NameError: name 'self' is not defined

Question

Why such structure

class A:
    def __init__(self, a):
        self.a = a

    def p(self, b=self.a):
        print b

gives an error

Answer 1

Default argument values are evaluated at function define-time, but self is an argument only available at function call time. Thus arguments in the argument list cannot refer each other.

It's a common pattern to default an argument to None and add a test for that in code:

def p(self, b=None):
    if b is None:
        b = self.a
    print b