Repository navigation
MappingProxyType can not be pickled #75392
Description
| BPO | 31209 |
|---|---|
| Nosy | @rhettinger |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
Show more details
GitHub fields:
assignee = None
closed_at = <Date 2017-08-28.09:56:42.114>
created_at = <Date 2017-08-15.04:49:11.844>
labels = ['invalid', 'type-bug', 'library']
title = 'MappingProxyType can not be pickled'
updated_at = <Date 2017-08-28.09:56:42.112>
user = 'https://bugs.python.org/AlexHayes'bugs.python.org fields:
activity = <Date 2017-08-28.09:56:42.112>
actor = 'Alex Hayes'
assignee = 'none'
closed = True
closed_date = <Date 2017-08-28.09:56:42.114>
closer = 'Alex Hayes'
components = ['Library (Lib)']
creation = <Date 2017-08-15.04:49:11.844>
creator = 'Alex Hayes'
dependencies = []
files = []
hgrepos = []
issue_num = 31209
keywords = []
message_count = 5.0
messages = ['300284', '300324', '300735', '300738', '300947']
nosy_count = 2.0
nosy_names = ['rhettinger', 'Alex Hayes']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31209'
versions = ['Python 3.6']
Activity
AlexHayes commentedon Aug 15, 2017
I imagine that this is by design (likely because it can't be implemented) however I wanted to check if this was the case.
Instances of types.MappingProxyType can't be pickled.
For example;
Raises: TypeError: can't pickle mappingproxy objects
----
Is this the desired behaviour or is it a bug?
rhettinger commentedon Aug 16, 2017
What would be the point of pickling a MappingProxy? Do you have use case or is it just a curiosity?
AlexHayes commentedon Aug 23, 2017
Some background. I'm not actually trying to pickle MappingProxyType, I'm using the schematics [1] library and recently it broke support for pickle [2] and the cause of that is because they are using MappingProxyType [3] internally (when they used to use dict).
The Python docs on MappingProxyType [4] provides an appropriate level of API documentation but what it doesn't cover is when you should or should not use MappingProxyType (understandably, it's most likely a complex answer). However, I imagine that it's being used in schematics because at the bottom of the Python dict documentation [5] it states;
To answer the question "What would be the point of pickling a MappingProxy?" I would probably have to ask, what is the intended use of MappingProxyType?
If one was unfamiliar with the way in which pickle and MappingProxyType works I imagine they would think there would be no reason why a "read-only view of a dict" can't be pickled.
So I guess my questions now are;
I imagine the answer to the first question is "no" given how
__setstate__works.----
[1] https://github.com/schematics/schematics
[2] schematics/schematics#510
[3] schematics/schematics#511
[4] https://docs.python.org/3/library/types.html#types.MappingProxyType
[5] https://docs.python.org/3/library/stdtypes.html#dict
rhettinger commentedon Aug 23, 2017
For the second question, I don't think so. AFAICT the only intention was to support the actual mapping views for regular dicts. Its likely that not much thought was given to other uses, so it doesn't make sense to document intention when none really exists ;-)
Given the answers above and that you have a PR solve your upstream issue with Schematics, can this bug report be closed?
AlexHayes commentedon Aug 28, 2017
Closing issue - thanks rhettinger
eroller commentedon Dec 13, 2023
I think some clever python developers may inadvertently use it as a default value for an argument with mypy annotation of Mapping hoping to workaround the age-old issue with having mutable default arguments in python. A workaround for default arguments with mypy annotation of Sequence exists by using a tuple which is inherently immutable.
For symmetry and to enable an immutable default argument for Mappings it would be nice if pickle could support MappingProxyType
See related: https://peps.python.org/pep-0416/