This question is asking the opposite of Inherit namedtuple from a base class in python , where the aim is to inherit a subclass from a namedtuple and not vice versa.
In normal inheritance, this works:
class Y(object):
def __init__(self, a, b, c):
self.a = a
self.b = b
self.c = c
class Z(Y):
def __init__(self, a, b, c, d):
super(Z, self).__init__(a, b, c)
self.d = d
[out]:
>>> Z(1,2,3,4)
<__main__.Z object at 0x10fcad950>
But if the baseclass is a namedtuple:
from collections import namedtuple
X = namedtuple('X', 'a b c')
class Z(X):
def __init__(self, a, b, c, d):
super(Z, self).__init__(a, b, c)
self.d = d
[out]:
>>> Z(1,2,3,4)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __new__() takes exactly 4 arguments (5 given)
The question, is it possible to inherit namedtuples as a base class in Python? Is so, how?
You can, but you have to override __new__ which is called implicitly before __init__:
class Z(X):
def __new__(cls, a, b, c, d):
self = super(Z, cls).__new__(cls, a, b, c)
self.d = d
return self
>>> z = Z(1, 2, 3, 4)
>>> z
Z(a=1, b=2, c=3)
>>> z.d
4
But d will be just an independent attribute!
>>> list(z)
[1, 2, 3]
I think you can achieve what you want by including all of the fields
in the original named tuple, then adjusting the number of arguments
using __new__ as schwobaseggl suggests above. For instance, to address
max's case, where some of the input values are to be calculated
rather than supplied directly, the following works:
from collections import namedtuple
class A(namedtuple('A', 'a b c computed_value')):
def __new__(cls, a, b, c):
computed_value = (a + b + c)
return super(A, cls).__new__(cls, a, b, c, computed_value)
>>> A(1,2,3)
A(a=1, b=2, c=3, computed_value=6)
I came here with the exact same problem, just two years later.
I personally thought that the @property decorator would fit in here better:
from collections import namedtuple
class Base:
@property
def computed_value(self):
return self.a + self.b + self.c
# inherits from Base
class A(Base, namedtuple('A', 'a b c')):
pass
cls = A(1, 2, 3)
print(cls.computed_value)
# 6