<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def exists(self, digest):
 """Check file existence in fsdb Returns: True if file exists under this instance of fsdb, false otherwise """

        if not isinstance(digest, string_types):
            raise TypeError("digest must be a string")
        return os.path.isfile(self.get_file_path(digest))