pop('max_length', None)
          self.allow_empty_file = kwargs.pop('allow_empty_file', False)
          if 'use_url' in kwargs:
              self.use_url = kwargs.pop('use_url')
          super(FileField, self).__init__(*args, **kwargs)
  
      def to_internal_value(self, data):
          try:
              # `UploadedFile` objects should have name and size attributes.
              file_name = data.name
              file_size = data.size
          except AttributeError:
              self.fail('invalid')
  
          if not file_name:
              self.fail('no_name')
          if not self.allow_empty_file and not file_size:
              self.fail('empty')
          if self.max_length and len(file_name) > self.max_length:
              self.fail('max_length', max_length=self.max_length, length=len(file_name))
  
          return data
  
      def to_representation(self, value):
          if not value:
              return None
  
          use_url = getattr(self, 'use_url', api_settings.UPLOADED_FILES_USE_URL)
  
          if use_url:
              if not getattr(value, 'url', None):
                  # If the file has not been saved it may not have a URL.
                  return None
              url = value.url
              request = self.context.get('request', None)
              if request is not None:
                  return request.build_absolute_uri(url)
              return url
          return value.name
  
  
  class ImageField(FileField):
      default_error_messages = {
          'invalid_image': _(
              'Upload a valid image. The file you uploaded was either not an image or a corrupted image.'
          ),
      }
  
      def __init__(self, *args, **kwargs):
          self._DjangoImageField = kwargs.pop('_DjangoImageField', DjangoImageField)
          super(ImageField, self).__init__(*args, **kwargs)
  
      def to_internal_value(self, data):
          # Image validation is a bit grungy, so we'll just outright
          # defer to Django's implementation so we don't need to
          # consider it, or treat PIL as a test dependency.
          file_object = super(ImageField, self).to_internal_value(data)
          django_field = self._DjangoImageField()
          django_field.error_messages = self.error_messages
          return django_field.clean(file_object)
  
  
  # Composite field types...
  
  class _UnvalidatedField(Field):
      def __init__(self, *args, **kwargs):
          super(_UnvalidatedField, self).__init__(*args, **kwargs)
          self.allow_blank = True
          self.allow_null = True
  
      def to_internal_value(self, data):
          return data
  
      def to_representation(self, value):
          return value
  
  
  class ListField(Field):
      child = _UnvalidatedField()
      initial = []
      default_error_messages = {
          'not_a_list': _('Expected a list of items but got type "{input_type}".'),
          'empty': _('This list may not be empty.'),
          'min_length': _('Ensure this field has at least {min_length} elements.'),
          'max_length': _('Ensure this field has no more than {max_length} elements.')
      }
  
      def __init__(self, *args, **kwargs):
          self.child = kwargs.pop('child', copy.deepcopy(self.child))
          self.allow_empty = kwargs.pop('allow_empty', True)
          self.max_length = kwargs.pop('max_length', None)
          self.min_length = kwargs.pop('min_length', None)
  
          assert not inspect.isclass(self.child), '`child` has not been instantiated.'
          assert self.child.source is None, (
              "The `source` argument is not meaningful when applied to a `child=` field. "
              "Remove `source=` from the field declaration."
          )
  
          super(ListField, self).__init__(*args, **kwargs)
          self.child.bind(field_name='', parent=self)
          if self.max_length is not None:
              message = self.error_messages['max_length'].format(max_length=self.max_length)
              self.validators.append(MaxLengthValidator(self.max_length, message=message))
          if self.min_length is not None:
              message = self.error_messages['min_length'].format(min_length=self.min_length)
              self.validators.append(MinLengthValidator(self.min_length, message=message))
  
      def get_value(self, dictionary):
          if self.field_name not in dictionary:
              if getattr(self.root, 'partial', False):
                  return empty
          # We override the default field access in order to support
          # lists in HTML forms.
          if html.is_html_input(dictionary):
              val = dictionary.getlist(self.field_name, [])
              if len(val) > 0:
                  # Support QueryDict lists in HTML input.
                  return val
              return html.parse_html_list(dictionary, prefix=self.field_name, default=empty)
  
          return dictionary.get(self.field_name, empty)
  
