如何在Python中将256位大端整数转换为小端？

Question

不太复杂，或者我希望如此。我有一个256位十六进制整数编码为大端，我需要转换为小端。 Python的struct模块通常就足够了，但是the official documentation没有列出的格式，其大小甚至接近我需要的格式。

使用struct的非长度特定类型（虽然我可能做错了）似乎不起作用：

>> x = 'ffffffffffffffffffffffffffffffffffffffffffffffffffffffff00000000'
>> y = struct.unpack('>64s', x)[0] # Unpacking as big-endian
>> z = struct.pack('<64s', y) # Repacking as little-endian
>> print z
'ffffffffffffffffffffffffffffffffffffffffffffffffffffffff00000000'

示例代码（应该发生什么）：

>> x = 'ffffffffffffffffffffffffffffffffffffffffffffffffffffffff00000000'
>> y = endianSwap(x)
>> print y
'00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff'

Answer 1

struct模块无法处理256位数。所以你必须手动编码。

首先，您应该将其转换为字节：

x = 'ffffffffffffffffffffffffffffffffffffffffffffffffffffffff00000000'
a = x # for having more successive variables
b = a.decode('hex')
print repr(b)
# -> '\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00'

这样您可以将其反转using @Lennart's method：

c = b[::-1]
# -> '\x00\x00\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff'

d = c.encode('hex')
z = d
print z
# -> 00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff

Answer 2

>>> big = 'ffffffffffffffffffffffffffffffffffffffffffffffffffffffff00000000'
>>> big[::-1]
'00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff'