Strange behaviour when inheriting from a list in python

Question

I was fiddling around with inheritance recently and I'm a little confused by the behaviour of the following:

class Foo(list):

    def method(self, thing):
        new = self + [thing]
        print(new)
        self = new
        print(self)

    def method2(self, thing):
        self += [thing]

>>> f = Foo([1, 2, 3, 4, 5])
>>> f.method(10)
[1, 2, 3, 4, 5, 10]
[1, 2, 3, 4, 5, 10]
>>> f
[1, 2, 3, 4, 5]
>>> f.method2(10)
>>> f
[1, 2, 3, 4, 5, 10]

Why does the in-place method method2 work but the first one doesn't?

glglgl · Accepted Answer · 2014-02-18 08:06:16Z

2

Because that's how in-place operators work.

self = self + [thing] creates a new list and put it into the local variable self, overriding the passed one. But it doesn't modify the object itself. Internally, it does self = self.__add__([thing]).

self += [thing], OTOH, modifies the list in-place. Internally it tries self = self.__iadd__([thing]) first. iadd stands for "inplace add". Only if that doesn't exist, self = self.__add__([thing]) is called.

The difference is that __add__() always creates a new object and leaves the others untouched. __iadd__(), however, is supposed to try first to modify the object it operates on. In this case, it returns it so that no change of object occurs, i. e. self refers to the same object as before. Only if this is not possible, it returns a new one which is then assigned.

edited Feb 18, 2014 at 8:06

answered Feb 17, 2014 at 20:42

glglgl

91.5k13 gold badges157 silver badges230 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Joel Cornett · Accepted Answer · 2014-02-17 20:41:47Z

0

list.__add__() (the + operator builtin) creates a new list, while list.__iadd__() (the += operator builtin) modifies the list in place.

answered Feb 17, 2014 at 20:41

Joel Cornett

24.8k9 gold badges69 silver badges90 bronze badges

Collectives™ on Stack Overflow

Strange behaviour when inheriting from a list in python

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related