if the current file exists
            returns the sha blob
        else
            returns None

def get_sha_blob(self):
        """
        if the current file exists
            returns the sha blob
        else
            returns None
        """
        r = requests.get(self.api_url, auth=self.get_auth_details())
        try:
            return r.json()['sha']
        except KeyError:
            return None