def diff(a: str, b: str, a_name: str, b_name: str) -> str:
    """Return a unified diff string between strings `a` and `b`."""
    import difflib

    a_lines = _splitlines_no_ff(a)
    b_lines = _splitlines_no_ff(b)
    diff_lines = []
    for line in difflib.unified_diff(
        a_lines, b_lines, fromfile=a_name, tofile=b_name, n=5
    ):
        # Work around https://bugs.python.org/issue2142
        # See:
        # https://www.gnu.org/software/diffutils/manual/html_node/Incomplete-Lines.html
        if line[-1] == "\n":
            diff_lines.append(line)
        else:
            diff_lines.append(line + "\n")
            diff_lines.append("\\ No newline at end of file\n")
    return "".join(diff_lines)
Return a unified diff string between strings `a` and `b`.
src/black/output.py