langchain/document_loaders/youtube.py/GoogleApiClient/_load_credentials
class GoogleApiClient:    def _load_credentials(self) -> Any:
        """Load credentials."""
        # Adapted from https://developers.google.com/drive/api/v3/quickstart/python
        try:
            from google.auth.transport.requests import Request
            from google.oauth2 import service_account
            from google.oauth2.credentials import Credentials
            from google_auth_oauthlib.flow import InstalledAppFlow
            from youtube_transcript_api import YouTubeTranscriptApi  # noqa: F401
        except ImportError:
            raise ImportError(
                "You must run"
                "`pip install --upgrade "
                "google-api-python-client google-auth-httplib2 "
                "google-auth-oauthlib "
                "youtube-transcript-api` "
                "to use the Google Drive loader"
            )

        creds = None
        if self.service_account_path.exists():
            return service_account.Credentials.from_service_account_file(
                str(self.service_account_path)
            )
        if self.token_path.exists():
            creds = Credentials.from_authorized_user_file(str(self.token_path), SCOPES)

        if not creds or not creds.valid:
            if creds and creds.expired and creds.refresh_token:
                creds.refresh(Request())
            else:
                flow = InstalledAppFlow.from_client_secrets_file(
                    str(self.credentials_path), SCOPES
                )
                creds = flow.run_local_server(port=0)
            with open(self.token_path, "w") as token:
                token.write(creds.to_json())

        return creds