What is the difference between "+=" and "=+"?

Question

Why is the output of this code 18?
def f(x=[]):
x+=[3]
return sum(x)
print (f()+f()+f())

But the output of this code is 9?
def f(x=[]):
x=x+[3]
return sum(x)
print (f()+f()+f())

Answer 1

There's a conceptional difference between x+=y and x = x+y.

The idea is that += changes 'this very object', while the other way creates a separate object.

How exactly this plays out - as often in Python - depends on how the type itself is defined.

With lists, += is basically synonymous with the method 'extend'. It changes that very object in place, adding another list to the end of it.

Now in this version...

def f(x=[]):
x+=[3]
return sum(x)

... the default list x stays the same, so it will still be the same list next time you call the function - stuff from last call still in it.

Which means that it will grow each time you call the function.

However in this version...

def f(x=[]):
x=x+[3]
return sum(x)

... you create a *new* object x+[3], and then paste the name x to it.

So x doesn't refer to the default object anymore - that one still exists, but it isn't influenced by what you do using the name x anymore.

So this time, you have a new list, just with a 3 in it, each call.

Answer 2

=+ ?
it is just an assignment operator:
=

where + does not do anything.
+6 == 6

Answer 3

So, x+=[3] and x=x+[3] should have the same output?

Answer 4

Maybe in this context it is also worth mentioning that it is better to avoid empty iterables as default arguments.

better use

def f(x=None):
if x is None:
x=[]
# some code...

that way it is garantied that an empty list is assigned to x.

Answer 5

Absolutely good to mention it.

Default lists are a good way to get accustomed to the whole reference issue which in Python you have to understand but can't see.

However probably most tutorials at some point warn: Don't do this in real life!

Answer 6

=+ would be just the assignment operator =. The + would be referring to a positive number/expresion relating to the right hand side operand.

Answer 7

Шамиль Эркенов Yes, but lists, and maybe some other iterables have a weird exception with it.