Break long with statements using backslashes if parentheses aren't usable

[tdegeus]

Describe the style change

Consider this example

        with g5.ExtendableList(output, "r", np.uint64) as dset_r,\
             g5.ExtendableList(output, "t", np.float64) as dset_t,\
             g5.ExtendableList(output, "dx", np.float64) as dset_dx,\
             g5.ExtendableList(output, "ds", np.int64) as dset_ds:

Examples in the current Black style

        with g5.ExtendableList(output, "r", np.uint64) as dset_r, g5.ExtendableList(
            output, "t", np.float64
        ) as dset_t, g5.ExtendableList(output, "dx", np.float64) as dset_dx, g5.ExtendableList(
            output, "ds", np.int64
        ) as dset_ds:

Desired style

For example:

        with g5.ExtendableList(output, "r", np.uint64) as dset_r,\
             g5.ExtendableList(output, "t", np.float64) as dset_t,\
             g5.ExtendableList(output, "dx", np.float64) as dset_dx,\
             g5.ExtendableList(output, "ds", np.int64) as dset_ds:

Additional context

Using with is crucial as some flushing is needed at the end.

[ichard26]

This is documented in the documentation as something we want to do. No one has picked this up though AFAIK (as far as I know).

[JelleZijlstra]

There is a PR for this, #3489.

[tdegeus]

Amazing !!

[ichard26]

While #3489 did improve the story for when the target version is 3.9 or higher, Black still poorly formats long with statements when parentheses are not an option. We would like to implement the style documented here (which happens to be the "desired style" in this issue too) but it's unlikely any one on the core team will pick this up anytime soon. External contributions would be appreciated!

[JelleZijlstra]

Given that the backslash syntax is only necessary in Python 3.8, which will go EOL in a few months, I don't think I'd accept a PR adding support for it at this point. We should consistently use the nicer 3.9+ syntax with parentheses where possible, and leave the current style alone otherwise.