How to pack a UUID into a struct in Python?

Question

I have a UUID that I was thinking of packing into a struct using UUID.int, which turns it into a 128-bit integer. But none of the struct format characters are large enough to store it, how to go about doing this?

Sample code:

s = struct.Struct('L')
unique_id = uuid.uuid4()    
tuple = (unique_id.int)
packed = s.pack(*tuple)

The problem is, struct format 'L' is only 4 bytes...I need to store 16. Storing it as a 32-char string is a bit much.

user_881224 · Answer

TL;DR

struct.pack('


Introduction
Even though Cat++'s answer is really great, it breaks the UUID in half to fit it into two unsigned long longs. I wanted to pack each field, which left me with the following:
def maxsize(size: typing.Union[int,str]):
    """ Useful for playing with different struct.pack formats """
    if isinstance(size, str):
        size = struct.calcsize(size)
    return 2 ** (4 * size) - 1

uuid_max = uuid.UUID('ffffffff-ffff-ffff-ffff-ffffffffffff')
tuple(maxsize(len(f)) for f in str(u).split('-'))
# (4294967295, 65535, 65535, 65535, 281474976710655)

uuid_max.fields
# (4294967295, 65535, 65535, 255, 255, 281474976710655)

uuid_foo = UUID('909822be-c5c4-432f-95db-da1be79cf067')
uuid_foo.fields
# (2425889470, 50628, 17199, 149, 219, 239813384794215)

The first five fields are easy since they already line up as unsigned 8, 4, 4, 2, 2 size integers. The last one required a little extra help from another answer.

Notes:
Padding is only automatically added between successive structure members. No padding is added at the beginning or the end of the encoded struct.
No padding is added when using non-native size and alignment, e.g. with ‘<’, ‘>’, ‘=’, and ‘!’.
To align the end of a structure to the alignment requirement of a particular type, end the format with the code for that type with a repeat count of zero. See Examples.

struct.pack('>LHHBBQ', *uuid_foo.fields)
# b'\x90\x98"\xbe\xc5\xc4C/\x95\xdb\x00\x00\xda\x1b\xe7\x9c\xf0g'
#                                    ^^  ^^ these empty bytes won't work!

The actual answer
Since the last field is size 12, you'll have to pack it and unpack it backwards, little endian. That'll leave zeros at the end, instead of between the fifth and sixth fields.
struct.unpack('

Regenerating this requires you reverse it one more time.
uuid_packed = struct.pack('

How to pack a UUID into a struct in Python?

3 Answers

TL;DR

Introduction

The actual answer