def __init__(self, descriptor_db=None):
    """Initializes a Pool of proto buffs.

    The descriptor_db argument to the constructor is provided to allow
    specialized file descriptor proto lookup code to be triggered on demand. An
    example would be an implementation which will read and compile a file
    specified in a call to FindFileByName() and not require the call to Add()
    at all. Results from this database will be cached internally here as well.

    Args:
      descriptor_db: A secondary source of file descriptors.
    """

    self._internal_db = descriptor_database.DescriptorDatabase()
    self._descriptor_db = descriptor_db
    self._descriptors = {}
    self._enum_descriptors = {}
    self._service_descriptors = {}
    self._file_descriptors = {}
    self._toplevel_extensions = {}
    # TODO(jieluo): Remove _file_desc_by_toplevel_extension after
    # maybe year 2020 for compatibility issue (with 3.4.1 only).
    self._file_desc_by_toplevel_extension = {}
    self._top_enum_values = {}
    # We store extensions in two two-level mappings: The first key is the
    # descriptor of the message being extended, the second key is the extension
    # full name or its tag number.
    self._extensions_by_name = collections.defaultdict(dict)
    self._extensions_by_number = collections.defaultdict(dict)
Initializes a Pool of proto buffs.

    The descriptor_db argument to the constructor is provided to allow
    specialized file descriptor proto lookup code to be triggered on demand. An
    example would be an implementation which will read and compile a file
    specified in a call to FindFileByName() and not require the call to Add()
    at all. Results from this database will be cached internally here as well.

    Args:
      descriptor_db: A secondary source of file descriptors.
    
deps/protobuf/python/google/protobuf/descriptor_pool.py