text: # -*- coding: utf-8 -*-
  """Field classes for various types of data."""
  
  from __future__ import absolute_import, unicode_literals
  
  import collections
  import datetime as dt
  import uuid
  import warnings
  import decimal
  from operator import attrgetter
  
  from marshmallow import validate, utils, class_registry
  from marshmallow.base import FieldABC, SchemaABC
  from marshmallow.utils import missing as missing_
  from marshmallow.compat import text_type, basestring
  from marshmallow.exceptions import ValidationError
  from marshmallow.validate import Validator
  
  __all__ = [
      'Field',
      'Raw',
      'Nested',
      'Dict',
      'List',
      'String',
      'UUID',
      'Number',
      'Integer',
      'Decimal',
      'Boolean',
      'FormattedString',
      'Float',
      'DateTime',
      'LocalDateTime',
      'Time',
      'Date',
      'TimeDelta',
      'Url',
      'URL',
      'Email',
      'Method',
      'Function',
      'Str',
      'Bool',
      'Int',
      'Constant',
  ]
  
  MISSING_ERROR_MESSAGE = (
      'ValidationError raised by `{class_name}`, but error key `{key}` does '
      'not exist in the `error_messages` dictionary.'
  )
  _RECURSIVE_NESTED = 'self'
  
  
  class Field(FieldABC):
      """Basic field from which other fields should extend. It applies no
      formatting by default, and should only be used in cases where
      data does not need to be formatted before being serialized or deserialized.
      On error, the name of the field will be returned.
  
      :param default: If set, this value will be used during serialization if the input value
          is missing. If not set, the field will be excluded from the serialized output if the
          input value is missing. May be a value or a callable.
      :param str attribute: The name of the attribute to get the value from. If
          `None`, assumes the attribute has the same name as the field.
      :param str load_from: Additional key to look for when deserializing. Will only
          be checked if the field's name is not found on the input dictionary. If checked,
          it will return this parameter on error.
      :param str dump_to: Field name to use as a key when serializing.
      :param callable validate: Validator or collection of validators that are called
          during deserialization. Validator takes a field's input value as
          its only parameter and returns a boolean.
          If it returns `False`, an :exc:`ValidationError` is raised.
      :param required: Raise a :exc:`ValidationError` if the field value
          is not supplied during deserialization.
      :param allow_none: Set this to `True` if `None` should be considered a valid value during
          validation/deserialization. If ``missing=None`` and ``allow_none`` is unset,
          will default to ``True``. Otherwise, the default is ``False``.
      :param bool load_only: If `True` skip this field during serialization, otherwise
          its value will be present in the serialized data.
      :param bool dump_only: If `True` skip this field during deserialization, otherwise
          its value will be present in the deserialized object. In the context of an
          HTTP API, this effectively marks the field as "read-only".
      :param missing: Default deserialization value for the field if the field is not
          found in the input data. May be a value or a callable.
      :param dict error_messages: Overrides for `Field.default_error_messages`.
      :param metadata: Extra arguments to be stored as metadata.
  
      .. versionchanged:: 2.0.0
          Removed `error` parameter. Use ``error_messages`` instead.
  
      .. versionchanged:: 2.0.0
          Added `allow_none` parameter, which makes validation/deserialization of `None`
          consistent across fields.
  
      .. versionchanged:: 2.0.0
          Added `load_only` and `dump_only` parameters, which allow field skipping
          during the (de)serialization process.
  
      .. versionchanged:: 2.0.0
          Added `missing` parameter, which indicates the value for a field if the field
          is not found during deserialization.
  
      .. versionchanged:: 2.0.0
          ``default`` value is only used if explicitly set. Otherwise, missing values
          inputs are excluded from serialized output.
      """
      # Some fields, such as Method fields and Function fields, are not expected
      #  to exists as attributes on the objects to serialize. Set this to False
      #  for those fields
      _CHECK_ATTRIBUTE = True
      _creation_index = 0  # Used for sorting
  
      #: Default error messages for various kinds of errors. The keys in this dictionary
      #: are passed to `Field.fail`. The values are error messages passed to
      #: :exc:`marshmallow.ValidationError`.
      default_error_messages = {
          'required': 'Missing data for required field.',
          'type': 'Invalid input type.', # used by Unmarshaller
          'null': 'Field may not be null.',
          'validator_failed': 'Invalid value.'
      }
  
      def __init__(self, default=missing_, attribute=None, load_from=None, dump_to=None,
                   error=None, validate=None, required=False, allow_none=None, load_only=False,
                   dump_only=False, missing=missing_, error_messages=None, **metadata):
          self.default = default
          self.attribute = attribute
          self.load_from = load_from  # this flag is used by Unmarshaller
          self.dump_to = dump_to  # this flag is used by Marshaller
          self.validate = validate
          if utils.is_iterable_but_not_string(validate):
              if not utils.is_generator(validate):
                  self.validators = validate
              else:
                  self.validators = list(validate)
          elif callable(validate):
              self.validators = [validate]
          elif validate is None:
              self.validators = []
          else:
              raise ValueError("The 'validate' parameter must be a callable "
                               "or a collection of callables.")
  
