code: def system(session, py):
      
  
      # Sanity check: Only run system tests if the environment variable is set.
      if not os.environ.get('GOOGLE_APPLICATION_CREDENTIALS', ''):
          session.skip('Credentials must be set via environment variable.')
  
      # Run the system tests against latest Python 2 and Python 3 only.
      session.interpreter = 'python{}'.format(py)
  
      # Set the virtualenv dirname.
      session.virtualenv_dirname = 'sys-' + py
  
      # Install all test dependencies.
      session.install('-r', 'requirements-test.txt')
  
      # Install dev packages into the virtualenv's dist-packages.
      _install_dev_packages(session)
  
      # Run py.test against the system tests.
      session.run(
          'py.test',
          '-s',
          'tests/system/',
          *session.posargs
      )
docs: Run the system test suite.