class A:
def __init__(self):
print("world")
class B(A):
def __init__(self):
print("hello")
B() # output: hello
In all other languages I've worked with the super constructor is invoked implicitly. How does one invoke it in Python? I would expect super(self)
but this doesn't work.
super().__init__(args...)
super()
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-3.x
class A(object):
def __init__(self):
print("world")
class B(A):
def __init__(self):
print("hello")
super().__init__()
Python-2.x
In python 2.x, you have to call the slightly more verbose version super(<containing classname>, self)
, which is equivalent to super()
as per the docs.
class A(object):
def __init__(self):
print "world"
class B(A):
def __init__(self):
print "hello"
super(B, self).__init__()
super()
returns a parent-like object in new-style classes:
class A(object):
def __init__(self):
print("world")
class B(A):
def __init__(self):
print("hello")
super(B, self).__init__()
B()
With Python 2.x old-style classes it would be this:
class A:
def __init__(self):
print "world"
class B(A):
def __init__(self):
print "hello"
A.__init__(self)
super
and not have to directly name the class you're inheriting from.
super
.
One way is to call A's constructor and pass self
as an argument, like so:
class B(A):
def __init__(self):
A.__init__(self)
print "hello"
The advantage of this style is that it's very clear. It call A's initialiser. The downside is that it doesn't handle diamond-shaped inheritance very well, since you may end up calling the shared base class's initialiser twice.
Another way is to use super(), as others have shown. For single-inheritance, it does basically the same thing as letting you call the parent's initialiser.
However, super() is quite a bit more complicated under-the-hood and can sometimes be counter-intuitive in multiple inheritance situations. On the plus side, super() can be used to handle diamond-shaped inheritance. If you want to know the nitty-gritty of what super() does, the best explanation I've found for how super() works is here (though I'm not necessarily endorsing that article's opinions).
A.__init__(self)
line.
A.__init__
actually does. It may initialize various internal attributes in a constant fashion (self.stuff = []
, e.g.), which may cause your instance of B
to not behave correctly if you don't call A.__init__
.
Short Answer
super(DerivedClass, self).__init__()
Long Answer
What does super()
do?
It takes specified class name, finds its base classes (Python allows multiple inheritance) and looks for the method (__init__
in this case) in each of them from left to right. As soon as it finds method available, it will call it and end the search.
How do I call init of all base classes?
Above works if you have only one base class. But Python does allow multiple inheritance and you might want to make sure all base classes are initialized properly. To do that, you should have each base class call init:
class Base1:
def __init__():
super(Base1, self).__init__()
class Base2:
def __init__():
super(Base2, self).__init__()
class Derived(Base1, Base2):
def __init__():
super(Derived, self).__init__()
What if I forget to call init for super?
The constructor (__new__
) gets invoked in a chain (like in C++ and Java). Once the instance is created, only that instance's initialiser (__init__
) is called, without any implicit chain to its superclass.
__new__
isn't chained, either. If all three classes in your example defined __new__
, but didn't use super().__new__()
, then Derived()
would call Derived.__new__
but not Base1.__new__
or Base2.__new__
. A __new__
that doesn't call its parent's __new__
is almost useless, though, because ultimately object.__new__
is the only thing that truly creates a new instance of anything.
Just to add an example with parameters:
class B(A):
def __init__(self, x, y, z):
A.__init__(self, x, y)
Given a derived class B that requires the variables x, y, z to be defined, and a superclass A that requires x, y to be defined, you can call the static method init of the superclass A with a reference to the current subclass instance (self) and then the list of expected arguments.
I use the following formula that extends previous answers:
class A(object):
def __init__(self):
print "world"
class B(A):
def __init__(self):
print "hello"
super(self.__class__, self).__init__()
B()
This way you don't have to repeat the name of the class in the call to super. It can come handy if you are coding a large number of classes, and want to make your code in the initialiser methods independent of the class name.
B
with C
, and self.__class__
points to C(B)
instead of B(A)
, so super(self.__class__, self)
points back to B
instead of A
.
B
. Do not use self.__class__
or type(self)
, either pass in an explicit class (B
here) or, in Python 3, use super()
without arguments.
Success story sharing
super(B,self)
require both B and self to be mentioned? isn't this redundant? shouldn't self contain a reference to B already?self
might actually be an instance ofC
, a child ofB
.super()
, you should be able to writesuper().__init__()
wothout arguments.