Python: variables, inheritance, and default arguments

Question

I think I have some misunderstandings of the usage of "class" and "inheritance' in Python. I'll simplify my question as the followings:

class A:
    def __init__(self):
        self.data = 100

class B(A):
    def b(self):
        print self.data

>>>B().b()
>>>100

OK, so far so good. However, if I create another class, something goes wrong, which is shown as the following:

class C(A):
    def c(self, num=self.data):
        print self.data

>>>C().c()
NameError: name 'self' is not defined

I want to set the default value of 'num' to self.data, which is '100'. Without 'class', it will be much simpler:

data = 100
def d(num = data):
    print num

>>>d()
>>>100

I've already googled some articles, but still stuck in this problem... Thanks in advance!

Answer 1

When you do this:

class C(A):
    def c(self, num=self.data):
        print self.data

you're doing something like:

>>> def d(num, data=num):
...     print(num, data)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'num' is not defined

And as you see the python compiler does not know what the second num is.

But there's nothing stopping you from doing something like:

class C(A):
    def c(self, num=None):
        print num or self.data

or with an explicit None check:

class C(A):
    def c(self, num=None):
        if num is None:
            num = self.data
        print num

Answer 2

This has nothing to do with inheritance. You can try to do the same in the base class A and it would fail in the same way.

For achieving what you want simply do:

def c(self, num=None):
    if num is None:
        num = self.data

Answer 3

Function parameter defaults are evaluated when the function is defined, not when it is called. At definition time, there is no name self .

The best course is to use None as a default, and then use logic in the function to interpret what that means:

class C(A):
    def c(self, num=None):
        if num is None:
            num = self.data
        #.. the rest of the function ..

Answer 4

You are misunderstanding class methods. Class methods are implicitly passed self. Self does not exist def c(self, num=HERE>>>>>self<<<<<HERE.data):

Why do you want to do what you are doing?

Answer 5

class C(A):
    def c(self,data=None):
        if data == None:
            print self.data
        else 
            print data 

The method of your class get arguments, you cannot pass self.something.

Answer 6

As already mentioned, you can't use 'self' variable like that. If you don't have to change the value of data later, you can do following:

class A:
    data = 100

class C(A):
    def me(self, num=A.data):
        print num