Behavior on all-whitespace files differs from Black

[akaihola]

As discovered in #164, Darker strips all-whitespace Python files which have been modified. This is different from Black which leaves them untouched. See also this comment to psf/black#779.

We could replace (in darker.black_diff.run_black()):

    return TextDocument.from_str(
        format_str(contents_for_black, mode=mode),
        encoding=src_contents.encoding,
        override_newline=src_contents.newline,
    )

with

    try:
        reformatted = format_file_contents(contents_for_black, fast=True, mode=mode)
    except NothingChanged:
        reformatted = contents_for_black
    return TextDocument.from_str(
        reformatted,
        encoding=src_contents.encoding,
        override_newline=src_contents.newline,
    )

to match black's behavior on all-whitespace files. The other option is to handle all-whitespace files in darker just like black does:

    contents_for_black = src_contents.string_with_newline("\n")
    if not contents_for_black.strip():
        return src_contents
    return TextDocument.from_str(
        format_str(contents_for_black, mode=mode),
        encoding=src_contents.encoding,
        override_newline=src_contents.newline,
    )

Which approach to choose should depend on Black's possible choice of a public Python API (psf/black#779).

[akaihola]

Actually, Black developers seem to agree in psf/black#2382 that the behavior is currently wrong, and Black should format all-whitespace files as just a single newline.

So instead of matching Black's current behavior, we could just fix Darker to truncate all-whitespace files to a single newline instead of a zero-length file.

[akaihola]

I've submitted pull request psf/black#2484 to Black. It fixes Black so it always reformats zero-length and all-whitespace files into one empty line.