Differences creating tuples and collections.namedtuples

John Reid j.reid at mail.cryst.bbk.ac.uk
Mon Feb 18 16:07:00 CET 2013


On 18/02/13 14:53, Oscar Benjamin wrote:
> On 18 February 2013 14:23, John Reid <j.reid at mail.cryst.bbk.ac.uk> wrote:
> [snip]
>> That said it would be nice to know the rationale for
>> namedtuple.__new__ to have a different signature to tuple.__new__. I'm
>> guessing namedtuple._make has a similar interface to tuple.__new__. Does
>> anyone know what the rationale was for this design?
> 
> Because namedtuples use names for the arguments in the constructor:
> 
>>>> from collections import namedtuple
>>>> Point = namedtuple('Point', 'x y')
>>>> p1 = Point(x=2, y=3)
>>>> p1
> Point(x=2, y=3)
>>>> p1.x
> 2
> 
That's a good point. I haven't used __new__ much before but wouldn't
something like this __new__() cater for both uses? (Example taken from
namedtuple docs
http://docs.python.org/2/library/collections.html#collections.namedtuple).

>>> Point = namedtuple('Point', ['x', 'y'], verbose=True)
class Point(tuple):
    'Point(x, y)'

    __slots__ = ()

    _fields = ('x', 'y')

    def __new__(_cls, *args, **kwds):
        'Create a new instance of Point(x, y)'
        if args:
            return _tuple.__new__(_cls, args)
        else:
            return _tuple.__new__(_cls, (kwds[f] for f in _fields))

    ...


Perhaps I could subclass namedtuple so that my namedtuples have the
correct signature for __new__.




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