Indeterminate-length big-endian to little-endian conversion in Python

Question

I have a need to convert wire encoding (big-endian) to little-endian in Python.

The conversion needs to happen as a separate function; I am not allowed to change the function that reads the byte stream because it will break a lot of other functions.

The best I can think of is this:

def be_to_le(value: int) -> int:
 assert isinstance(value, int)
 numbytes = len(f"0{value:x}") >> 1
 if numbytes == 1:
 return value
 rslt = 0
 for b in value.to_bytes(numbytes, byteorder="little"):
 rslt <<= 8
 rslt |= b
 return rslt

Is there a better way?

Answer 1

To calculate number of bytes, better use:

numbytes = math.ceil(value.bit_length() / 8)

And then you can just:

def be_to_le(value: int) -> int:
 numbytes = math.ceil(value.bit_length() / 8)
 return int.from_bytes(value.to_bytes(numbytes, byteorder="little"), byteorder="big")

• pepoluan: Ooohhh... so int has a bit_length() method? That's news to me... Thanks for telling me that! I'll probably not use math.ceil though. (n + 7) >> 3 might be better since it keeps everything in int land. Anyways, thanks!