      def to_internal_value(self, data):
          """
          List of dicts of native values <- List of dicts of primitive datatypes.
          """
          if html.is_html_input(data):
              data = html.parse_html_list(data, default=[])
          if isinstance(data, type('')) or isinstance(data, collections.Mapping) or not hasattr(data, '__iter__'):
              self.fail('not_a_list', input_type=type(data).__name__)
          if not self.allow_empty and len(data) == 0:
              self.fail('empty')
          return self.run_child_validation(data)
  
      def to_representation(self, data):
          """
          List of object instances -> List of dicts of primitive datatypes.
          """
          return [self.child.to_representation(item) if item is not None else None for item in data]
  
      def run_child_validation(self, data):
          result = []
          errors = OrderedDict()
  
          for idx, item in enumerate(data):
              try:
                  result.append(self.child.run_validation(item))
              except ValidationError as e:
                  errors[idx] = e.detail
  
          if not errors:
              return result
          raise ValidationError(errors)
  
  
  class DictField(Field):
      child = _UnvalidatedField()
      initial = {}
      default_error_messages = {
          'not_a_dict': _('Expected a dictionary of items but got type "{input_type}".')
      }
  
      def __init__(self, *args, **kwargs):
          self.child = kwargs.pop('child', copy.deepcopy(self.child))
  
          assert not inspect.isclass(self.child), '`child` has not been instantiated.'
          assert self.child.source is None, (
              "The `source` argument is not meaningful when applied to a `child=` field. "
              "Remove `source=` from the field declaration."
          )
  
          super(DictField, self).__init__(*args, **kwargs)
          self.child.bind(field_name='', parent=self)
  
      def get_value(self, dictionary):
          # We override the default field access in order to support
          # dictionaries in HTML forms.
          if html.is_html_input(dictionary):
              return html.parse_html_dict(dictionary, prefix=self.field_name)
          return dictionary.get(self.field_name, empty)
  
      def to_internal_value(self, data):
          """
          Dicts of native values <- Dicts of primitive datatypes.
          """
          if html.is_html_input(data):
              data = html.parse_html_dict(data)
          if not isinstance(data, dict):
              self.fail('not_a_dict', input_type=type(data).__name__)
          return self.run_child_validation(data)
  
      def to_representation(self, value):
          return {
              six.text_type(key): self.child.to_representation(val) if val is not None else None
              for key, val in value.items()
          }
  
      def run_child_validation(self, data):
          result = {}
          errors = OrderedDict()
  
          for key, value in data.items():
              key = six.text_type(key)
  
              try:
                  result[key] = self.child.run_validation(value)
              except ValidationError as e:
                  errors[key] = e.detail
  
          if not errors:
              return result
          raise ValidationError(errors)
  
  
  class HStoreField(DictField):
      child = CharField(allow_blank=True, allow_null=True)
  
      def __init__(self, *args, **kwargs):
          super(HStoreField, self).__init__(*args, **kwargs)
          assert isinstance(self.child, CharField), (
              "The `child` argument must be an instance of `CharField`, "
              "as the hstore extension stores values as strings."
          )
  
  
  class JSONField(Field):
      default_error_messages = {
          'invalid': _('Value must be valid JSON.')
      }
  
      def __init__(self, *args, **kwargs):
          self.binary = kwargs.pop('binary', False)
          super(JSONField, self).__init__(*args, **kwargs)
  
      def get_value(self, dictionary):
          if html.is_html_input(dictionary) and self.field_name in dictionary:
              # When HTML form input is used, mark up the input
              # as being a JSON string, rather than a JSON primitive.
              class JSONString(six.text_type):
                  def __new__(self, value):
                      ret = six.text_type.__new__(self, value)
                      ret.is_json_string = True
                      return ret
              return JSONString(dictionary[self.field_name])
          return dictionary.get(self.field_name, empty)
  
      def to_internal_value(self, data):
          try:
              if self.binary or getattr(data, 'is_json_string', False):
                  if isinstance(data, bytes):
                      data = data.decode('utf-8')
                  return json.loads(data)
              else:
                  json.dumps(data)
          except (TypeError, ValueError):
              self.fail('invalid')
          return data
  
      def to_representation(self, value):
          if self.binary:
              value = json.dumps(value)
              # On python 2.x the return type for json.dumps() is underspecified.
              # On python 3.x json.dumps() returns unicode strings.
              if isinstance(value, six.text_type):
                  value = bytes(value.encode('utf-8'))
          return value
  
  
  # Miscellaneous field types...
  
  class ReadOnlyField(Field):
      """
      A read-only field that simply returns the field value.
  