          self.required = required
          # If missing=None, None should be considered valid by default
          if allow_none is None:
              if missing is None:
                  self.allow_none = True
              else:
                  self.allow_none = False
          else:
              self.allow_none = allow_none
          self.load_only = load_only
          self.dump_only = dump_only
          self.missing = missing
          self.metadata = metadata
          self._creation_index = Field._creation_index
          Field._creation_index += 1
  
          # 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 __repr__(self):
          return ('<fields.{ClassName}(default={self.default!r}, '
                  'attribute={self.attribute!r}, '
                  'validate={self.validate}, required={self.required}, '
                  'load_only={self.load_only}, dump_only={self.dump_only}, '
                  'missing={self.missing}, allow_none={self.allow_none}, '
                  'error_messages={self.error_messages})>'
                  .format(ClassName=self.__class__.__name__, self=self))
  
      def get_value(self, attr, obj, accessor=None, default=missing_):
          """Return the value for a given key from an object."""
          # NOTE: Use getattr instead of direct attribute access here so that
          # subclasses aren't required to define `attribute` member
          attribute = getattr(self, 'attribute', None)
          accessor_func = accessor or utils.get_value
          check_key = attr if attribute is None else attribute
          return accessor_func(check_key, obj, default)
  
      def _validate(self, value):
          """Perform validation on ``value``. Raise a :exc:`ValidationError` if validation
          does not succeed.
          """
          errors = []
          kwargs = {}
          for validator in self.validators:
              try:
                  r = validator(value)
                  if not isinstance(validator, Validator) and r is False:
                      self.fail('validator_failed')
              except ValidationError as err:
                  kwargs.update(err.kwargs)
                  if isinstance(err.messages, dict):
                      errors.append(err.messages)
                  else:
                      errors.extend(err.messages)
          if errors:
              raise ValidationError(errors, **kwargs)
  
      # Hat tip to django-rest-framework.
      def fail(self, key, **kwargs):
          """A helper method that simply raises a `ValidationError`.
          """
          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)
          if isinstance(msg, basestring):
              msg = msg.format(**kwargs)
          raise ValidationError(msg)
  
      def _validate_missing(self, value):
          """Validate missing values. Raise a :exc:`ValidationError` if
          `value` should be considered missing.
          """
          if value is missing_:
              if hasattr(self, 'required') and self.required:
                  self.fail('required')
          if value is None:
              if hasattr(self, 'allow_none') and self.allow_none is not True:
                  self.fail('null')
  
      def serialize(self, attr, obj, accessor=None):
          """Pulls the value for the given key from the object, applies the
          field's formatting and returns the result.
  
          :param str attr: The attibute or key to get from the object.
          :param str obj: The object to pull the key from.
          :param callable accessor: Function used to pull values from ``obj``.
          :raise ValidationError: In case of formatting problem
          """
          if self._CHECK_ATTRIBUTE:
              value = self.get_value(attr, obj, accessor=accessor)
              if value is missing_:
                  if hasattr(self, 'default'):
                      if callable(self.default):
                          return self.default()
                      else:
                          return self.default
          else:
              value = None
          return self._serialize(value, attr, obj)
  
      def deserialize(self, value, attr=None, data=None):
          """Deserialize ``value``.
  
          :raise ValidationError: If an invalid value is passed or if a required value
              is missing.
          """
          # Validate required fields, deserialize, then validate
          # deserialized value
          self._validate_missing(value)
          if getattr(self, 'allow_none', False) is True and value is None:
              return None
          output = self._deserialize(value, attr, data)
          self._validate(output)
          return output
  
      # Methods for concrete classes to override.
  
      def _add_to_schema(self, field_name, schema):
          """Update field with values from its parent schema. Called by
              :meth:`__set_field_attrs <marshmallow.Schema.__set_field_attrs>`.
  
          :param str field_name: Field name set in schema.
          :param Schema schema: Parent schema.
          """
          self.parent = self.parent or schema
          self.name = self.name or field_name
  
      def _serialize(self, value, attr, obj):
          """Serializes ``value`` to a basic Python datatype. Noop by default.
          Concrete :class:`Field` classes should implement this method.
  
          Example: ::
  
              class TitleCase(Field):
                  def _serialize(self, value, attr, obj):
                      if not value:
                          return ''
                      return unicode(value).title()
  
          :param value: The value to be serialized.
          :param str attr: The attribute or key on the object to be serialized.
          :param object obj: The object the value was pulled from.
          :raise ValidationError: In case of formatting or validation failure.
          :return: The serialized value
          """
          return value
  
      def _deserialize(self, value, attr, data):
          """Deserialize value. Concrete :class:`Field` classes should implement this method.
  
          :param value: The value to be deserialized.
          :param str attr: The attribute/key in `data` to be deserialized.
          :param dict data: The raw input data passed to the `Schema.load`.
          :raise ValidationError: In case of formatting or validation failure.
          :return: The deserialized value.
  
