Format catenated unified diffs

[make-github-pseudonymous-again]

What is the proper way to catenate unified diffs? When I use the following python loop

for path in paths:
	file_a = os.path.join(dir_a, path)
	with open(file_a) as fd:
		sa = fd.read().splitlines(keepends=True)

	file_b = os.path.join(dir_b, path)
	with open(file_b) as fd:
		sb = fd.read().splitlines(keepends=True)

	lines = unified_diff(
		sa,
		sb,
		fromfile=file_a,
		tofile=file_b
	)

	sys.stdout.writelines(lines)

then pipe it to diff-so-fancy, the first --- file_a +++ file_b header is correctly formatted, but the others are formatted as diff lines -- file_a ++ file_b (removing the first -/+ symbol) and the line annotations always refer to the first file.

[OJFord]

You're missing the header lines like:

diff --git a/foo b/bar
[optional line e.g. 'deleted file mode ..']
index 0f24aeb..fe5efa 100644

or similar.

[scottchiefbaker]

Any updates on this? Can we close this issue?

[make-github-pseudonymous-again]

You're missing the header lines like:

diff --git a/foo b/bar
[optional line e.g. 'deleted file mode ..']
index 0f24aeb..fe5efa 100644

or similar.

I sort of understand how to construct the first line, but what about the other two lines? Is there some utility function that can produce that header?

[OJFord]

Well the middle one is optional; the third is just the git SHAs that the diff is between, and the file mode, which d-s-f will strip (and not meaningfully use) so you could put whatever, probably just best to keep it real-ish so you don't rely too much on exactly how the parsing happens to work today (i.e. this is not minimal, the content of the third line doesn't seem to matter today):

$ echo -e 'diff --git a/foo b/foo\nindex dead..beef 100644\n--- a/foo\n+++b/foo' | diff-so-fancy
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
modified: foo
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
$

[make-github-pseudonymous-again]

Thanks for the help! Here is how I fixed my script following your advice:

for path in paths:
    file_a = os.path.join(dir_a, path)
    with open(file_a) as fd:
        sa = fd.read().splitlines(keepends=True)

    file_b = os.path.join(dir_b, path)
    with open(file_b) as fd:
        sb = fd.read().splitlines(keepends=True)

    lines = unified_diff(
        sa,
        sb,
        fromfile=file_a,
        tofile=file_b
    )

    try:
        first_line = next(lines)
        sys.stdout.writelines([
            'diff --git a/{} b/{}\n'.format(file_a, file_b),
            'index dead..beef 100644\n',
            first_line
        ])
        sys.stdout.writelines(lines)
    except StopIteration:
        pass

@scottchiefbaker Feel free to close this issue.