Work with bytes in python

Question

I have to get bytes array and send it into socket.

The structure looks like: 1 byte + 2 bytes + 2 bytes.

First byte is number '5', second 2 bytes should be taken from variable first, third 2 bytes should be taken from variable second. What's the right way to do this in python?

id = 5      # Fill as 1 byte
first = 42  # Fill as 2 bytes
second = 58 # The same as first

Martijn Pieters · Accepted Answer · 2025-04-04 14:06:49Z

8

Use the struct module:

>>> import struct
>>> id, first, second = 5, 42, 58
>>> struct.pack('>bhh', id, first, second)
'\x05\x00*\x00:'

You may want to figure out if your data is a) little or big endian and b) is signed or unsigned; the example above use big-endian ordering and signed values.

The result (in python 3) is a bytes object.

edited Apr 4 at 14:06

answered Aug 22, 2012 at 12:30

Martijn Pieters

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4 Comments

Martijn Pieters Over a year ago

@Ockonal: then use the '>' format marker at the start; my example now uses that.

cleong Over a year ago

Consider creating a Struct object (from the same module) if the byte-packing occurs repeatedly.

Ray Over a year ago

According to the question the variable named 'second' should be two bytes too.

Martijn Pieters Apr 4 at 14:08

@Ray: ah, indeed, I fixed my answer now. Slightly confusing wording there, first could be read as referring to the first value, but there is also a variable with the same name. The description of the structure bytes is clearer however.

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