          .. versionchanged:: 2.0.0
              Added ``attr`` and ``data`` parameters.
          """
          return value
  
      # Properties
  
      @property
      def context(self):
          """The context dictionary for the parent :class:`Schema`."""
          return self.parent.context
  
      @property
      def root(self):
          """Reference to the `Schema` that this field belongs to even if it is buried in a `List`.
          Return `None` for unbound fields.
          """
          ret = self
          while hasattr(ret, 'parent') and ret.parent:
              ret = ret.parent
          return ret if isinstance(ret, SchemaABC) else None
  
  class Raw(Field):
      """Field that applies no formatting or validation."""
      pass
  
  class Nested(Field):
      """Allows you to nest a :class:`Schema <marshmallow.Schema>`
      inside a field.
  
      Examples: ::
  
          user = fields.Nested(UserSchema)
          user2 = fields.Nested('UserSchema')  # Equivalent to above
          collaborators = fields.Nested(UserSchema, many=True, only='id')
          parent = fields.Nested('self')
  
      When passing a `Schema <marshmallow.Schema>` instance as the first argument,
      the instance's ``exclude``, ``only``, and ``many`` attributes will be respected.
  
      Therefore, when passing the ``exclude``, ``only``, or ``many`` arguments to `fields.Nested`,
      you should pass a `Schema <marshmallow.Schema>` class (not an instance) as the first argument.
  
      ::
  
          # Yes
          author = fields.Nested(UserSchema, only=('id', 'name'))
  
          # No
          author = fields.Nested(UserSchema(), only=('id', 'name'))
  
      :param Schema nested: The Schema class or class name (string)
          to nest, or ``"self"`` to nest the :class:`Schema` within itself.
      :param default: Default value to if attribute is missing or None
      :param tuple exclude: A list or tuple of fields to exclude.
      :param required: Raise an :exc:`ValidationError` during deserialization
          if the field, *and* any required field values specified
          in the `nested` schema, are not found in the data. If not a `bool`
          (e.g. a `str`), the provided value will be used as the message of the
          :exc:`ValidationError` instead of the default message.
      :param only: A tuple or string of the field(s) to marshal. If `None`, all fields
          will be marshalled. If a field name (string) is given, only a single
          value will be returned as output instead of a dictionary.
          This parameter takes precedence over ``exclude``.
      :param bool many: Whether the field is a collection of objects.
      :param kwargs: The same keyword arguments that :class:`Field` receives.
      """
  
      default_error_messages = {
          'type': 'Invalid type.',
      }
  
      def __init__(self, nested, default=missing_, exclude=tuple(), only=None, **kwargs):
          self.nested = nested
          self.only = only
          self.exclude = exclude
          self.many = kwargs.get('many', False)
          self.__schema = None  # Cached Schema instance
          self.__updated_fields = False
          super(Nested, self).__init__(default=default, **kwargs)
  
      @property
      def schema(self):
          """The nested Schema object.
  
          .. versionchanged:: 1.0.0
              Renamed from `serializer` to `schema`
          """
          if not self.__schema:
              # Ensure that only parameter is a tuple
              if isinstance(self.only, basestring):
                  only = (self.only,)
              else:
                  only = self.only
  
              # Inherit context from parent.
              context = getattr(self.parent, 'context', {})
              if isinstance(self.nested, SchemaABC):
                  self.__schema = self.nested
                  self.__schema.context.update(context)
              elif isinstance(self.nested, type) and \
                      issubclass(self.nested, SchemaABC):
                  self.__schema = self.nested(many=self.many,
                          only=only, exclude=self.exclude, context=context,
                          load_only=self._nested_normalized_option('load_only'),
                          dump_only=self._nested_normalized_option('dump_only'))
              elif isinstance(self.nested, basestring):
                  if self.nested == _RECURSIVE_NESTED:
                      parent_class = self.parent.__class__
                      self.__schema = parent_class(many=self.many, only=only,
                              exclude=self.exclude, context=context,
                              load_only=self._nested_normalized_option('load_only'),
                              dump_only=self._nested_normalized_option('dump_only'))
                  else:
                      schema_class = class_registry.get_class(self.nested)
                      self.__schema = schema_class(many=self.many,
                              only=only, exclude=self.exclude, context=context,
                              load_only=self._nested_normalized_option('load_only'),
                              dump_only=self._nested_normalized_option('dump_only'))
              else:
                  raise ValueError('Nested fields must be passed a '
                                   'Schema, not {0}.'.format(self.nested.__class__))
              self.__schema.ordered = getattr(self.parent, 'ordered', False)
          return self.__schema
  
      def _nested_normalized_option(self, option_name):
          nested_field = '%s.' % self.name
          return [field.split(nested_field, 1)[1]
                  for field in getattr(self.root, option_name, set())
                  if field.startswith(nested_field)]
  
      def _serialize(self, nested_obj, attr, obj):
          # Load up the schema first. This allows a RegistryError to be raised
          # if an invalid schema name was passed
          schema = self.schema
          if nested_obj is None:
              return None
          if not self.__updated_fields:
              schema._update_fields(obj=nested_obj, many=self.many)
              self.__updated_fields = True
          ret, errors = schema.dump(nested_obj, many=self.many,
                  update_fields=not self.__updated_fields)
          if isinstance(self.only, basestring):  # self.only is a field name
              if self.many:
                  return utils.pluck(ret, key=self.only)
              else:
                  return ret[self.only]
          if errors:
              raise ValidationError(errors, data=ret)
          return ret
  
