Question

class Ball:   a = []   def __init__(self):     pass    def add(self,thing):     self.a.append(thing)    def size(self):     print len(self.a)  for i in range(3):   foo = Ball()   foo.add(1)   foo.add(2)   foo.size() 

I would expect a return of :

2 2 2 

But I get :

2 4 6 

Why is this? I’ve found that by doing a=[] in the init, I can route around this behavior, but I’m less than clear why.

Answer 1

doh

I just figured out why.

In the above case, the a is a class attribute, not a data attribute – those are shared by all Balls(). Commenting out the a=[] and placing it into the init block means that it’s a data attribute instead. (And, I couldn’t access it then with foo.a, which I shouldn’t do anyhow.) It seems like the class attributes act like static attributes of the class, they’re shared by all instances.

Whoa.

One question though : CodeCompletion sucks like this. In the foo class, I can’t do self.(variable), because it’s not being defined automatically – it’s being defined by a function. Can I define a class variable and replace it with a data variable?