The sys.float_repr_style should be read-only

[skirpichev]

Bug report

Bug description:

It's possible to "change" the repr style for floats:

Python 3.14.0a2+ (heads/long_export-decimal:4bb9ad04f5, Dec 17 2024, 05:39:57) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.float_repr_style
'short'
>>> sys.float_repr_style = 'legacy'
>>> sys.float_repr_style
'legacy'

Of course, this has no effect, as this variable just shows build-time settings. That certainly can misguide people, e.g. see this: #128005 (comment)

It could be nice to make this variable read-only (unfortunately, PEP 726 was rejected and it's might be not so easy). Or, at least, improve docs to mention that it's intended to be read-only (we emphasize this e.g. for int_info fields).

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-128017: Make a note that variables that are provided by sys are read-only #128887
• [3.13] gh-128017: Make a note that sys variables are read-only (GH-128887) #128908
• [3.12] gh-128017: Make a note that sys variables are read-only (GH-128887) #128909

[srinivasreddy]

@skirpichev Are you working on it? Shall i take it?

[srinivasreddy]

So final resolution is to improve the docs that float_repr_style is only a read only field, and that any change to other values such as legacy has no effect on the formatting?

[skirpichev]

Are you working on it?

No. I'm not sure yet how it should be solved.

Shall i take it?

Sure.

So final resolution is to improve the docs that float_repr_style is only a read only field

The "problem" actually span not only this variable in the sys module. You can "change" everything:

>>> sys.maxsize
9223372036854775807
>>> sys.maxsize = 123
>>> sys.maxsize
123
>>> sys.int_info = "Spam"
>>> sys.int_info
'Spam'

Current docs looks ok for me. But it seems some people are misguided by such possibility. Maybe some generic note might help them.

[vstinner]

https://peps.python.org/pep-0726/ was a good solution to this problem but it was rejected: https://discuss.python.org/t/pep-726-module-setattr-and-delattr/32640

[vstinner]

It could be nice to make this variable read-only

I don't think that we can easily solve this issue and I suggest to close it.

[skirpichev]

@vstinner, does it make sense for you as a documentation issue?

[vstinner]

Which kind of documentation addition are you thinking about? Add a note to each sys attribute? Add a global note to the sys module documentation?

[skirpichev]

Add a global note to the sys module documentation?

Yes, something like

diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index dd6293c722..1aedb277ef 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -8,7 +8,9 @@
 
 This module provides access to some variables used or maintained by the
 interpreter and to functions that interact strongly with the interpreter. It is
-always available.
+always available.  Except when explicitly noted otherwise, all variables are
+only for read access.
+
 
 
 .. data:: abiflags