如何在Python中将256位大端数整数转换为小端数？

Question

不太复杂，所以我希望。我有一个256位的十六进制整数编码为大端，我需要转换为小端。 Python的结构模块通常就足够了，但the official documentation没有列出的格式，其大小甚至接近我需要的大小。

使用结构的非长度特定类型的（虽然我可以做这个错误）似乎不工作：

>> 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（会发生什么）：

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

Answer 1

struct模块无法处理256位数字。所以你必须手动做你的编码。

首先，你应该把它转换为字节：

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' 

这样你就可以扭转这种局面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'