[Python-ideas] namedtuple fields with default values

[Barry Warsaw]

On Jul 16, 2015, at 01:03 PM, Russell Kaplan wrote:

>I'm using a namedtuple to keep track of several fields, only some of which
>ever need to be specified during instantiation. However, there is no
>Pythonic way to create a namedtuple with fields that have default values.

I don't know about "Pythonic" but there's a not too horrible way to do it:

_Record = namedtuple('Record', 'url destination checksum')('', '', '')

def Record(url, destination, checksum=''):
    return _Record._replace(
        url=url, destination=destination, checksum=checksum)

Now you only need to provide 'url', and 'destination' when you create a
Record.  Okay, sure _Record is the actual namedtuple, but I usually don't
care.

>Having to assign to Foo.__new__.__defaults__ is a bit ugly. I think it
>would be easier and more readable to support syntax like:
>>>> Foo = namedtuple('Foo', ['optional_bar=None', 'optional_baz=None'])

That would mean you couldn't ever have an actual parameter called
'optional_bar'.

>This suggestion is fully backwards compatible and allows for cleaner
>definitions of nametuples with default-value fields. Thanks for considering.

Not that I think anything really needs to be done, but a few other approaches
could be:

* Allow for arbitrary keyword arguments at the end of the signature to define
  default values.

  Record = namedtuple('Record', 'url destination checksum', checksum='')

* Extend the semantics of field_names to allow for a dictionary of attributes
  mapping to their default values, though you'd need a marker to be able to
  specify a required field:

  Record = namedtuple('Record', dict(url=Required, destination=Required,
                                     checksum=''))

I suppose I'd prefer the former, although that might cut off the ability to
add other controlling arguments to the namedtuple() API.

Cheers,
-Barry
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