The more general issue is the difference between the __add__ and __iadd__ special methods.

The __iadd__ special method is for an in-place addition, that is it mutates the object that it acts on. The __add__ special method returns a new object and is used for the standard + operator.

When the += operator is used on an object which has an __iadd__ defined it is used, and the object is modified in place. If there isn't an __iadd__ it will instead try to use the plain __add__ and return a new object.

That is why for mutable types += changes the object's value, whereas for immutable types a new object is returned instead (a += b becomes equivalent to a = a + b).

For mutable type that support both __iadd__ and __add__ you therefore have to be careful which one you use. a += b will call __iadd__ and mutate a, whereas a = a + b will create a new object and assign it to a.

For immutable types (where you don't have an __iadd__) a += b and a = a + b are equivalent. This is what lets you use += on immutable types, which might seem a strange design decision until you consider that otherwise you couldn't use += on immutable types like numbers!

The general answer is that += tries to call the __iadd__ special method, and if that isn't available it tries to use __add__ instead. So the issue is with the difference between these special methods.

The __iadd__ special method is for an in-place addition, that is it mutates the object that it acts on. The __add__ special method returns a new object and is also used for the standard + operator.

So when the += operator is used on an object which has an __iadd__ defined the object is modified in place. Otherwise it will instead try to use the plain __add__ and return a new object.

That is why for mutable types like lists += changes the object's value, whereas for immutable types like tuples, strings and integers a new object is returned instead (a += b becomes equivalent to a = a + b).

For types that support both __iadd__ and __add__ you therefore have to be careful which one you use. a += b will call __iadd__ and mutate a, whereas a = a + b will create a new object and assign it to a. They are not the same operation!

>>> a1 = a2 = [1, 2] >>> b1 = b2 = [1, 2] >>> a1 += [3] # Uses __iadd__, modifies a1 in-place >>> b1 = b1 + [3] # Uses __add__, creates new list, assigns it to b1 >>> a2 [1, 2, 3] # a1 and a2 are still the same list >>> b2 [1, 2] # whereas only b1 was changed 

For immutable types (where you don't have an __iadd__) a += b and a = a + b are equivalent. This is what lets you use += on immutable types, which might seem a strange design decision until you consider that otherwise you couldn't use += on immutable types like numbers!