copy.copy(bytes) is slow

[frankmillman]

BPO	20791
Nosy	@pitrou, @vstinner, @bitdancer, @1st1
Files	copy_bytes.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-02-27.21:16:28.389>
created_at = <Date 2014-02-27.08:58:03.176>
labels = ['library', 'performance']
title = 'copy.copy(bytes) is slow'
updated_at = <Date 2014-02-27.23:35:06.626>
user = 'https://bugs.python.org/frankmillman'

bugs.python.org fields:

activity = <Date 2014-02-27.23:35:06.626>
actor = 'python-dev'
assignee = 'none'
closed = True
closed_date = <Date 2014-02-27.21:16:28.389>
closer = 'pitrou'
components = ['Library (Lib)']
creation = <Date 2014-02-27.08:58:03.176>
creator = 'frankmillman'
dependencies = []
files = ['34244']
hgrepos = []
issue_num = 20791
keywords = ['patch']
message_count = 12.0
messages = ['212340', '212344', '212345', '212354', '212377', '212383', '212388', '212389', '212392', '212393', '212394', '212395']
nosy_count = 7.0
nosy_names = ['peter.otten', 'pitrou', 'vstinner', 'frankmillman', 'r.david.murray', 'python-dev', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue20791'
versions = ['Python 3.4', 'Python 3.5']

[frankmillman]

Using copy.copy on a byte string returns a new copy instead of the original immutable object. Using copy.deepcopy returns the original, as expected. Testing with timeit, copy.copy is much slower than copy.deepcopy.

>>> import copy
>>>
>>> a = 'a'*1000  # string
>>> copy.copy(a) is a
True
>>> copy.deepcopy(a) is a
True
>>>
>>> b = b'b'*1000  # bytes
>>> copy.copy(b) is b
False
>>> copy.deepcopy(b) is b
True

[peterotten]

Assuming that the current behaviour is by accident here's a patch that adds the bytes type to the _copy_dispatch registry.

[vstinner]

bytes is immutable, I don't why we would duplicate it.

[bitdancer]

We probably just forgot to add it when the bytes type was added, and there was of course no test that would fail since it was a new type.

[pitrou]

Thanks for the patch, Peter. Could you sign a contributor's agreement? You can do so online at http://www.python.org/psf/contrib/contrib-form/

[peterotten]

I did sign today (and received the confirmation email).

[python-dev]

New changeset c2523bca50d7 by Antoine Pitrou in branch 'default':
Issue bpo-20791: copy.copy() now doesn't make a copy when the input is a bytes object. Initial patch by Peter Otten.
http://hg.python.org/cpython/rev/c2523bca50d7

[pitrou]

Ok, thank you! This will be in 3.4.1 and 3.5.

[vstinner]

Why not fixing this bug in Python 3.3 too?

[pitrou]

Is Python 3.3 still allowing bug fixes?
If yes, this is a good idea. Do you want to backport it?

[bitdancer]

3.3 will get fixes until its final release, which will be some time after the release of 3.4.0.

[python-dev]

New changeset b3fb7828b6fc by Antoine Pitrou in branch '3.3':
Issue bpo-20791: copy.copy() now doesn't make a copy when the input is a bytes object. Initial patch by Peter Otten.
http://hg.python.org/cpython/rev/b3fb7828b6fc