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[EDIT] FILE: api_implementation.py

# Protocol Buffers - Google's data interchange format
# Copyright 2008 Google Inc.  All rights reserved.
# https://developers.google.com/protocol-buffers/
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
#
#     * Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
#     * Redistributions in binary form must reproduce the above
# copyright notice, this list of conditions and the following disclaimer
# in the documentation and/or other materials provided with the
# distribution.
#     * Neither the name of Google Inc. nor the names of its
# contributors may be used to endorse or promote products derived from
# this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

"""Determine which implementation of the protobuf API is used in this process.
"""

import importlib
import os
import sys
import warnings

def _ApiVersionToImplementationType(api_version):
  if api_version == 2:
    return 'cpp'
  if api_version == 1:
    raise ValueError('api_version=1 is no longer supported.')
  if api_version == 0:
    return 'python'
  return None

_implementation_type = None
try:
  # pylint: disable=g-import-not-at-top
  from google.protobuf.internal import _api_implementation
  # The compile-time constants in the _api_implementation module can be used to
  # switch to a certain implementation of the Python API at build time.
  _implementation_type = _ApiVersionToImplementationType(
      _api_implementation.api_version)
except ImportError:
  pass  # Unspecified by compiler flags.

def _CanImport(mod_name):
  try:
    mod = importlib.import_module(mod_name)
    # Work around a known issue in the classic bootstrap .par import hook.
    if not mod:
      raise ImportError(mod_name + ' import succeeded but was None')
    return True
  except ImportError:
    return False

if _implementation_type is None:
  if _CanImport('google._upb._message'):
    _implementation_type = 'upb'
  elif _CanImport('google.protobuf.pyext._message'):
    _implementation_type = 'cpp'
  else:
    _implementation_type = 'python'

# This environment variable can be used to switch to a certain implementation
# of the Python API, overriding the compile-time constants in the
# _api_implementation module. Right now only 'python', 'cpp' and 'upb' are
# valid values. Any other value will raise error.
_implementation_type = os.getenv('PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION',
                                 _implementation_type)

if _implementation_type not in ('python', 'cpp', 'upb'):
  raise ValueError('PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION {0} is not '
                   'supported. Please set to \'python\', \'cpp\' or '
                   '\'upb\'.'.format(_implementation_type))

if 'PyPy' in sys.version and _implementation_type == 'cpp':
  warnings.warn('PyPy does not work yet with cpp protocol buffers. '
                'Falling back to the python implementation.')
  _implementation_type = 'python'

_c_module = None

if _implementation_type == 'cpp':
  try:
    # pylint: disable=g-import-not-at-top
    from google.protobuf.pyext import _message
    sys.modules['google3.net.proto2.python.internal.cpp._message'] = _message
    _c_module = _message
    del _message
  except ImportError:
    # TODO(jieluo): fail back to python
    warnings.warn(
        'Selected implementation cpp is not available.')
    pass

if _implementation_type == 'upb':
  try:
    # pylint: disable=g-import-not-at-top
    from google._upb import _message
    _c_module = _message
    del _message
  except ImportError:
    warnings.warn('Selected implementation upb is not available. '
                  'Falling back to the python implementation.')
    _implementation_type = 'python'
    pass

# Detect if serialization should be deterministic by default
try:
  # The presence of this module in a build allows the proto implementation to
  # be upgraded merely via build deps.
  #
  # NOTE: Merely importing this automatically enables deterministic proto
  # serialization for C++ code, but we still need to export it as a boolean so
  # that we can do the same for `_implementation_type == 'python'`.
  #
  # NOTE2: It is possible for C++ code to enable deterministic serialization by
  # default _without_ affecting Python code, if the C++ implementation is not in
  # use by this module.  That is intended behavior, so we don't actually expose
  # this boolean outside of this module.
  #
  # pylint: disable=g-import-not-at-top,unused-import
  from google.protobuf import enable_deterministic_proto_serialization
  _python_deterministic_proto_serialization = True
except ImportError:
  _python_deterministic_proto_serialization = False

# Usage of this function is discouraged. Clients shouldn't care which
# implementation of the API is in use. Note that there is no guarantee
# that differences between APIs will be maintained.
# Please don't use this function if possible.
def Type():
  return _implementation_type

# See comment on 'Type' above.
# TODO(jieluo): Remove the API, it returns a constant. b/228102101
def Version():
  return 2

# For internal use only
def IsPythonDefaultSerializationDeterministic():
  return _python_deterministic_proto_serialization

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