      def _deserialize(self, value, attr, data):
          if self.many and not utils.is_collection(value):
              self.fail('type', input=value, type=value.__class__.__name__)
  
          data, errors = self.schema.load(value)
          if errors:
              raise ValidationError(errors, data=data)
          return data
  
      def _validate_missing(self, value):
          """Validate missing values. Raise a :exc:`ValidationError` if
          `value` should be considered missing.
          """
          if value is missing_ and hasattr(self, 'required'):
              if self.nested == _RECURSIVE_NESTED:
                  self.fail('required')
              errors = self._check_required()
              if errors:
                  raise ValidationError(errors)
          else:
              super(Nested, self)._validate_missing(value)
  
      def _check_required(self):
          errors = {}
          if self.required:
              for field_name, field in self.schema.fields.items():
                  if not field.required:
                      continue
                  error_field_name = field.load_from or field_name
                  if (
                      isinstance(field, Nested) and
                      self.nested != _RECURSIVE_NESTED and
                      field.nested != _RECURSIVE_NESTED
                  ):
                      errors[error_field_name] = field._check_required()
                  else:
                      try:
                          field._validate_missing(field.missing)
                      except ValidationError as ve:
                          errors[error_field_name] = ve.messages
              if self.many and errors:
                  errors = {0: errors}
              # No inner errors; just raise required error like normal
              if not errors:
                  self.fail('required')
          return errors
  
  
  class List(Field):
      """A list field, composed with another `Field` class or
      instance.
  
      Example: ::
  
          numbers = fields.List(fields.Float())
  
      :param Field cls_or_instance: A field class or instance.
      :param bool default: Default value for serialization.
      :param kwargs: The same keyword arguments that :class:`Field` receives.
  
      .. versionchanged:: 2.0.0
          The ``allow_none`` parameter now applies to deserialization and
          has the same semantics as the other fields.
      """
      default_error_messages = {
          'invalid': 'Not a valid list.',
      }
  
      def __init__(self, cls_or_instance, **kwargs):
          super(List, self).__init__(**kwargs)
          if isinstance(cls_or_instance, type):
              if not issubclass(cls_or_instance, FieldABC):
                  raise ValueError('The type of the list elements '
                                             'must be a subclass of '
                                             'marshmallow.base.FieldABC')
              self.container = cls_or_instance()
          else:
              if not isinstance(cls_or_instance, FieldABC):
                  raise ValueError('The instances of the list '
                                             'elements must be of type '
                                             'marshmallow.base.FieldABC')
              self.container = cls_or_instance
  
      def get_value(self, attr, obj, accessor=None):
          """Return the value for a given key from an object."""
          value = super(List, self).get_value(attr, obj, accessor=accessor)
          if self.container.attribute:
              if utils.is_collection(value):
                  return [
                      self.container.get_value(self.container.attribute, each)
                      for each in value
                  ]
              return self.container.get_value(self.container.attribute, value)
          return value
  
      def _add_to_schema(self, field_name, schema):
          super(List, self)._add_to_schema(field_name, schema)
          self.container.parent = self
          self.container.name = field_name
  
      def _serialize(self, value, attr, obj):
          if value is None:
              return None
          if utils.is_collection(value):
              return [self.container._serialize(each, attr, obj) for each in value]
          return [self.container._serialize(value, attr, obj)]
  
      def _deserialize(self, value, attr, data):
          if not utils.is_collection(value):
              self.fail('invalid')
  
          result = []
          errors = {}
          for idx, each in enumerate(value):
              try:
                  result.append(self.container.deserialize(each))
              except ValidationError as e:
                  result.append(e.data)
                  errors.update({idx: e.messages})
  
          if errors:
              raise ValidationError(errors, data=result)
  
          return result
  
  class String(Field):
      """A string field.
  
      :param kwargs: The same keyword arguments that :class:`Field` receives.
      """
  
      default_error_messages = {
          'invalid': 'Not a valid string.',
          'invalid_utf8': 'Not a valid utf-8 string.'
      }
  
      def _serialize(self, value, attr, obj):
          if value is None:
              return None
          return utils.ensure_text_type(value)
  
      def _deserialize(self, value, attr, data):
          if not isinstance(value, basestring):
              self.fail('invalid')
          try:
              return utils.ensure_text_type(value)
          except UnicodeDecodeError:
              self.fail('invalid_utf8')
  
  
  class UUID(String):
      """A UUID field."""
      default_error_messages = {
          'invalid_uuid': 'Not a valid UUID.',
          'invalid_guid': 'Not a valid UUID.'  # TODO: Remove this in marshmallow 3.0
      }
  
      def _validated(self, value):
          """Format the value or raise a :exc:`ValidationError` if an error occurs."""
          if value is None:
              return None
          if isinstance(value, uuid.UUID):
              return value
          try:
              return uuid.UUID(value)
          except (ValueError, AttributeError):
              self.fail('invalid_uuid')
  
      def _serialize(self, value, attr, obj):
          validated = str(self._validated(value)) if value is not None else None
          return super(String, self)._serialize(validated, attr, obj)
  
      def _deserialize(self, value, attr, data):
          return self._validated(value)
  
  
  class Number(Field):
      """Base class for number fields.
  
