I'm begginer of python. I can't understand inheritance and __init__()
.
class Num:
def __init__(self,num):
self.n1 = num
class Num2(Num):
def show(self):
print self.n1
mynumber = Num2(8)
mynumber.show()
RESULT: 8
This is OK. But I replace Num2
with
class Num2(Num):
def __init__(self,num):
self.n2 = num*2
def show(self):
print self.n1,self.n2
RESULT: Error. Num2 has no attribute "n1".
In this case, how can Num2
access n1
?
This question is related to
python
inheritance
init
In the first situation, Num2
is extending the class Num
and since you are not redefining the special method named __init__()
in Num2
, it gets inherited from Num
.
When a class defines an
__init__()
method, class instantiation automatically invokes__init__()
for the newly-created class instance.
In the second situation, since you are redefining __init__()
in Num2
you need to explicitly call the one in the super class (Num
) if you want to extend its behavior.
class Num2(Num):
def __init__(self,num):
Num.__init__(self,num)
self.n2 = num*2
A simple change in Num2 class like this:
super().__init__(num)
It works in python3.
class Num:
def __init__(self,num):
self.n1 = num
class Num2(Num):
def __init__(self,num):
super().__init__(num)
self.n2 = num*2
def show(self):
print (self.n1,self.n2)
mynumber = Num2(8)
mynumber.show()
Since you don't call Num.__init__
, the field "n1" never gets created. Call it and then it will be there.
When you override the init you have also to call the init of the parent class
super(Num2, self).__init__(num)
Source: Stackoverflow.com