Python Variable Access

Question

class A:
 def __init__(self):
 self.i = 0
 def demo(self):
 self.a=1
class B(A):
 def __init__(self, j = 0):
 super().__init__()
 self.j = j
 print(self.i)
 self.demo()
 def demo(self):
 print(self.a)
def main():
 b = B()
 print(b.i)
 print(b.j)
main()

why am i not able to access self.a inside class b does prefixing a variable with self. will make it an instance variable Thanks

• Pax0r: As question was already anwsered I would only like to point that in almost every case you SHOULD declare all instance parameters in init, even with simple self.a = None. Fist - you will never get AttributeError, second - you will have cleaner code with all your instance variables declared in the same place.

Answer 1

When you include a demo method for both classes, the most recently-defined one masks the others. Since you define B after you define A but before you call any of the methods in A, demo will try to access a variable that was never defined. You should either call demo within A (in __init__, probably), or change the name of demo in B to something unique, which will allow you to access both methods (probably the best approach, since they do different things and you want to make use of both).

• BLACKMAMBA: Thanks what will be the scope of variable a is that instance or local
• TigerhawkT3: It is an instance variable, as its reference is self.a.
• user2357112: Note that calling self.demo() in A wouldn't address the issue, since it'd still get B.demo. If you were to go with that route, you'd need to explicitly invoke A.demo(self). I'd say using different names is a better solution, but the code is contrived enough and has enough other design issues that it's hard to talk about what it "should" do.

Answer 2

Because you overwrite demo method on B class.
If you want to access self.a add it to __init__ method of A class or call parent demo method like this:

 def demo(self):
 super().demo()
 print(self.a)

Answer 3

It is because instance variable b is not initiated within __init__ of A

Answer 4

Because class A.demo() is not executed:
class A: def init(self): self.i = 0 def demo(self): self.a=1

class B(A):
 def __init__(self, j = 0):
 super().__init__()
 self.j = j
 print(self.i)
 super().demo()
 self.demo()
 def demo(self):
 print(self.a)
def main():
 b = B()
 print(b.i)
 print(b.j)
main()