compress a string in python 3?

Question

I don't understand in 2.X it worked :

import zlib
zlib.compress('Hello, world')

now i have a :

zlib.compress("Hello world!")
TypeError: must be bytes or buffer, not str

How can i compress my string ? Regards Bussiere

Compression always works on sequences of bytes, but you need to convert to them first (i.e., pick an encoding for the characters as bytes). — Donal Fellows
– Donal Fellows, Commented Oct 1, 2010 at 13:01
possible duplicate of TypeError: 'str' does not support the buffer interface — NoDataDumpNoContribution
– NoDataDumpNoContribution, Commented Oct 6, 2014 at 19:59

relet · Accepted Answer · 2010-10-01 13:01:48Z

20

This is meant to enforce that you actually have a defined encoding.

zlib.compress("Hello, world".encode("utf-8"))
b'x\x9c\xf3H\xcd\xc9\xc9\xd7Q(\xcf/\xcaI\x01\x00\x1b\xd4\x04i'
zlib.compress("Hello, world".encode("ascii"))
b'x\x9c\xf3H\xcd\xc9\xc9\xd7Q(\xcf/\xcaI\x01\x00\x1b\xd4\x04i'

The same string could describe different byte sequences otherwise. But it is actually a byte sequence that will be encoded by zlib.

>>> zlib.compress("Hello, wørld".encode("utf-16"))
b'x\x9c\xfb\xff\xcf\x83!\x95!\x07\x08\xf3\x19t\x18\x14\x18\xca\x19~0\x14\x01y)\x0c\x00n\xa6\x06\xef'
>>> zlib.compress("Hello, wørld".encode("utf-8"))
b"x\x9c\xf3H\xcd\xc9\xc9\xd7Q(?\xbc\xa3('\x05\x00#\x7f\x05u"

answered Oct 1, 2010 at 13:01

relet

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4 Comments

Manoj Govindan Over a year ago

Curious: why does this work for me (Python 3.0.1, Ubuntu Jaunty) without calling encode?

Dana the Sane Over a year ago

Check the release docs, this change might be enforced in a later version of 3.

user743873 Over a year ago

Wow, 2019 and this answer is still helpful. Thanks for posting all those years ago.

relet Over a year ago

You are welcome. Isn't it great if your basic implementation details don't change every few years. :D

Douglas Leeder · Accepted Answer · 2010-10-01 13:33:49Z

19

In python 2.x strings are bytes string by default. In python 3.x they are unicode strings.

Compressing needs a byte string.

answered Oct 1, 2010 at 13:33

Douglas Leeder

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