      :param bool as_string: If True, format the serialized value as a string.
      :param kwargs: The same keyword arguments that :class:`Field` receives.
      """
  
      num_type = float
      default_error_messages = {
          'invalid': 'Not a valid number.'
      }
  
      def __init__(self, as_string=False, **kwargs):
          self.as_string = as_string
          super(Number, self).__init__(**kwargs)
  
      def _format_num(self, value):
          """Return the number value for value, given this field's `num_type`."""
          if value is None:
              return None
          return self.num_type(value)
  
      def _validated(self, value):
          """Format the value or raise a :exc:`ValidationError` if an error occurs."""
          try:
              return self._format_num(value)
          except (TypeError, ValueError) as err:
              self.fail('invalid')
  
      def serialize(self, attr, obj, accessor=None):
          """Pulls the value for the given key from the object and returns the
          serialized number representation. Return a string if `self.as_string=True`,
          othewise return this field's `num_type`. Receives the same `args` and `kwargs`
          as `Field`.
          """
          ret = Field.serialize(self, attr, obj, accessor=accessor)
          return self._to_string(ret) if (self.as_string and ret not in (None, missing_)) else ret
  
      def _to_string(self, value):
          return str(value)
  
      def _serialize(self, value, attr, obj):
          return self._validated(value)
  
      def _deserialize(self, value, attr, data):
          return self._validated(value)
  
  
  class Integer(Number):
      """An integer field.
  
      :param kwargs: The same keyword arguments that :class:`Number` receives.
      """
  
      num_type = int
      default_error_messages = {
          'invalid': 'Not a valid integer.'
      }
  
  
  class Decimal(Number):
      """A field that (de)serializes to the Python ``decimal.Decimal`` type.
      It's safe to use when dealing with money values, percentages, ratios
      or other numbers where precision is critical.
  
      .. warning::
  
          This field serializes to a `decimal.Decimal` object by default. If you need
          to render your data as JSON, keep in mind that the `json` module from the
          standard library does not encode `decimal.Decimal`. Therefore, you must use
          a JSON library that can handle decimals, such as `simplejson`, or serialize
          to a string by passing ``as_string=True``.
  
      .. warning::
  
          If a JSON `float` value is passed to this field for deserialization it will
          first be cast to its corresponding `string` value before being deserialized
          to a `decimal.Decimal` object. The default `__str__` implementation of the
          built-in Python `float` type may apply a destructive transformation upon
          its input data and therefore cannot be relied upon to preserve precision.
          To avoid this, you can instead pass a JSON `string` to be deserialized
          directly.
  
      :param int places: How many decimal places to quantize the value. If `None`, does
          not quantize the value.
      :param rounding: How to round the value during quantize, for example
          `decimal.ROUND_UP`. If None, uses the rounding value from
          the current thread's context.
      :param bool allow_nan: If `True`, `NaN`, `Infinity` and `-Infinity` are allowed,
          even though they are illegal according to the JSON specification.
      :param bool as_string: If True, serialize to a string instead of a Python
          `decimal.Decimal` type.
      :param kwargs: The same keyword arguments that :class:`Number` receives.
  
      .. versionadded:: 1.2.0
      """
  
      num_type = decimal.Decimal
  
      default_error_messages = {
          'special': 'Special numeric values are not permitted.',
      }
  
      def __init__(self, places=None, rounding=None, allow_nan=False, as_string=False, **kwargs):
          self.places = decimal.Decimal((0, (1,), -places)) if places is not None else None
          self.rounding = rounding
          self.allow_nan = allow_nan
          super(Decimal, self).__init__(as_string=as_string, **kwargs)
  
      # override Number
      def _format_num(self, value):
          if value is None:
              return None
  
          num = decimal.Decimal(str(value))
  
          if self.allow_nan:
              if num.is_nan():
                  return decimal.Decimal('NaN')  # avoid sNaN, -sNaN and -NaN
          else:
              if num.is_nan() or num.is_infinite():
                  self.fail('special')
  
          if self.places is not None and num.is_finite():
              num = num.quantize(self.places, rounding=self.rounding)
  
          return num
  
      # override Number
      def _validated(self, value):
          try:
              return super(Decimal, self)._validated(value)
          except decimal.InvalidOperation:
              self.fail('invalid')
  
      # override Number
      def _to_string(self, value):
          return format(value, 'f')
  
  
  class Boolean(Field):
      """A boolean field.
  
      :param kwargs: The same keyword arguments that :class:`Field` receives.
      """
      #: Values that will (de)serialize to `True`. If an empty set, any non-falsy
      #  value will deserialize to `True`.
      truthy = set(('t', 'T', 'true', 'True', 'TRUE', '1', 1, True))
      #: Values that will (de)serialize to `False`.
      falsy = set(('f', 'F', 'false', 'False', 'FALSE', '0', 0, 0.0, False))
  
      default_error_messages = {
          'invalid': 'Not a valid boolean.'
      }
  
      def _serialize(self, value, attr, obj):
          if value is None:
              return None
          elif value in self.truthy:
              return True
          elif value in self.falsy:
              return False
  
          return bool(value)
  
