text: """
  Provides various throttling policies.
  """
  import time
  
  from django.core.cache import cache as default_cache
  from django.core.exceptions import ImproperlyConfigured
  
  from rest_framework.settings import api_settings
  
  
  class BaseThrottle:
      """
      Rate throttling of requests.
      """
  
      def allow_request(self, request, view):
          """
          Return `True` if the request should be allowed, `False` otherwise.
          """
          raise NotImplementedError('.allow_request() must be overridden')
  
      def get_ident(self, request):
          """
          Identify the machine making the request by parsing HTTP_X_FORWARDED_FOR
          if present and number of proxies is > 0. If not use all of
          HTTP_X_FORWARDED_FOR if it is available, if not use REMOTE_ADDR.
          """
          xff = request.META.get('HTTP_X_FORWARDED_FOR')
          remote_addr = request.META.get('REMOTE_ADDR')
          num_proxies = api_settings.NUM_PROXIES
  
          if num_proxies is not None:
              if num_proxies == 0 or xff is None:
                  return remote_addr
              addrs = xff.split(',')
              client_addr = addrs[-min(num_proxies, len(addrs))]
              return client_addr.strip()
  
          return ''.join(xff.split()) if xff else remote_addr
  
      def wait(self):
          """
          Optionally, return a recommended number of seconds to wait before
          the next request.
          """
          return None
  
  
  class SimpleRateThrottle(BaseThrottle):
      """
      A simple cache implementation, that only requires `.get_cache_key()`
      to be overridden.
  
      The rate (requests / seconds) is set by a `rate` attribute on the View
      class.  The attribute is a string of the form 'number_of_requests/period'.
  
      Period should be one of: ('s', 'sec', 'm', 'min', 'h', 'hour', 'd', 'day')
  
      Previous request information used for throttling is stored in the cache.
      """
      cache = default_cache
      timer = time.time
      cache_format = 'throttle_%(scope)s_%(ident)s'
      scope = None
      THROTTLE_RATES = api_settings.DEFAULT_THROTTLE_RATES
  
      def __init__(self):
          if not getattr(self, 'rate', None):
              self.rate = self.get_rate()
          self.num_requests, self.duration = self.parse_rate(self.rate)
  
      def get_cache_key(self, request, view):
          """
          Should return a unique cache-key which can be used for throttling.
          Must be overridden.
  
          May return `None` if the request should not be throttled.
          """
          raise NotImplementedError('.get_cache_key() must be overridden')
  
      def get_rate(self):
          """
          Determine the string representation of the allowed request rate.
          """
          if not getattr(self, 'scope', None):
              msg = ("You must set either `.scope` or `.rate` for '%s' throttle" %
                     self.__class__.__name__)
              raise ImproperlyConfigured(msg)
  
          try:
              return self.THROTTLE_RATES[self.scope]
          except KeyError:
              msg = "No default throttle rate set for '%s' scope" % self.scope
              raise ImproperlyConfigured(msg)
  
      def parse_rate(self, rate):
          """
          Given the request rate string, return a two tuple of:
          <allowed number of requests>, <period of time in seconds>
          """
          if rate is None:
              return (None, None)
          num, period = rate.split('/')
          num_requests = int(num)
          if not period.isdigit():
              duration = {'s': 1, 'm': 60, 'h': 3600, 'd': 86400}[period[0]]
          else:
              duration = int(period)
          return (num_requests, duration)
  
      def allow_request(self, request, view):
          """
          Implement the check to see if the request should be throttled.
  
          On success calls `throttle_success`.
          On failure calls `throttle_failure`.
          """
          if self.rate is None:
              return True
  
          self.key = self.get_cache_key(request, view)
          if self.key is None:
              return True
  
          self.history = self.cache.get(self.key, [])
          self.now = self.timer()
  
          # Drop any requests from the history which have now passed the
          # throttle duration
          while self.history and self.history[-1] <= self.now - self.duration:
              self.history.pop()
          if len(self.history) >= self.num_requests:
              return self.throttle_failure()
          return self.throttle_success()
  
      def throttle_success(self):
          """
          Inserts the current request's timestamp along with the key
          into the cache.
          """
          self.history.insert(0, self.now)
          self.cache.set(self.key, self.history, self.duration)
          return True
  
      def throttle_failure(self):
          """
          Called when a request to the API has failed due to throttling.
          """
          return False
  
      def wait(self):
          """
          Returns the recommended next request time in seconds.
          """
          if self.history:
              remaining_duration = self.duration - (self.now - self.history[-1])
          else:
              remaining_duration = self.duration
  
          available_requests = self.num_requests - len(self.history) + 1
          if available_requests <= 0:
              return None
  
          return remaining_duration / float(available_requests)
  
  
  class AnonRateThrottle(SimpleRateThrottle):
      """
      Limits the rate of API calls that may be made by a anonymous users.
  
      The IP address of the request will be used as the unique cache key.
      """
      scope = 'anon'
  
      def get_cache_key(self, request, view):
          if request.user.is_authenticated:
              return None  # Only throttle unauthenticated requests.
  
          return self.cache_format % {
              'scope': self.scope,
              'ident': self.get_ident(request)
          }
  
  
  class UserRateThrottle(SimpleRateThrottle):
      """
      Limits the rate of API calls that may be made by a given user.
  
      The user id will be used as a unique cache key if the user is
      authenticated.  For anonymous requests, the IP address of the request will
      be used.
      """
      scope = 'user'
  
      def get_cache_key(self, request, view):
          if request.user.is_authenticated:
              ident = request.user.pk
          else:
              ident = self.get_ident(request)
  
          return self.cache_format % {
              'scope': self.scope,
              'ident': ident
          }
  
  
  class ScopedRateThrottle(SimpleRateThrottle):
      """
      Limits the rate of API calls by different amounts for various parts of
      the API.  Any view that has the `throttle_scope` property set will be
      throttled.  The unique cache key will be generated by concatenating the
      user id of the request, and the scope of the view being accessed.
      """
      scope_attr = 'throttle_scope'
  
      def __init__(self):
          # Override the usual SimpleRateThrottle, because we can't determine
          # the rate until called by the view.
          pass
  
      def allow_request(self, request, view):
          # We can only determine the scope once we're called by the view.
          self.scope = getattr(view, self.scope_attr, None)
  
          # If a view does not have a `throttle_scope` always allow the request
          if not self.scope:
              return True
  
          # Determine the allowed request rate as we normally would during
          # the `__init__` call.
          self.rate = self.get_rate()
          self.num_requests, self.duration = self.parse_rate(self.rate)
  
          # We can now proceed as normal.
          return super().allow_request(request, view)
  
      def get_cache_key(self, request, view):
          """
          If `view.throttle_scope` is not set, don't apply this throttle.
  
          Otherwise generate the unique cache key by concatenating the user id
          with the '.throttle_scope` property of the view.
          """
          if request.user.is_authenticated:
              ident = request.user.pk
          else:
              ident = self.get_ident(request)
  
          return self.cache_format % {
              'scope': self.scope,
              'ident': ident
          }
  