Deepcopying property objects results in unexpected TypeError

[GudniNatan]

BPO	38293
Nosy	@serhiy-storchaka, @eamanu, @miss-islington, @GudniNatan
PRs	bpo-38293: Allow shallow and deep copying of property objects #16438 [3.7] bpo-38293: Allow shallow and deep copying of property objects (GH-16438) #17968 [3.8] bpo-38293: Allow shallow and deep copying of property objects (GH-16438) #17969

^{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 2020-01-12.17:43:50.420>
created_at = <Date 2019-09-27.16:16:43.481>
labels = ['3.7', '3.8', 'type-bug', 'library', '3.9']
title = 'Deepcopying property objects results in unexpected TypeError'
updated_at = <Date 2020-01-12.18:04:21.916>
user = 'https://github.com/GudniNatan'

bugs.python.org fields:

activity = <Date 2020-01-12.18:04:21.916>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-12.17:43:50.420>
closer = 'cheryl.sabella'
components = ['Library (Lib)']
creation = <Date 2019-09-27.16:16:43.481>
creator = 'GudniNatan'
dependencies = []
files = []
hgrepos = []
issue_num = 38293
keywords = ['patch']
message_count = 9.0
messages = ['353375', '353376', '353377', '353383', '353388', '353428', '359857', '359860', '359861']
nosy_count = 4.0
nosy_names = ['serhiy.storchaka', 'eamanu', 'miss-islington', 'GudniNatan']
pr_nums = ['16438', '17968', '17969']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38293'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[GudniNatan]

Currently, attempting to deepcopy a property object will result in an unexpected TypeError:

>>> import copy
>>> obj = property()
>>> new_obj = copy.deepcopy(obj)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.7_3.7.1264.0_x64__qbz5n2kfra8p0\lib\copy.py", line 169, in deepcopy
    rv = reductor(4)
TypeError: can't pickle property objects

What I believe is happening here is that since property objects are not treated by the copy module as atomic, they are passed off to be pickled and so our error is raised.
This can be fixed in a similar manner to how it works for type objects, function objects and more.
Adding a single line of code to Lib/copy.py after line 208:

d[property] = _deepcopy_atomic

Means that property objects will be treated as atomic, and therefore returned as-is.

[GudniNatan]

A small change:

The fix should go to Lib/copy.py:198, not line 208.

[serhiy-storchaka]

But the property object is not atomic. It's attribute __doc__ is writeable.

[GudniNatan]

Function objects are considered "atomic" here and I believe you can also write to their __doc__ (among other attributes).

[GudniNatan]

This bug appears to also affect shallow copies and can be reproduced with the following code:

>>> import copy
>>> obj = property()
>>> copy.copy(obj)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Program Files\WindowsApps\PythonSoftwareFoundation.Python.3.7_3.7.1264.0_x64__qbz5n2kfra8p0\lib\copy.py", line 96, in copy
    rv = reductor(4)
TypeError: can't pickle property objects

[eamanu]

I can confirm this behavior also on python 3.6 3.8 3.9

[miss-islington]

New changeset 9f3fc6c by Miss Islington (bot) (Guðni Natan Gunnarsson) in branch 'master':
bpo-38293: Allow shallow and deep copying of property objects (GH-16438)
9f3fc6c

[miss-islington]

New changeset 4be9726 by Miss Islington (bot) in branch '3.7':
bpo-38293: Allow shallow and deep copying of property objects (GH-16438)
4be9726

[miss-islington]

New changeset 3043ec7 by Miss Islington (bot) in branch '3.8':
bpo-38293: Allow shallow and deep copying of property objects (GH-16438)
3043ec7