Failure to parse string templates with similar quotes

[andrewvaughan]

Describe the bug

When using a ternary statement within a string template, Black fails to parse the string if the quotes used within the template are the same as the template string.

Edit: Any template string that contains quotes of the same style as the parent template will fail.

To Reproduce

This fails:

cmd.append(f"--{"no-" if not do_continue else ""}continue")

With the following error (per the playground):

Cannot parse: 1:17: cmd.append(f"--{"no-" if not do_continue else ""}continue")

But this will pass:

cmd.append(f"--{'no-' if not do_continue else ''}continue")

EDIT:

These also all fail (but are correctable by switching quote types), showing it has to do with template string parsing, not ternary:

cmd.append(f"--ffmpeg-location {pipes.quote(cfg["ffmpeg"])}")
cmd.append(f"--example {some_dict["key"]}")

These are all valid Python, however.

Expected behavior

Black should enforce quotes consistently, including within template strings (or, at minimum, not fail at parsing and rather throw a formatting error). Per my configurations, this should be the proper format:

cmd.append(f"--{"no-" if not do_continue else ""}continue")

But I am forced to use single-quotes to make Black function.

EDIT:

"Working" examples from addition:

cmd.append(f"--ffmpeg-location {pipes.quote(cfg['ffmpeg'])}")
cmd.append(f"--example {some_dict['key']}")

These should, at minimum, throw a validation error and not fail to parse, if enforcing different quote-styles is the intent (although, I would argue switching quote-styles is detrimental).

Environment

• Black's version: Current Black playground (Stable: 24.1.0 | Main: @ed770b)
• OS and Python version: N/A

Additional context

N/A

[andrewvaughan]

I have also tested that nested template strings work, but only if all nested templates have opposite quote styles:

cmd.append(f"--add-header {pipes.quote(f'User-Agent:{browser['user-agent']}')}")

I was somewhat expecting this to be an "unsolvable" situation, but it did parse correctly when all internal quotes were single-quotes (') if the parent template was double-quotes (").

[JelleZijlstra]

Duplicate of #3746

[andrewvaughan]

Duplicate of #3746

Ah, thanks. Did a cursory search but didn't see it - I'm still getting into the habit of checking pins and not going straight to search.

[JelleZijlstra]

And just to explain the behavior you're seeing: Black's parser currently treats f-strings just like normal strings, treating whatever is inside as just a string literal. Your cmd.append(f"--add-header {pipes.quote(f'User-Agent:{browser['user-agent']}')}") is accepted because Black just ignores whatever happens until it sees the closing double quote.