How to use default arguments with namedtuple

I was running into some trouble earlier today trying to figure out how to use default argument with namedtuple.

According to the docs:

Default values can be implemented by using _replace() to customize a prototype instance:

>>> Account = namedtuple('Account', 'owner balance transaction_count')
>>> default_account = Account('<owner name>', 0.0, 0)
>>> johns_account = default_account._replace(owner='John')

This is of course not really what I was looking for.

Searching around some more led to the Python mailing list where a software engineer by the name of Issac Morland suggests exactly what I was looking for.

2. It would be nice to be able to have default values for named tuple fields.
Using Signature it’s easy to do this – I just specify a dictionary of defaults
at named tuple class creation time.

To which Raymond Hettinger (the creator of collections) responds:

This came-up before, but I’ll take another look at it this weekend. If it
significantly complicates the API or impacts performance, then it is a
non-starter; otherwise, I’ll put together an implementation and float it on
ASPN for comments.

And the trail ends. Not sure what the full story is.

In any case, the solution is fairly simple. Just subclass the result of namedtuple and override __new__ as follows:

from collections import namedtuple
class Move(namedtuple('Move', 'piece start to captured promotion')):
    def __new__(cls, piece, start, to, captured=None, promotion=None):
        # add default values
        return super(Move, cls).__new__(cls, piece, start, to, captured, promotion)

As you’ll notice, super is being used a little weird here, with cls being passed to both super and __new__. I’m not quite sure why that’s neccessary, but I can say I’ve tested it and it does work.

EDIT: It appears this question on SO explains the weirdness with using super together with __new__. tldr; __new__ is a static method, not a class method, so it requires an explicit passing of cls.