Merge implicitly concatenated string literals that fit on one line

[max-sixty]

Black could make single-line strings over multiple lines (i.e. a number of single quotes strings on multiple lines surrounded by parentheses) more efficient, by resizing them to the full length of the line.

Even if that was overreach, there's a peculiar situation where you end up with multiple strings on the same line, like below:

- warnings.warn('Dataset.sel_points is deprecated: use Dataset.sel()'
- 'instead.', DeprecationWarning, stacklevel=2)
-
+ warnings.warn(
+ 'Dataset.sel_points is deprecated: use Dataset.sel()' 'instead.',
+ DeprecationWarning,
+ stacklevel=2,
+ )

[ambv]

In this case Black is suggesting that you should merge the two strings into one and the result is more readable that way.

I don't do this automatically (yet?) because it gets complicated if the two strings don't share the same prefix (for example r'something' f'another thing'). This is where user action after the formatting is probably best.

[max-sixty]

Right, that makes sense. I think whether the strings on the same line are merge-able is clear (i.e. do they have the same prefix), but yes it's a rare case; feel free to close

And changing beyond that may require discretion (e.g. turning 6 lines of 2/3-long lines into 4 full length lines)

[ambv]

There's another related problem: if I merged string literals, I am now making semantic changes to the AST. I'm not opposed to those but this will make safety checks after reformatting trickier.

Let's leave this open for the time being, it's an interesting problem.

[zsol]

I'm not even sure what I would expect black to do with code that implicit-concatenates two differently prefixed strings to be honest. I think the path of least surprise is just leaving them alone.

[ambv]

Yeah, it they are different prefixes, leave them alone. If they share a prefix and they end up on the same line, they should be merged.

If you really want to be correct here the implementation is going to be hard in the following edge case:

• two strings like "STR1" "STR2" don't fit on one line because the closing quote of STR1, the space, and the opening quote of STR2 are the 3 characters that cause the entire thing to not fit in a single line. So you will keep them on two lines.
• but if you knew that it's safe to concatenate them, it would fit in a single line (without those 3 extra characters).

I'm inclined not to touch this edge case since that makes it tricky where to perform the merge.

Another small edge case which I'm inclined to avoid is this:

a = (
 "a"
 "bb"
 "ccc"
 "dddd"
 "eeeee"
 "ffffff"
 "ggggggg"
 "hhhhhhhh"
 "iiiiiiiii"
 "jjjjjjjjjj"
 "kkkkkkkkkkk"
 "llllllllllll"
 "mmmmmmmmmmmmmmmm"
)

Technically Black could implement the "fill" algorithm for this case that Prettier also has for JSX. But I think what it currently does is fine for simplicity and obvious for users to recognize.