def calcHA1(pszAlg, pszUserName, pszRealm, pszPassword, pszNonce, pszCNonce,
            preHA1=None):
    """
    Compute H(A1) from RFC 2617.

    @param pszAlg: The name of the algorithm to use to calculate the digest.
        Currently supported are md5, md5-sess, and sha.
    @param pszUserName: The username
    @param pszRealm: The realm
    @param pszPassword: The password
    @param pszNonce: The nonce
    @param pszCNonce: The cnonce

    @param preHA1: If available this is a str containing a previously
       calculated H(A1) as a hex string.  If this is given then the values for
       pszUserName, pszRealm, and pszPassword must be L{None} and are ignored.
    """

    if (preHA1 and (pszUserName or pszRealm or pszPassword)):
        raise TypeError(("preHA1 is incompatible with the pszUserName, "
                         "pszRealm, and pszPassword arguments"))

    if preHA1 is None:
        # We need to calculate the HA1 from the username:realm:password
        m = algorithms[pszAlg]()
        m.update(pszUserName)
        m.update(b":")
        m.update(pszRealm)
        m.update(b":")
        m.update(pszPassword)
        HA1 = hexlify(m.digest())
    else:
        # We were given a username:realm:password
        HA1 = preHA1

    if pszAlg == b"md5-sess":
        m = algorithms[pszAlg]()
        m.update(HA1)
        m.update(b":")
        m.update(pszNonce)
        m.update(b":")
        m.update(pszCNonce)
        HA1 = hexlify(m.digest())

    return HA1

    Compute H(A1) from RFC 2617.

    @param pszAlg: The name of the algorithm to use to calculate the digest.
        Currently supported are md5, md5-sess, and sha.
    @param pszUserName: The username
    @param pszRealm: The realm
    @param pszPassword: The password
    @param pszNonce: The nonce
    @param pszCNonce: The cnonce

    @param preHA1: If available this is a str containing a previously
       calculated H(A1) as a hex string.  If this is given then the values for
       pszUserName, pszRealm, and pszPassword must be L{None} and are ignored.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/_digest.py