      def _deserialize(self, value, attr, data):
          if not self.truthy:
              return bool(value)
          else:
              try:
                  if value in self.truthy:
                      return True
                  elif value in self.falsy:
                      return False
              except TypeError:
                  pass
          self.fail('invalid')
  
  class FormattedString(Field):
      """Interpolate other values from the object into this field. The syntax for
      the source string is the same as the string `str.format` method
      from the python stdlib.
      ::
  
          class UserSchema(Schema):
              name = fields.String()
              greeting = fields.FormattedString('Hello {name}')
  
          ser = UserSchema()
          res = ser.dump(user)
          res.data  # => {'name': 'Monty', 'greeting': 'Hello Monty'}
      """
      default_error_messages = {
          'format': 'Cannot format string with given data.'
      }
      _CHECK_ATTRIBUTE = False
  
      def __init__(self, src_str, *args, **kwargs):
          Field.__init__(self, *args, **kwargs)
          self.src_str = text_type(src_str)
  
      def _serialize(self, value, attr, obj):
          try:
              data = utils.to_marshallable_type(obj)
              return self.src_str.format(**data)
          except (TypeError, IndexError) as error:
              self.fail('format')
  
  
  class Float(Number):
      """
      A double as IEEE-754 double precision string.
  
      :param bool as_string: If True, format the value as a string.
      :param kwargs: The same keyword arguments that :class:`Number` receives.
      """
  
      num_type = float
  
  
  class DateTime(Field):
      """A formatted datetime string in UTC.
  
      Example: ``'2014-12-22T03:12:58.019077+00:00'``
  
      Timezone-naive `datetime` objects are converted to
      UTC (+00:00) by :meth:`Schema.dump <marshmallow.Schema.dump>`.
      :meth:`Schema.load <marshmallow.Schema.load>` returns `datetime`
      objects that are timezone-aware.
  
      :param str format: Either ``"rfc"`` (for RFC822), ``"iso"`` (for ISO8601),
          or a date format string. If `None`, defaults to "iso".
      :param kwargs: The same keyword arguments that :class:`Field` receives.
  
      """
  
      DATEFORMAT_SERIALIZATION_FUNCS = {
          'iso': utils.isoformat,
          'iso8601': utils.isoformat,
          'rfc': utils.rfcformat,
          'rfc822': utils.rfcformat,
      }
  
      DATEFORMAT_DESERIALIZATION_FUNCS = {
          'iso': utils.from_iso,
          'iso8601': utils.from_iso,
          'rfc': utils.from_rfc,
          'rfc822': utils.from_rfc,
      }
  
      DEFAULT_FORMAT = 'iso'
  
      localtime = False
      default_error_messages = {
          'invalid': 'Not a valid datetime.',
          'format': '"{input}" cannot be formatted as a datetime.',
      }
  
      def __init__(self, format=None, **kwargs):
          super(DateTime, self).__init__(**kwargs)
          # Allow this to be None. It may be set later in the ``_serialize``
          # or ``_desrialize`` methods This allows a Schema to dynamically set the
          # dateformat, e.g. from a Meta option
          self.dateformat = format
  
      def _add_to_schema(self, field_name, schema):
          super(DateTime, self)._add_to_schema(field_name, schema)
          self.dateformat = self.dateformat or schema.opts.dateformat
  
      def _serialize(self, value, attr, obj):
          if value is None:
              return None
          self.dateformat = self.dateformat or self.DEFAULT_FORMAT
          format_func = self.DATEFORMAT_SERIALIZATION_FUNCS.get(self.dateformat, None)
          if format_func:
              try:
                  return format_func(value, localtime=self.localtime)
              except (AttributeError, ValueError) as err:
                  self.fail('format', input=value)
          else:
              return value.strftime(self.dateformat)
  
      def _deserialize(self, value, attr, data):
          if not value:  # Falsy values, e.g. '', None, [] are not valid
              raise self.fail('invalid')
          self.dateformat = self.dateformat or self.DEFAULT_FORMAT
          func = self.DATEFORMAT_DESERIALIZATION_FUNCS.get(self.dateformat)
          if func:
              try:
                  return func(value)
              except (TypeError, AttributeError, ValueError):
                  raise self.fail('invalid')
          elif self.dateformat:
              try:
                  return dt.datetime.strptime(value, self.dateformat)
              except (TypeError, AttributeError, ValueError):
                  raise self.fail('invalid')
          elif utils.dateutil_available:
              try:
                  return utils.from_datestring(value)
              except TypeError:
                  raise self.fail('invalid')
          else:
              warnings.warn('It is recommended that you install python-dateutil '
                            'for improved datetime deserialization.')
              raise self.fail('invalid')
  
  
  class LocalDateTime(DateTime):
      """A formatted datetime string in localized time, relative to UTC.
  
          ex. ``"Sun, 10 Nov 2013 08:23:45 -0600"``
  
      Takes the same arguments as :class:`DateTime <marshmallow.fields.DateTime>`.
      """
      localtime = True
  
  
  class Time(Field):
      """ISO8601-formatted time string.
  
