old_contents: # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #    http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
  # implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  from __future__ import absolute_import, division, print_function
  
  import pytest
  
  from cryptography.hazmat.backends import _available_backends
  
  from .utils import check_backend_support, select_backends
  
  
  def pytest_generate_tests(metafunc):
      names = metafunc.config.getoption("--backend")
      selected_backends = select_backends(names, _available_backends())
  
      if "backend" in metafunc.fixturenames:
          filtered_backends = []
          for backend in selected_backends:
              try:
                  required = metafunc.function.requires_backend_interface
              except AttributeError:
                  # function does not have requires_backend_interface decorator
                  filtered_backends.append(backend)
              else:
                  required_interfaces = tuple(
                      mark.kwargs["interface"] for mark in required
                  )
                  if isinstance(backend, required_interfaces):
                      filtered_backends.append(backend)
  
          if not filtered_backends:
              pytest.skip(
                  "No backends provided supply the interface: {0}".format(
                      ", ".join(iface.__name__ for iface in required_interfaces)
                  )
              )
          else:
              metafunc.parametrize("backend", filtered_backends)
  
  
  @pytest.mark.trylast
  def pytest_runtest_setup(item):
      check_backend_support(item)
  
  
  def pytest_addoption(parser):
      parser.addoption(
          "--backend", action="store", metavar="NAME",
          help="Only run tests matching the backend NAME."
      )
  
new_contents: # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #    http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
  # implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  from __future__ import absolute_import, division, print_function
  
  import pytest
  
  from cryptography.hazmat.backends import _available_backends
  
  from .utils import check_backend_support, select_backends
  
  
  def pytest_generate_tests(metafunc):
      names = metafunc.config.getoption("--backend")
      selected_backends = select_backends(names, _available_backends())
  
      if "backend" in metafunc.fixturenames:
          filtered_backends = []
          for backend in selected_backends:
              try:
                  required = metafunc.function.requires_backend_interface
              except AttributeError:
                  # function does not have requires_backend_interface decorator
                  filtered_backends.append(backend)
              else:
                  required_interfaces = tuple(
                      mark.kwargs["interface"] for mark in required
                  )
                  if isinstance(backend, required_interfaces):
                      filtered_backends.append(backend)
  
          if not filtered_backends:
              # If you pass an empty list to parametrize Bad Things(tm) happen
              # as of pytest 2.6.4 when the test also has a parametrize decorator
              pytest.skip(
                  "No backends provided supply the interface: {0}".format(
                      ", ".join(iface.__name__ for iface in required_interfaces)
                  )
              )
          else:
              metafunc.parametrize("backend", filtered_backends)
  
  
  @pytest.mark.trylast
  def pytest_runtest_setup(item):
      check_backend_support(item)
  
  
  def pytest_addoption(parser):
      parser.addoption(
          "--backend", action="store", metavar="NAME",
          help="Only run tests matching the backend NAME."
      )
  
subject: add comment explaining some weirdness