Reinvent the wheel: implemente base64 encoding and decoing in Python.

base64.py

CHARS = b'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
NUMS = [62, 0, 0, 0, 63,
        *range(52, 62),
        0, 0, 0, 0, 0, 0, 0,
        *range(26),
        0, 0, 0, 0, 0, 0,
        *range(26, 52)]


def to_val(c):
    return NUMS[c - ord('+')]


def group(s, step):
    i = 0
    while i + step < len(s):
        yield s[i:i + step]
        i += step
    yield list(s[i:]) + [0 for _ in range(i + step - len(s))]


def base64_encode(s):
    rv = []
    for i, j, k in group(s, 3):
        a = i >> 2
        b = ((i & 0b11) << 4) + (j >> 4)
        c = ((j & 0b1111) << 2) + (k >> 6)
        d = k & 0b111111
        rv.extend([CHARS[a], CHARS[b], CHARS[c], CHARS[d]])
    # 最后的 A 或者 AA 需要替换为 =
    # 如果 Base64 编码字符串不会相互拼接在传输，最后的=也可以省略。
    # 在解码时发现长度不能被4整除，则先补充=号，再解码。
    if len(s) % 3:
        for i in range(3 - len(s) % 3):
            rv[-(i + 1)] = ord('=')
    return bytes(rv)


def base64_decode(s):
    nequal = len(s) - s.find(b'=')
    rv = []
    for a, b, c, d in group(s, 4):
        a, b, c, d = to_val(a), to_val(b), to_val(c), to_val(d)
        i = (a << 2) + (b >> 4)
        j = ((b & 0b1111) << 4) + (c >> 2)
        k = ((c & 0b11) << 6) + d
        rv.extend([i, j, k])
    for _ in range(nequal):
        rv.pop()
    return bytes(rv)


if __name__ == '__main__':
    print(base64_decode(b'YQ=='))