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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_client_side_certificate_subject():
 """Return the DN from the client side certificate as a D1 subject if a client side cert has been configured. Else return None. """

    subject = django.core.cache.cache.get('client_side_certificate_subject')
    if subject is not None:
        return subject
    cert_pem = _get_client_side_certificate_pem()
    if cert_pem is None:
        return None
    subject = _extract_subject_from_pem(cert_pem)
    django.core.cache.cache.set('client_side_certificate_subject', subject)
    return subject