Python Class scope & lists

Question

I\'m still fairly new to Python, and my OO experience comes from Java. So I have some code I\'ve written in Python that\'s acting very unusual to me, given the following code:<

Answer 1

tlayton's answer is part of the story, but it doesn't explain everything.

Add a

print MyClass.mynum

to become even more confused :). It will print '0'. Why? Because the line

self.mynum += 1

creates an instance variable and subsequently increases it. It doesn't increase the class variable.

The story of the mylist is different.

self.mylist.append("Hey!")

will not create a list. It expects a variable with an 'append' function to exist. Since the instance doesn't have such a variable, it ends up referring the one from the class, which does exist, since you initialized it. Just like in Java, an instance can 'implicitly' reference a class variable. A warning like 'Class fields should be referenced by the class, not by an instance' (or something like that; it's been a while since I saw it in Java) would be in order. Add a line

print MyClass.mylist

to verify this answer :).

In short: you are initializing class variables and updating instance variables. Instances can reference class variables, but some 'update' statements will automagically create the instance variables for you.