functools.partial is not compatible between 2.7 and 3.5

[naoyuki-kamo]

BPO	29532
Nosy	@rhettinger, @stevendaprano, @methane, @serhiy-storchaka, @naoyuki-kamo
PRs	bpo-29532: Altering a kwarg dictionary passed to functools.partial() #190 [3.6] bpo-29532: Altering a kwarg dictionary passed to functools.partial() no longer affects a partial object after creation. #198 [3.6] bpo-29532: Altering a kwarg dictionary passed to functools.partial() … #209 [3.5] bpo-29532: Altering a kwarg dictionary passed to functools.partial() no longer affects a partial object after creation. #222 [Do Not Merge] Sample of CPython life with blurb. #703 [Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552
Files	partial-copy-kwargs.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 2017-02-22.09:53:25.891>
created_at = <Date 2017-02-11.07:20:01.590>
labels = ['3.7', 'type-bug', 'library']
title = 'functools.partial is not compatible between 2.7 and 3.5'
updated_at = <Date 2017-03-31.16:36:21.417>
user = 'https://github.com/naoyuki-kamo'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:21.417>
actor = 'dstufft'
assignee = 'none'
closed = True
closed_date = <Date 2017-02-22.09:53:25.891>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2017-02-11.07:20:01.590>
creator = 'naoyuki'
dependencies = []
files = ['46633']
hgrepos = []
issue_num = 29532
keywords = ['patch']
message_count = 8.0
messages = ['287594', '287595', '287598', '287673', '287832', '288197', '288309', '288347']
nosy_count = 5.0
nosy_names = ['rhettinger', 'steven.daprano', 'methane', 'serhiy.storchaka', 'naoyuki']
pr_nums = ['190', '198', '209', '222', '703', '552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29532'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']

[naoyuki-kamo]

The code:
from functools import partial
def f(a):
print(a)
d = {'a': 3}
g = partial(f, **d)
g()
d['a'] = 5
g()

On python2.7, gets
3
but on python3.5, gets
5

is it a bug?

[stevendaprano]

Confirmed that in Python 2.7 calling g() before and after modifying the dict prints 3 both times; calling g() before modifying the dict prints 3, then after modifying it prints 5.

Python 3.3 behaves like 2.7, so this sounds like a regression in 3.5 or maybe 3.4.

[serhiy-storchaka]

Yes, this can be considered a bug. Following patch fixes it.

[methane]

patch LGTM.

[rhettinger]

Agree that this is a bug. Patch and test looks good.

[serhiy-storchaka]

New changeset 9639e4a by GitHub in branch 'master':
bpo-29532: Altering a kwarg dictionary passed to functools.partial() (#190)
9639e4a

[serhiy-storchaka]

New changeset e48fd93 by GitHub in branch '3.6':
bpo-29532: Altering a kwarg dictionary passed to functools.partial() no longer affects a partial object after creation. (#209)
e48fd93

[serhiy-storchaka]

New changeset 5010a77 by GitHub in branch '3.5':
[3.5] bpo-29532: Altering a kwarg dictionary passed to functools.partial() no longer affects a partial object after creation. (#222)
5010a77