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gh-142108: Document Decimal formatting behavior differences by Yashp002 · Pull Request #142117 · python/cpython · GitHub

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gh-142108: Document Decimal formatting behavior differences - #142117

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gh-142108: Document Decimal formatting behavior differences#142117
Yashp002 wants to merge 6 commits into
python:mainfrom
Yashp002:formatdoc

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Yashp002 commented Nov 30, 2025 •
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This PR clarifies how Decimal behaves differently when formatted using __format__ (f-strings, format(), str.format()) versus % formatting (old-style string formatting).

Changes:

  • Added a note in the Format Specification Mini-Language section explaining:
    • For __format__: Decimal uses maximum precision only when precision is omitted in f/e/g format types; explicit precision is respected
    • For % formatting: Decimal values are converted to float first, resulting in potential precision loss

Documentation preview: (will be auto-generated)

Fixes #142108

📚 Documentation preview 📚: https://cpython-previews--142117.org.readthedocs.build/

bedevere-app Bot added awaiting review docs Documentation in the Doc dir skip news labels Nov 30, 2025
github-project-automation Bot moved this to Todo in Docs PRs Nov 30, 2025
bedevere-app Bot mentioned this pull request Nov 30, 2025
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StanFromIreland requested changes Nov 30, 2025
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

You have multiple different changes mixed together, please split it out.

skirpichev requested changes Nov 30, 2025
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I'll mark this a draft. Sorry, but current approach seems unacceptable for me.

PS: Please also don't alter commit history in any way (force-pushes, etc).

Comment thread Doc/c-api/descriptor.rst
.. c:function:: PyObject* PyDescr_NewMember(PyTypeObject *type, struct PyMemberDef *meth)


.. c:var:: PyTypeObject PyMemberDescr_Type

skirpichev Nov 30, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Changes in Doc/c-api/ are not related to this pr, please revert.

Comment thread Doc/library/string.rst
Comment on lines +637 to +638
* When using ``__format__`` (f-strings, ``format()``, ``str.format()``),
Decimal formats with maximum precision only when precision is **omitted**

skirpichev Nov 30, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This is already documented, please revert. Also, the whole section of documentation is about string formatting, it's pointless to say that you are using __format__().

Comment thread Doc/library/string.rst
Comment on lines +642 to +643
* When using ``%`` formatting (old-style string formatting with the ``%`` operator),
Decimal values are first converted to :class:`float`, which may result in

skirpichev Nov 30, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality
  1. This note not belongs to this section, it's about printf-like formatting.
  2. In the issue thread it was explained, that there is nothing special about Decimal's here. If we have to document something, it should be about behavior of printf-like formatting for non-builtin types.

skirpichev marked this pull request as draft November 30, 2025 21:16

Yashp002 commented Dec 1, 2025

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Apologies for the confusion—I had an out-of-sync branch that mixed commits from multiple PRs.

After reviewing the feedback, I realize:

  • The note placement was incorrect (this section is about printf-like formatting)
  • This behavior may already be documented elsewhere

Closing this PR. I'll discuss on the issue thread whether any documentation changes are still needed before opening a fresh PR.

Thanks for the review.

Yashp002 closed this Dec 1, 2025
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skirpichev skirpichev requested changes
StanFromIreland StanFromIreland requested changes
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Improve printf-style formatting docs for non-built-in types

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