What and how to pass credential using using Python Client Library for gcp compute API

The credentials that you need are called "Service Account JSON Key File". These are created in the Google Cloud Console under IAM & Admin / Service Accounts. Create a service account and download the key file. In the example below this is service-account.json.

Example code that uses a service account:

from googleapiclient import discovery
from google.oauth2 import service_account

scopes = ['https://www.googleapis.com/auth/cloud-platform']
sa_file = 'service-account.json'
zone = 'us-central1-a'
project_id = 'my_project_id' # Project ID, not Project Name

credentials = service_account.Credentials.from_service_account_file(sa_file, scopes=scopes)

# Create the Cloud Compute Engine service object
service = discovery.build('compute', 'v1', credentials=credentials)

request = service.instances().list(project=project_id, zone=zone)
while request is not None:
    response = request.execute()

    for instance in response['items']:
        # TODO: Change code below to process each `instance` resource:
        print(instance)

    request = service.instances().list_next(previous_request=request, previous_response=response)