[Python-ideas] namedtuple fields with default values

Eric V. Smith eric at trueblade.com
Fri Jul 17 16:59:05 CEST 2015

Hash: SHA1

On 07/17/2015 10:44 AM, Barry Warsaw wrote:
> 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.

I've implemented default parameters to namedtuples (and namedlists a
mutable version) in https://pypi.python.org/pypi/namedlist

The syntax is not great, but none of the options are awesome.

>>> Point = namedlist.namedtuple('Point', [('x', 0), ('y', 100)])
>>> p = Point() assert p.x == 0 assert p.y == 100

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