Can't set attribute for subclasses of namedtuple

Named tuples are immutable, so you cannot manipulate them in the __init__ initializer. Your only option is to override the __new__ method:

class C(namedtuple('C', 'x, y')):    __slots__ = ()    def __new__(cls, obj):        return super(C, cls).__new__(cls, obj.x, obj.y)

Note that because __new__ is a factory method for new instances, you do need to return the newly created instance. If you do not use return in the __new__ method, the default return value is None, which gives you your error.

Demo with an object with x and y attributes:

>>> class C(namedtuple('C', 'x, y')):...     __slots__ = ()...     def __new__(cls, obj):...         return super(C, cls).__new__(cls, obj.x, obj.y)... >>> O.x, O.y(10, 20)>>> C(O)C(x=10, y=20)

Python does not support method overloading; generally you either use optional keyword arguments or extra class methods as factory methods.

The datetime module, for example, has several such factory methods to let you create objects that do not fit the standard constructor. datetime.datetime.fromtimestamp() creates a datetime.datetime instance from a single numeric value, and so does datetime.datetime.fromordinal(); except that they interpret the number in different ways.

If you wanted to support variable arguments, do:

class C(namedtuple('C', 'x, y')):    __slots__ = ()    def __new__(cls, x, y=None):        if y is None:            # assume attributes            x, y = x.x, x.y        return super(C, cls).__new__(cls, x, y)

Here, y is an optional argument, defaulting to None if not supplied by the caller:

>>> C(3, 5):C(x=3, y=5)>>> C(O)C(x=10, y=20)

The alternative, using a class method, would be:

class C(namedtuple('C', 'x, y')):    @classmethod    def from_attributes(cls, obj):        return cls(obj.x, obj.y)

Now there are two factory methods; one default and one named:

>>> C(3, 5):C(x=3, y=5)>>> C.from_attributes(O)C(x=10, y=20)

I suggest you use the the _replace method

from collections import namedtupleC = namedtuple('C', 'x, y')c = C(x=10, y=20)# c.x = 30 won't workc = c._replace(x=30)

Two things: one, you're not really getting much out of namedtuple here, as far as i can tell. So maybe you should just switch to a normal class. Also, you can't overload the

Second, other possibilities which might help with your problem:

Factory design pattern - instead of putting the different parameters in the constructor, have a class that takes different kinds of parameters and calls the constructor with appropriate arguments, outside the object.recordtype - a mutable namedtuple, that allows defaults but would also let you write your subclass the way you originally wanted.bunch - not exactly a named tuple, but lets you create somewhat arbitrary objects.