In [1]: import functools

In [2]: def add(x, y) -> int:
   ...:     return x + y
   ...:

In [3]: newadd = functools.partial(add, y=5)

In [4]: newadd(7)
Out[4]: 12

In [5]: newadd(7, y=6)
Out[5]: 13

In [6]: newadd(y=10, x=6)
Out[6]: 16

In [7]: import inspect

In [8]: inspect.signature(newadd)
Out[8]: <Signature (x, *, y=5) -> int>

In [9]: def partial(func, *args, **keywords):
   ...:     def newfunc(*fargs, **fkeywords):
   ...:         newkeywords = keywords.copy()
   ...:         newkeywords.update(fkeywords)
   ...:         return func(*(args + fargs), **newkeywords)
   ...:     newfunc.func = func
   ...:     newfunc.args = args
   ...:     newfunc.keywords = keywords
   ...:     return newfunc
   ...:

In [10]: def add(x, y):
    ...:     return x + y
    ...:

In [11]: foo = partial(add, 4)

In [12]: foo(5)
Out[12]: 9

from collections import OrderedDict

base_tbl = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
alphabet = OrderedDict(zip(base_tbl, range(64)))

def base64decode(src:bytes):
    ret = bytearray()
    length = len(src)

    step = 4 # 对齐，每次取4个
    for offset in range(0, length, step):
        tmp = 0x00
        block = src[offset:offset + step]

        # 开始移位计算
        for i in range(4):
            index = alphabet.get(block[-i-1])
            if index is not None:
                tmp += index << i*6 

        ret.extend(tmp.to_bytes(3, 'big'))
    return bytes(ret.rstrip(b'\x00'))

txt = 'TWFu'

txt = txt.encode()
print(txt)

print(base64decode(txt).decode())

>>>
b'TWFu'
Man