code: def _readCredentials(self):
          
          self._googleJson = os.getenv('GOOGLE_APPLICATION_CREDENTIALS')
          if not self._googleJson:
              raise RuntimeError('GOOGLE_APPLICATION_CREDENTIALS not set.')
          try:
              with open(self._googleJson) as jsonFile:
                  self.googleConnectionParams = json.loads(jsonFile.read())
          except:
               raise RuntimeError('GCEProvisioner: Could not parse the Google service account json file %s'
                                  % self._googleJson)
  
          self._projectId = self.googleConnectionParams['project_id']
          self._clientEmail = self.googleConnectionParams['client_email']
          self._credentialsPath = self._googleJson
          self._masterPublicKey = None
          self._gceDriver = self._getDriver()
docs: Get the credentials from the file specified by GOOGLE_APPLICATION_CREDENTIALS.