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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _GetAccessToken(self):
 """Return an OAuth 2.0 access token for Google Storage."""

    service_accounts = self.watcher.GetMetadata(metadata_key=self.metadata_key)
    try:
      return service_accounts[self.service_account]['token']['access_token']
    except KeyError:
      return None