def _cert_callback(callback, der_cert, reason):
if not callback:<EOL><INDENT>return<EOL><DEDENT>callback(x509.Certificate.load(der_cert), reason)<EOL>
Constructs an asn1crypto.x509.Certificate object and calls the export
callback

:param callback:
    The callback to call

:param der_cert:
    A byte string of the DER-encoded certificate

:param reason:
    None if cert is being exported, or a unicode string of the reason it
    is not being exported