<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def from_file(cls, filename):
 """ Create a public blob from a ``-cert.pub``-style file on disk. """

        with open(filename) as f:
            string = f.read()
        return cls.from_string(string)