text: # This file is dual licensed under the terms of the Apache License, Version
  # 2.0, and the BSD License. See the LICENSE file in the root of this repository
  # for complete details.
  
  from __future__ import absolute_import, division, print_function
  
  from cryptography import utils
  from cryptography.exceptions import (
      AlreadyFinalized, UnsupportedAlgorithm, _Reasons
  )
  from cryptography.hazmat.backends.interfaces import HMACBackend
  from cryptography.hazmat.primitives import hashes, mac
  
  
  @utils.register_interface(mac.MACContext)
  @utils.register_interface(hashes.HashContext)
  class HMAC(object):
      def __init__(self, key, algorithm, backend, ctx=None):
          if not isinstance(backend, HMACBackend):
              raise UnsupportedAlgorithm(
                  "Backend object does not implement HMACBackend.",
                  _Reasons.BACKEND_MISSING_INTERFACE
              )
  
          if not isinstance(algorithm, hashes.HashAlgorithm):
              raise TypeError("Expected instance of hashes.HashAlgorithm.")
          self._algorithm = algorithm
  
          self._backend = backend
          self._key = key
          if ctx is None:
              self._ctx = self._backend.create_hmac_ctx(key, self.algorithm)
          else:
              self._ctx = ctx
  
      algorithm = utils.read_only_property("_algorithm")
  
      def update(self, data):
          if self._ctx is None:
              raise AlreadyFinalized("Context was already finalized.")
          if not isinstance(data, bytes):
              raise TypeError("data must be bytes.")
          self._ctx.update(data)
  
      def copy(self):
          if self._ctx is None:
              raise AlreadyFinalized("Context was already finalized.")
          return HMAC(
              self._key,
              self.algorithm,
              backend=self._backend,
              ctx=self._ctx.copy()
          )
  
      def finalize(self):
          if self._ctx is None:
              raise AlreadyFinalized("Context was already finalized.")
          digest = self._ctx.finalize()
          self._ctx = None
          return digest
  
      def verify(self, signature):
          if not isinstance(signature, bytes):
              raise TypeError("signature must be bytes.")
          if self._ctx is None:
              raise AlreadyFinalized("Context was already finalized.")
  
          ctx, self._ctx = self._ctx, None
          ctx.verify(signature)
  