Calculate the length in bytes of a base64 string.

base64.py

def b64len(b64str):
    """
    Calculate the length in bytes of a base64 string.

    This function could decode the base64 string to a binary blob and count its
    number of bytes with len(). But, that's inefficient and requires more
    memory that really needed.

    Base64 encodes three bytes to four characters. Sometimes, padding is added
    in the form of one or two '=' characters.

    So, the following formula allows to know the number of bytes of a base64
    string without decoding it::

        (3 * (length_in_chars / 4)) - (number_of_padding_chars)

    :param str b64str: A base64 encoded string.

    :return: Length, in bytes, of the binary blob encoded in base64.
    :rtype: int
    """
    return (3 * (len(b64str) / 4)) - b64str[-2:].count('=')