      :param kwargs: The same keyword arguments that :class:`Field` receives.
      """
      default_error_messages = {
          'invalid': 'Not a valid time.',
          'format': '"{input}" cannot be formatted as a time.',
      }
  
      def _serialize(self, value, attr, obj):
          if value is None:
              return None
          try:
              ret = value.isoformat()
          except AttributeError:
              self.fail('format', input=value)
          if value.microsecond:
              return ret[:15]
          return ret
  
      def _deserialize(self, value, attr, data):
          """Deserialize an ISO8601-formatted time to a :class:`datetime.time` object."""
          if not value:   # falsy values are invalid
              self.fail('invalid')
              raise err
          try:
              return utils.from_iso_time(value)
          except (AttributeError, TypeError, ValueError):
              self.fail('invalid')
  
  class Date(Field):
      """ISO8601-formatted date string.
  
      :param kwargs: The same keyword arguments that :class:`Field` receives.
      """
      default_error_messages = {
          'invalid': 'Not a valid date.',
          'format': '"{input}" cannot be formatted as a date.',
      }
  
      def _serialize(self, value, attr, obj):
          if value is None:
              return None
          try:
              return value.isoformat()
          except AttributeError:
              self.fail('format', input=value)
          return value
  
      def _deserialize(self, value, attr, data):
          """Deserialize an ISO8601-formatted date string to a
          :class:`datetime.date` object.
          """
          if not value:  # falsy values are invalid
              self.fail('invalid')
          try:
              return utils.from_iso_date(value)
          except (AttributeError, TypeError, ValueError):
              self.fail('invalid')
  
  
  class TimeDelta(Field):
      """A field that (de)serializes a :class:`datetime.timedelta` object to an
      integer and vice versa. The integer can represent the number of days,
      seconds or microseconds.
  
      :param str precision: Influences how the integer is interpreted during
          (de)serialization. Must be 'days', 'seconds' or 'microseconds'.
      :param str error: Error message stored upon validation failure.
      :param kwargs: The same keyword arguments that :class:`Field` receives.
  
      .. versionchanged:: 2.0.0
          Always serializes to an integer value to avoid rounding errors.
          Add `precision` parameter.
      """
  
      DAYS = 'days'
      SECONDS = 'seconds'
      MICROSECONDS = 'microseconds'
  
      default_error_messages = {
          'invalid': 'Not a valid period of time.',
          'format': '{input!r} cannot be formatted as a timedelta.'
      }
  
      def __init__(self, precision='seconds', error=None, **kwargs):
          precision = precision.lower()
          units = (self.DAYS, self.SECONDS, self.MICROSECONDS)
  
          if precision not in units:
              msg = 'The precision must be "{0}", "{1}" or "{2}".'.format(*units)
              raise ValueError(msg)
  
          self.precision = precision
          super(TimeDelta, self).__init__(error=error, **kwargs)
  
      def _serialize(self, value, attr, obj):
          if value is None:
              return None
          try:
              days = value.days
              if self.precision == self.DAYS:
                  return days
              else:
                  seconds = days * 86400 + value.seconds
                  if self.precision == self.SECONDS:
                      return seconds
                  else:  # microseconds
                      return seconds * 10**6 + value.microseconds  # flake8: noqa
          except AttributeError:
              self.fail('format', input=value)
  
      def _deserialize(self, value, attr, data):
          try:
              value = int(value)
          except (TypeError, ValueError):
              self.fail('invalid')
  
          kwargs = {self.precision: value}
  
          try:
              return dt.timedelta(**kwargs)
          except OverflowError:
              self.fail('invalid')
  
  
  class Dict(Field):
      """A dict field. Supports dicts and dict-like objects.
  
      .. note::
          This field is only appropriate when the structure of
          nested data is not known. For structured data, use
          `Nested`.
  
      .. versionadded:: 2.1.0
      """
  
      default_error_messages = {
          'invalid': 'Not a valid mapping type.'
      }
  
      def _deserialize(self, value, attr, data):
          if isinstance(value, collections.Mapping):
              return value
          else:
              self.fail('invalid')
  
  
  class ValidatedField(Field):
      """A field that validates input on serialization."""
  
      def _validated(self, value):
          raise NotImplementedError('Must implement _validate method')
  
      def _serialize(self, value, *args, **kwargs):
          ret = super(ValidatedField, self)._serialize(value, *args, **kwargs)
          return self._validated(ret)
  
  
  class Url(ValidatedField, String):
      """A validated URL field. Validation occurs during both serialization and
      deserialization.
  
      :param default: Default value for the field if the attribute is not set.
      :param str attribute: The name of the attribute to get the value from. If
          `None`, assumes the attribute has the same name as the field.
      :param bool relative: Allow relative URLs.
      :param kwargs: The same keyword arguments that :class:`String` receives.
      """
      default_error_messages = {'invalid': 'Not a valid URL.'}
  
      def __init__(self, relative=False, schemes=None, **kwargs):
          String.__init__(self, **kwargs)
  
          self.relative = relative
          # Insert validation into self.validators so that multiple errors can be
          # stored.
          self.validators.insert(0, validate.URL(
              relative=self.relative,
              schemes=schemes,
              error=self.error_messages['invalid']
          ))
  
      def _validated(self, value):
          if value is None:
              return None
          return validate.URL(
              relative=self.relative,
              error=self.error_messages['invalid']
          )(value)
  
  
  class Email(ValidatedField, String):
      """A validated email field. Validation occurs during both serialization and
      deserialization.
  
