Black causes W1404 Implicit string concatenation found in assignment (implicit-str-concat)

[vertti]

Describe the bug

When Black decides a multiline string can be put to single line, it leaves the concatenation in place rather than merging them to a single string. This is considered a bug pattern by Pylint and does seem to be a bug rather than a feature from Black.

To Reproduce Steps to reproduce the behavior:

Having code something like this:

foo = (
  "not really super long line "
  "but enough that I split it"
)

will, when line length allows, be pulled by Black to:

foo = "not really super long line " "but enough that I split it"

rather than a single string.

Expected behavior

In the above situation I would have expected the result to be:

foo = "not really super long line but enough that I split it"

instead of single-line implicit concatenation that is by default flagged by Pylint (W1404)

**Environment **

• Version: master
• OS and Python version: Ubuntu 20.10, Python 3.8.5

Does this bug also happen on master?

Yes

[JelleZijlstra]

I believe the --experimental-string-processing flag fixes this (we'll turn this on by default once it's stable enough).

[vertti]

Excellent. Thanks!

[jontwo]

We're still seeing this in v23.3.0. The --experimental-string-processing should have been removed by now, yes?

[JelleZijlstra]

It is not yet enabled by default; it's still part of --preview. Hopefully it will be the default in Black 24.

[tylerlaprade]

I have an f-string that looks like f"blah blah blah ... blah blah blah {my_variable}". This is getting auto-fixed when using --preview to

"blah blah blah ... blah blah blah"
f" {my_variable}"

which reports the violations reportImplicitStringConcatenation in Pyright and ISC002 in Ruff/Flake8.

[ichard26]

That's intentional. If you like this changed, please upvote/comment here: #2553.