If the environment variable GOOGLE_APPLICATION_CREDENTIALS is set to the path of a valid service account JSON private key file, then it is loaded and returned. The project ID returned is the project ID defined in the service account file if available (some older files do not contain project ID information).

If the environment variable is set to the path of a valid external account JSON configuration file (workload identity federation), then the configuration file is used to determine and retrieve the external credentials from the current environment (AWS, Azure, etc). These will then be exchanged for Google access tokens via the Google STS endpoint. The project ID returned in this case is the one corresponding to the underlying workload identity pool resource if determinable.

If the Google Cloud SDK is installed and has application default credentials set they are loaded and returned.

To enable application default credentials with the Cloud SDK run:

gcloud auth application-default login

If the Cloud SDK has an active project, the project ID is returned. The active project can be set using:

gcloud config set project

If the application is running in the App Engine standard environment (first generation) then the credentials and project ID from the App Identity Service are used.

If the application is running in Compute Engine or Cloud Run or the App Engine flexible environment or the App Engine standard environment (second generation) then the credentials and project ID are obtained from the Metadata Service.

If no credentials are found, DefaultCredentialsError will be raised.