Compound assignment to Python class and instance variables

Question

I\'ve been trying to understand Python\'s handling of class and instance variables. In particular, I found this answer quite helpful. Basically it says that if you declare a

Answer 1

bar.num += 1

creates a new instance variable 'num' on the 'bar' instance because it doesn't yet exist (and then adds 1 to this value)

an example:

class Foo:
  def __init__(self):
    self.num= 1

bar = Foo()
print bar.num

this prints 1

print bar.foo

this gives an error as expected: Traceback (most recent call last): File "", line 1, in AttributeError: Foo instance has no attribute 'foo'

bar.foo = 5
print bar.foo

now this prints 5

so what happens in your example: bar.num is resolved as Foo.num because there's only an class attribute. then foo.num is actually created because you assign a value to it.