code: def _implicit_credentials_from_files():
          
          credentials_filename = _get_environment_variable_file()
          if not credentials_filename:
              credentials_filename = _get_well_known_file()
              if os.path.isfile(credentials_filename):
                  extra_help = (' (produced automatically when running'
                                ' "gcloud auth login" command)')
              else:
                  credentials_filename = None
          else:
              extra_help = (' (pointed to by ' + GOOGLE_APPLICATION_CREDENTIALS +
                            ' environment variable)')
  
          if not credentials_filename:
              return
  
          # If we can read the credentials from a file, we don't need to know
          # what environment we are in.
          SETTINGS.env_name = DEFAULT_ENV_NAME
  
          try:
              return _get_application_default_credential_from_file(
                  credentials_filename)
          except (ApplicationDefaultCredentialsError, ValueError) as error:
              _raise_exception_for_reading_json(credentials_filename,
                                                extra_help, error)
docs: Attempts to get implicit credentials from local credential files.
  
          First checks if the environment variable GOOGLE_APPLICATION_CREDENTIALS
          is set with a filename and then falls back to a configuration file (the
          "well known" file) associated with the 'gcloud' command line tool.
  
          Returns:
              Credentials object associated with the
              GOOGLE_APPLICATION_CREDENTIALS file or the "well known" file if
              either exist. If neither file is define, returns None, indicating
              no credentials from a file can detected from the current
              environment.