Issue 42836: docs: struct: clarify struct caching behaviour

➜

This issue tracker has been migrated to GitHub, and is currently read-only.
For more information, see the GitHub FAQs in the Python's Developer Guide.

This issue has been migrated to GitHub: https://github.com/python/cpython/issues/87002

classification

Title:	docs: struct: clarify struct caching behaviour
Type:	enhancement	Stage:	patch review
Components:	Documentation	Versions:	Python 3.10, Python 3.9, Python 3.8, Python 3.7

process

Status:	open	Resolution:
Dependencies:		Superseder:
Assigned To:	docs@python	Nosy List:	docs@python, python-dev, uniq10
Priority:	normal	Keywords:	patch

Created on 2021-01-06 09:05 by uniq10, last changed 2022-04-11 14:59 by admin.

Pull Requests
URL	Status	Linked	Edit
PR 24164	open	python-dev, 2021-01-08 06:05

Messages (1)
msg384476 - (view)	Author: Sandeep Subramanian (uniq10) *	Date: 2021-01-06 09:05
As per docs in :https://docs.python.org/3.7/library/struct.html#struct.Struct > The compiled versions of the most recent format strings passed to Struct and the module-level functions are cached, so programs that use only a few format strings needn’t worry about reusing a single Struct instance. This statement seems to imply that the format string passed as `struct.Struct(format)` is also cached. This doesn't appear to be the case. Only format strings passed to the module level functions seems to be cached. `cache_struct_converter` at https://github.com/python/cpython/blob/v3.7.7/Modules/_struct.c#L2052 appears to be used only in the top module level functions `pack` and `pack_into` and is not used in `Struct.__init__`. Would it be better to change the doc to: > The compiled versions of the most recent format strings passed to the module-level functions are cached, so programs that use only a few format strings needn’t worry about reusing a single Struct instance. This issue was introduced by https://github.com/python/cpython/commit/3666b3c1f695a145adab1bf644c22e564e8eb0ee and is present in every version since. Refs: https://github.com/python/cpython/pull/7700 https://bugs.python.org/issue33032

msg384476 - (view)

Author: Sandeep Subramanian (uniq10) *

Date: 2021-01-06 09:05

As per docs in :https://docs.python.org/3.7/library/struct.html#struct.Struct

> The compiled versions of the most recent format strings passed to Struct and the module-level functions are cached, so programs that use only a few format strings needn’t worry about reusing a single Struct instance.

This statement seems to imply that the format string passed as `struct.Struct(format)` is also cached. This doesn't appear to be the case.

Only format strings passed to the module level functions seems to be cached.

`cache_struct_converter` at https://github.com/python/cpython/blob/v3.7.7/Modules/_struct.c#L2052 
appears to be used only in the top module level functions `pack` and `pack_into` and is *not* used in `Struct.__init__`.


Would it be better to change the doc to:

> The compiled versions of the most recent format strings passed to the module-level functions are cached, so programs that use only a few format strings needn’t worry about reusing a single Struct instance.

This issue was introduced by https://github.com/python/cpython/commit/3666b3c1f695a145adab1bf644c22e564e8eb0ee

and is present in every version since.

Refs:
https://github.com/python/cpython/pull/7700
https://bugs.python.org/issue33032

History
Date	User	Action	Args
2022-04-11 14:59:39	admin	set	github: 87002
2021-01-08 06:05:25	python-dev	set	keywords: + patch nosy: + python-dev pull_requests: + pull_request22992 stage: patch review
2021-01-06 09:05:31	uniq10	create