CODEWITHSUNDEEP
pythonpython-3.x
George
George

Reputation: 87

How to use struct.unpack to extract int from list of bytes in python 3?

Assuming, I have a list bites with constant length

>>> chunk = [b'\xd0', b'\x00', b'\xd5', b'\x00', b'\xdf', b'\x00', b'\xaa', b'U']
>>> print(type(chunk))
<class 'list'>
>>> print(chunk)
[b'\xd0', b'\x00', b'\xd5', b'\x00', b'\xdf', b'\x00', b'\xaa', b'U']

And I want to get a single int from it, which could be done this way:

>>> a = int.from_bytes(chunk[2], byteorder='little')
>>> b = int.from_bytes(chunk[3], byteorder='little')
>>> decoded = a * 256 + b
>>> print(decoded)
213

How do I use struct.unpack for that purpose? Several ways I failed:

>>> print(struct.unpack('<xxHxxxx', chunk))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: a bytes-like object is required, not 'list'

>>> tmp = bytes(chunk)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'bytes' object cannot be interpreted as an integer

>>> tmp = bytes([chunk])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'list' object cannot be interpreted as an integer

>>> tmp = bytearray(chunk)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'bytes' object cannot be interpreted as an integer

The initial motivation was to check if struct.unpack is any faster then already working code. But now I want to figure out the right way to use it.

Oh, and if there is any more efficient way, I would be glad to know that too!

Upvotes: 0

Views: 2395

Answers (1)

urban
urban

Reputation: 5702

The issue is in how you convert a list of bytes to bytes. You can do this with b''.join(list) and then apply your struct unpack.

Example:

>>> import struct
# your list of bytes
>>> lst = [b'\xd0', b'\x00', b'\xd5', b'\x00', b'\xdf', b'\x00', b'\xaa', b'U']

# Convert into `bytes`
>>> bts = b''.join(lst)

# Unpack
>>> print(struct.unpack('<xxHxxxx', bts))
(213,)

Note that you can also unpack the returned tuple with:

>>> (a,) = struct.unpack('<xxHxxxx', bts)
>>> a
213

Upvotes: 1

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