      If the field is a method with no parameters, the method will be called
      and its return value used as the representation.
  
      For example, the following would call `get_expiry_date()` on the object:
  
      class ExampleSerializer(Serializer):
          expiry_date = ReadOnlyField(source='get_expiry_date')
      """
  
      def __init__(self, **kwargs):
          kwargs['read_only'] = True
          super(ReadOnlyField, self).__init__(**kwargs)
  
      def to_representation(self, value):
          return value
  
  
  class HiddenField(Field):
      """
      A hidden field does not take input from the user, or present any output,
      but it does populate a field in `validated_data`, based on its default
      value. This is particularly useful when we have a `unique_for_date`
      constraint on a pair of fields, as we need some way to include the date in
      the validated data.
      """
      def __init__(self, **kwargs):
          assert 'default' in kwargs, 'default is a required argument.'
          kwargs['write_only'] = True
          super(HiddenField, self).__init__(**kwargs)
  
      def get_value(self, dictionary):
          # We always use the default value for `HiddenField`.
          # User input is never provided or accepted.
          return empty
  
      def to_internal_value(self, data):
          return data
  
  
  class SerializerMethodField(Field):
      """
      A read-only field that get its representation from calling a method on the
      parent serializer class. The method called will be of the form
      "get_{field_name}", and should take a single argument, which is the
      object being serialized.
  
      For example:
  
      class ExampleSerializer(self):
          extra_info = SerializerMethodField()
  
          def get_extra_info(self, obj):
              return ...  # Calculate some data to return.
      """
      def __init__(self, method_name=None, **kwargs):
          self.method_name = method_name
          kwargs['source'] = '*'
          kwargs['read_only'] = True
          super(SerializerMethodField, self).__init__(**kwargs)
  
      def bind(self, field_name, parent):
          # In order to enforce a consistent style, we error if a redundant
          # 'method_name' argument has been used. For example:
          # my_field = serializer.SerializerMethodField(method_name='get_my_field')
          default_method_name = 'get_{field_name}'.format(field_name=field_name)
          assert self.method_name != default_method_name, (
              "It is redundant to specify `%s` on SerializerMethodField '%s' in "
              "serializer '%s', because it is the same as the default method name. "
              "Remove the `method_name` argument." %
              (self.method_name, field_name, parent.__class__.__name__)
          )
  
          # The method name should default to `get_{field_name}`.
          if self.method_name is None:
              self.method_name = default_method_name
  
          super(SerializerMethodField, self).bind(field_name, parent)
  
      def to_representation(self, value):
          method = getattr(self.parent, self.method_name)
          return method(value)
  
  
  class ModelField(Field):
      """
      A generic field that can be used against an arbitrary model field.
  
      This is used by `ModelSerializer` when dealing with custom model fields,
      that do not have a serializer field to be mapped to.
      """
      default_error_messages = {
          'max_length': _('Ensure this field has no more than {max_length} characters.'),
      }
  
      def __init__(self, model_field, **kwargs):
          self.model_field = model_field
          # The `max_length` option is supported by Django's base `Field` class,
          # so we'd better support it here.
          max_length = kwargs.pop('max_length', None)
          super(ModelField, self).__init__(**kwargs)
          if max_length is not None:
              message = lazy(
                  self.error_messages['max_length'].format,
                  six.text_type)(max_length=self.max_length)
              self.validators.append(
                  MaxLengthValidator(self.max_length, message=message))
  
      def to_internal_value(self, data):
          rel = self.model_field.remote_field
          if rel is not None:
              return rel.model._meta.get_field(rel.field_name).to_python(data)
          return self.model_field.to_python(data)
  
      def get_attribute(self, obj):
          # We pass the object instance onto `to_representation`,
          # not just the field attribute.
          return obj
  
      def to_representation(self, obj):
          value = self.model_field.value_from_object(obj)
          if is_protected_type(value):
              return value
          return self.model_field.value_to_string(obj)
  