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 NameError: name 'self' is not defined?

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 

Update 2022: Python developers are now considering late-bound argument defaults for future Python versions.