Jens Jens - 5 months ago 74
Python Question

Can't set attribute for subclasses of namedtuple

It looks like this or this are somewhat related threads, but still haven't figured things out :)

I'm trying to create a subclass of

namedtuple
and provide different initializers so that I can construct objects in different ways. For example:

>>> from collections import namedtuple
>>> class C(namedtuple("C", "x, y")) :
... __slots__ = ()
... def __init__(self, obj) : # Initialize a C instance by copying values from obj
... self.x = obj.a
... self.y = obj.b
... def __init__(self, x, y) : # Initialize a C instance from the parameters
... self.x = x
... self.y = y


However, that doesn't work:

>>> c = C(1, 2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 7, in __init__
AttributeError: can't set attribute


After some poking around (for example, see this thread) I tried to use constructors instead of initializers:

>>> from collections import namedtuple
>>> class C(namedtuple("C", "x, y")) :
... __slots__ = ()
... def __new__(cls, obj) :
... self = super(C, cls).__new__(cls, obj.a, obj.b)
... def __new__(cls, x, y) :
... self = super(C, cls).__new__(cls, x, y)


which seemed to construct an object but then I can't read its attributes:

>>> c = C(1,2)
>>> c.x, c.y
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'x'


Where am I going wrong here? How can I create a subclass with multiple constructors or initializers?

Answer

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)
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