cannot init an instance of a subclassed namedtuple

namedtuple, python

Solution

Both the instance constructor (`__new__`) and your instance initializer (`__init__`) need to accept the same number of arguments.

Your `__new__` needs 4 arguments, but your `__init__` method only accepts 2. Adjust one or the other to accept the same number, or use a `*args` catch-all argument in your `__init__` method.

For example, using the following `__new__` method would make things work:

def __new__(cls, a):
    self = super(PDsub, cls).__new__(cls, a, a, a)
    return self

in which case you no longer need your `__init__` initializer at all.

Demo:

>>> from collections import namedtuple
>>> PD = namedtuple('PD', 'x y z')
>>> class PDsub(PD):
...     __slots__ = ()
...     def __new__(cls, a):
...         self = super(PDsub, cls).__new__(cls, a, a, a)
...         return self
...     def __str__(self):
...         return 'Foo'
... 
>>> p2 = PDsub(5)
>>> p2.x, p2.y, p2.z
(5, 5, 5)
>>> str(p2)
'Foo'

An immutable type like a tuple often uses a `__new__` constructor instead of a `__init__` initializer; all the built-in immutables (`frozenset`, `str`, `tuple`) do so.

Problem

I think I have not undertood how to define a class subclassed from a namedtuple : ``` from collections import namedtuple PD = namedtuple('PD', 'x y z') p1 = PD(0, 'u', 1) print p1.x #<== this works class PDsub(PD): __slots__ = () def __new__(cls, x, y, z): self = super(PDsub, cls).__new__(cls, x, y, z) return self def __init__(self, a): self.x, self.y, self.z = a, a, a def __str__(self): return 'Foo' p2 = PDsub(5) #<== this does not work ``` This code raises `TypeError : __new__() takes exactly 4 arguments (2 given)`. Any ideas why?

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