[llvm] [llvm][Docs] Show how to format Python files changed in a commit (PR #66227)

[David Spickett via llvm-commits]

https://github.com/DavidSpickett created https://github.com/llvm/llvm-project/pull/66227:

black will always work on whole files and darker seems to be the only way to get it to work on chunks of files as you would with clang-format and https://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting.

This adds a one liner for the common use case. In theory you can not list files and use `.` as the path instead but it seems to find all python files, then see if they have changed. Instead of starting with the change list. So it takes ages on llvm-project/.

>From afce6a8ec1cf867b4068f0a813c84b8b3b22bfb7 Mon Sep 17 00:00:00 2001
From: David Spickett <david.spickett at linaro.org>
Date: Wed, 13 Sep 2023 17:15:21 +0100
Subject: [PATCH] [llvm][Docs] Show how to format Python files changed in a
 commit

black will always work on whole files and darker seems to be the only
way to get it to work on chunks of files as you would with
clang-format and https://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting.

This adds a one liner for the common use case. In theory you can not list
files and use `.` as the path instead but it seems to find all python files, then
see if they have changed. Instead of starting with the change list. So
it takes ages on llvm-project/.
---
 llvm/docs/CodingStandards.rst | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/llvm/docs/CodingStandards.rst b/llvm/docs/CodingStandards.rst
index 9d87142b0984fda..8662908ec003fcd 100644
--- a/llvm/docs/CodingStandards.rst
+++ b/llvm/docs/CodingStandards.rst
@@ -120,6 +120,14 @@ For consistency and to limit churn, code should be automatically formatted with
 rules based on major version. In order to avoid unnecessary churn in the formatting rules
 we currently use black version 23.x in LLVM.
 
+To format only the lines changed in a patch you can use
+`darker <https://pypi.org/project/darker/>`_. To format the changes in the most recent
+commit:
+
+::
+
+  darker --revision HEAD~1 $(git diff-tree --no-commit-id --name-only HEAD -r)
+
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