title: python firestore issue with authentication
body: <p>I am using python with firestore, and trying to create a client in the backend. I am following <a href="https://firebase.google.com/docs/firestore/quickstart" rel="nofollow noreferrer">this</a> tutorial </p>
  
  <p>with the following code </p>
  
  <pre><code>import firebase_admin
  from firebase_admin import credentials, firestore
  cred = credentials.Certificate("cred_file.json")
  firebase_admin.initialize_app(cred)
  db = firestore.Client()
  ref = db.collections(u'table')
  </code></pre>
  
  <p>and getting the following error </p>
  
  <blockquote>
    <p>google.auth.exceptions.DefaultCredentialsError: Could not
    automatically determine credentials. Please set
    GOOGLE_APPLICATION_CREDENTIALS or explicitly create credentials and
    re-run the application. For more information</p>
  </blockquote>
  
  <p>I guess that is something wrong with the default credentials, what I don't get is, if I am initializing the credentials in my code, why does the app keep throwing an error for the default credentials? I am giving it the proper cred file explicitly.</p>