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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?

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1 Answer 1

Reset to default
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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")
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  • \$\begingroup\$ 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! \$\endgroup\$ Commented Jun 14, 2019 at 17:50

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