<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_hash(self, data, hasher=None):
 """ Get the hash of some data, using a particular hash algorithm, if specified. :param data: The data to be hashed. :type data: bytes :param hasher: The name of a hash implementation, supported by hashlib, or ``None``. Examples of valid values are ``'sha1'``, ``'sha224'``, ``'sha384'``, '``sha256'``, ``'md5'`` and ``'sha512'``. If no hasher is specified, the ``hasher`` attribute of the :class:`InstalledDistribution` instance is used. If the hasher is determined to be ``None``, MD5 is used as the hashing algorithm. :returns: The hash of the data. If a hasher was explicitly specified, the returned hash will be prefixed with the specified hasher followed by '='. :rtype: str """

        if hasher is None:
            hasher = self.hasher
        if hasher is None:
            hasher = hashlib.md5
            prefix = ''
        else:
            hasher = getattr(hashlib, hasher)
            prefix = '%s=' % self.hasher
        digest = hasher(data).digest()
        digest = base64.urlsafe_b64encode(digest).rstrip(b'=').decode('ascii')
        return '%s%s' % (prefix, digest)