how to refer to a parent method in python?

Question

Suppose I have two classes (one a parent and one a subclass). How do I refer to a method in the parent class if the method is also defined in the subclass different?

Here is the code:

class A:
    def __init__(self, num):
        self.value=num
    def f(self, num):
        return self.value+2
class B(A):
    def f(self, num):
        return 7*self.f(num)

In the very last line, I want to refer to the parent class A with the "self.f(num)" command, not the method itself in B which would create an infinite recursion. Thank you in advance.

score 42 · Accepted Answer · answered Feb 19 '12 at 07:24

42

If you know you want to use A you can also explicitly refer to A in this way:

class B(A):
    def f(self,num): 
        return 7 * A.f(self,num)

remember you have to explicitly give the self argument to the member function A.f()

answered Feb 19 '12 at 07:24

jimifiki

5,377
2
34
60

I had an error with doing that, when I tried to call a method, which was implemented in C. It expected an instance of A as first parameter and didn't recognize B as a child class. I suppose that using super() might get around that, but I'm not sure yet. – erikbstack Jun 25 '13 at 09:16

score 41 · Answer 2 · answered Feb 19 '12 at 07:07

41

Use super:

return 7 * super(B, self).f(num)

Or in python 3, it's just:

return 7 * super().f(num)

answered Feb 19 '12 at 07:07

tzaman

46,925
11
90
115

this is theoretically right, but for some reason, I get: TypeError: super() argument 1 must be type, not classobj – user1111042 Feb 19 '12 at 07:14
6

@user1111042: Let me guess - you are using Python 2.x? Then your _class A_ should inherit from _object_, only then you get a new style class. New style classes are instances of class _type_, whereas old style classes are just instances of _classobj_. _super()_ only works on new style classes. – pillmuncher Feb 19 '12 at 07:23
Yes, I am using Python 2.x and super as you said does not work in 2.x – user1111042 Feb 19 '12 at 07:29
3

Oh, right. Old-style classes, I forgot. You should really be defining A as `class A(object)`, after which this will work. – tzaman Feb 19 '12 at 07:36

score 6 · Answer 3 · answered Nov 12 '15 at 00:50

In line with the other answers, there are multiple ways to call super class methods (including the constructor), however in Python-3.x the process has been simplified:

Python-2.x

class A(object):
 def __init__(self):
   print "world"

class B(A):
 def __init__(self):
   print "hello"
   super(B, self).__init__()

Python-3.x

class A(object):
 def __init__(self):
   print "world"

class B(A):
 def __init__(self):
   print "hello"
   super().__init__()

super() is now equivalent to super(<containing classname>, self) as per the docs.

score 2 · Answer 4 · answered Feb 19 '12 at 07:25

2

Why not keep it simple?

class B(A):
    def f(self, num):
        return 7 * A.f(self, num)

answered Feb 19 '12 at 07:25

pillmuncher

10,094
2
35
33

score 1 · Answer 5 · answered Feb 19 '12 at 07:08

1

class B(A):
    def f(self, num):
        return 7 * super(B, self).f(num)

answered Feb 19 '12 at 07:08

wong2

34,358
48
134
179

score 1 · Answer 6 · answered Feb 19 '12 at 07:29

1

you can use super or if you can be more explicit and do something like this.

class B(A):
  def f(self, num):
    return 7 * A.f(self, num)

answered Feb 19 '12 at 07:29

Doboy

10,411
11
40
48

score 0 · Answer 7 · edited May 23 '17 at 12:10

0

Check out my answer at Call a parent class's method from child class in Python?.

It's a slight twist on some others here (that don't use super).

edited May 23 '17 at 12:10

Community

1
1

answered Jan 21 '17 at 19:17

BuvinJ

10,221
5
83
96

how to refer to a parent method in python?

7 Answers7

Linked