Python 3 named parameters

named.md

Let's say you want to write a comparator function that takes two objects.

def is_newer(older, newer):
    return older.timestamp < newer.timestamp

def keep_newer(file1, file2):
    if is_newer(file1, file2):
        os.unlink(file1)
    else:
        os.unlink(file2)

The function name is_newer() is ambiguous whether the first or second argument is considered the "newer" one. Reviewers of the function keep_newer() cannot trivially verify that the logic is correct without looking up the internals of is_newer().

In Python 3, you can use keyword-only parameters to force callers to name required parameters.

def is_newer(*, older, newer):
    # Everything after * must be a keyword parameter.
    return older.timestamp < newer.timestamp

def keep_newer(file1, file2):
    if is_newer(older=file1, newer=file2):
        os.unlink(file1)
    else:
        os.unlink(file2)

Now, someone who reads keep_newer() has a pretty good idea of what is going on at the point what is now a self-documenting function call. It's overkill to use this everywhere, but can be useful when extra clarity is needed.

In Python 2, you could get close with keyword arguments:

def is_newer(older=None, newer=None):
    assert older is not None
    assert newer is not None
    return older.timestamp < newer.timestamp

However, it's not quite as clean, and there might not be illegal default values that make sense in every case.

• Reference: Python 3 Glossary, search for "keyword-only"
• Also a StackOverflow answer on this topic.