Black's treatment of trailing commas depends on previous statements.

[JohnADawson]

Describe the bug

Black's treatment of trailing commas depends on previous statements.

To Reproduce

If file0.py is

print(
    *[],
    "Once",
    "there",
    "were",
    "four",
    "children",
    "whose",
    "names",
    "were",
    "Peter,",
    "Susan,",
    "Edmund",
    "and",
    "Lucy."
)

running

black --diff file0.py

produces

All done! ✨ 🍰 ✨
1 file would be left unchanged.

If file1.py is

f""
print(
    *[],
    "Once",
    "there",
    "were",
    "four",
    "children",
    "whose",
    "names",
    "were",
    "Peter,",
    "Susan,",
    "Edmund",
    "and",
    "Lucy."
)

running

black --diff file1.py

produces

--- file1.py	2024-07-13 22:34:48.876534+00:00
+++ file1.py	2024-07-13 22:35:00.795624+00:00
@@ -11,7 +11,7 @@
     "were",
     "Peter,",
     "Susan,",
     "Edmund",
     "and",
-    "Lucy."
+    "Lucy.",
 )
would reformat file1.py

All done! ✨ 🍰 ✨
1 file would be reformatted.

Expected behavior

The previous statement f"" does not affect the trailing comma after "Lucy.".

Environment

• Black's version: 24.4.2
• OS and Python version: Ubuntu 24.04, Python 3.12.3

The same behaviour is exhibited by https://black.vercel.app/?version=main.

[JelleZijlstra]

I believe this is because a trailing comma in this context is only legal in 3.6+. If you use an f-string, we know the file is 3.6+, so we can use other 3.6+ syntax.

However, Python 3.5 is long dead. We can probably get rid of this behavior now.

[JohnADawson]

@JelleZijlstra, as your explanation predicts, running

black --diff --target-version=py312 file0.py 

produces

--- file0.py	2024-07-13 21:54:24.113436+00:00
+++ file0.py	2024-07-13 22:52:31.502265+00:00
@@ -10,7 +10,7 @@
     "were",
     "Peter,",
     "Susan,",
     "Edmund",
     "and",
-    "Lucy."
+    "Lucy.",
 )
would reformat file0.py

All done! ✨ 🍰 ✨
1 file would be reformatted.