      :param args: The same positional arguments that :class:`String` receives.
      :param kwargs: The same keyword arguments that :class:`String` receives.
      """
      default_error_messages = {'invalid': 'Not a valid email address.'}
      def __init__(self, *args, **kwargs):
          String.__init__(self, *args, **kwargs)
          # Insert validation into self.validators so that multiple errors can be
          # stored.
          self.validators.insert(0, validate.Email(error=self.error_messages['invalid']))
  
      def _validated(self, value):
          if value is None:
              return None
          return validate.Email(
              error=self.error_messages['invalid']
          )(value)
  
  
  class Method(Field):
      """A field that takes the value returned by a `Schema` method.
  
      :param str method_name: The name of the Schema method from which
          to retrieve the value. The method must take an argument ``obj``
          (in addition to self) that is the object to be serialized.
      :param str deserialize: Optional name of the Schema method for deserializing
          a value The method must take a single argument ``value``, which is the
          value to deserialize.
  
      .. versionchanged:: 2.0.0
          Removed optional ``context`` parameter on methods. Use ``self.context`` instead.
      .. versionchanged:: 2.3.0
          Deprecated ``method_name`` parameter in favor of ``serialize`` and allow
          ``serialize`` to not be passed at all.
      """
      _CHECK_ATTRIBUTE = False
  
      def __init__(self, serialize=None, deserialize=None, method_name=None, **kwargs):
          if method_name is not None:
              warnings.warn('"method_name" argument of fields.Method is deprecated. '
                            'Use the "serialize" argument instead.', DeprecationWarning)
  
          self.serialize_method_name = self.method_name = serialize or method_name
          self.deserialize_method_name = deserialize
          super(Method, self).__init__(**kwargs)
  
      def _serialize(self, value, attr, obj):
          if not self.serialize_method_name:
              return missing_
  
          method = utils.callable_or_raise(
              getattr(self.parent, self.serialize_method_name, None)
          )
          try:
              return method(obj)
          except AttributeError:
              pass
          return missing_
  
      def _deserialize(self, value, attr, data):
          if self.deserialize_method_name:
              try:
                  method = utils.callable_or_raise(
                      getattr(self.parent, self.deserialize_method_name, None)
                  )
                  return method(value)
              except AttributeError:
                  pass
          return value
  
  
  class Function(Field):
      """A field that takes the value returned by a function.
  
      :param callable serialize: A callable from which to retrieve the value.
          The function must take a single argument ``obj`` which is the object
          to be serialized. It can also optionally take a ``context`` argument,
          which is a dictionary of context variables passed to the serializer.
          If no callable is provided then the ```load_only``` flag will be set
          to True.
      :param callable deserialize: A callable from which to retrieve the value.
          The function must take a single argument ``value`` which is the value
          to be deserialized. It can also optionally take a ``context`` argument,
          which is a dictionary of context variables passed to the deserializer.
          If no callable is provided then ```value``` will be passed through
          unchanged.
      :param callable func: This argument is to be deprecated. It exists for
          backwards compatiblity. Use serialize instead.
  
      .. versionchanged:: 2.3.0
          Deprecated ``func`` parameter in favor of ``serialize``.
      """
      _CHECK_ATTRIBUTE = False
  
      def __init__(self, serialize=None, deserialize=None, func=None, **kwargs):
          if func:
              warnings.warn('"func" argument of fields.Function is deprecated. '
                            'Use the "serialize" argument instead.', DeprecationWarning)
              serialize = func
          super(Function, self).__init__(**kwargs)
          self.serialize_func = self.func = serialize and utils.callable_or_raise(serialize)
          self.deserialize_func = deserialize and utils.callable_or_raise(deserialize)
  
      def _serialize(self, value, attr, obj):
          try:
              return self._call_or_raise(self.serialize_func, obj, attr)
          except AttributeError:  # the object is not expected to have the attribute
              pass
          return missing_
  
      def _deserialize(self, value, attr, data):
          if self.deserialize_func:
              return self._call_or_raise(self.deserialize_func, value, attr)
          return value
  
      def _call_or_raise(self, func, value, attr):
          if len(utils.get_func_args(func)) > 1:
              if self.parent.context is None:
                  msg = 'No context available for Function field {0!r}'.format(attr)
                  raise ValidationError(msg)
              return func(value, self.parent.context)
          else:
              return func(value)
  
  
  
  class Constant(Field):
      """A field that (de)serializes to a preset constant.  If you only want the
      constant added for serialization or deserialization, you should use
      ``dump_only=True`` or ``load_only=True`` respectively.
  
      :param constant: The constant to return for the field attribute.
  
      .. versionadded:: 2.0.0
      """
      _CHECK_ATTRIBUTE = False
  
      def __init__(self, constant, **kwargs):
          super(Constant, self).__init__(**kwargs)
          self.constant = constant
          self.missing = constant
          self.default = constant
  
      def _serialize(self, value, *args, **kwargs):
          return self.constant
  
      def _deserialize(self, value, *args, **kwargs):
          return self.constant
  
  
  # Aliases
  URL = Url
  Str = String
  Bool = Boolean
  Int = Integer
  