<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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