Some of the converted strings into fstrings are no longer processed by babel

I’m using babel via a custom function _(string,language) that outputs the translated version.

When running pyupgrade over the source files:

@@ -174,7 +173,7 @@ def irccommands(msgdetail):
                         )
                     else:
                         text = _(
-                            "Error opping users from chat %s: %s" % (chat_id, result),
+                            f"Error opping users from chat {chat_id}: {result}",
                             language=mylang,
                         )
 

It causes babel to segfault as per https://github.com/python-babel/babel/issues/715

I would expect pyupgrade not to touch the code in a way that makes it later fail.

[asottile]

you’re misusing translation – those strings aren’t being translated. if anything babel should produce an error for accidentally using string formatting inside of a translation function

babel will scan those (incorrectly) as "Error opping users from chat %s: %s" – then at runtime it will try and translate "Error opping users from chat whatever1: whatever2" which will not match any of your translated IDs so you will always end up with english text. (we had a linter when I was at yelp which prevented this sort of translation misuse – but I believe it was proprietary / lost to closed source)

[hugovk]

Sounds like Babel doesn’t support f-strings, which is a Babel bug, not pyupgrade.

But can you use the pyupgrade --keep-percent-format option?