Python int vs array class variable

Question

I played with Python classes and came up with the following example, in which two variables, which appear to be static class variables, change differently.

What's going on here? My first instinct is that something complicated happens with links.

class Foo: a = [] n = 0 def bar(self): self.a.append('foo') self.n += 1 x = Foo() print xa, xn ([] 0) x.bar() print xa, xn (['foo', 1]) y = Foo() print ya, yn (['foo', 0]) y.bar() print ya, yn (['foo', 'foo'], 1) 

Answer 1

You are right - in the case of Foo.a access to self.a really refers to Foo.a , which is shared between all instances of Foo . However, when you update self.n with += , you actually create an instance level variable on self that obscures Foo.n :

 >>> import dis >>> dis.dis(Foo.bar) 5 0 LOAD_FAST 0 (self) 3 LOAD_ATTR 0 (a) 6 LOAD_ATTR 1 (append) 9 LOAD_CONST 1 ('foo') 12 CALL_FUNCTION 1 15 POP_TOP 6 16 LOAD_FAST 0 (self) 19 DUP_TOP 20 LOAD_ATTR 2 (n) 23 LOAD_CONST 2 (1) 26 INPLACE_ADD 27 ROT_TWO 28 STORE_ATTR 2 (n) 31 LOAD_CONST 0 (None) 34 RETURN_VALUE 

In other words, when you execute self.a.append('some value') , the interpreter extracts a from memory through the name in Foo , and then mutates the list pointed to by Foo.a

On the other hand, when you execute the self.n += 1 interpreter:

• Selects n from Foo (because it cannot find n on self )
• Creates a new value n + 1
• Saves the new value in the n attribute to self