Why is there a difference between a = a + b and a += b

Question

I just encountered this bug:

def fn_that_uses_a_list(list):
  if (list[-1] < 0): list += [0]
  for item in list:
    print(item)

l = [-4, -2]
fn_that_uses_a_list(l)
# Now suddenly l has three items.

However, if I change list += [0] to list = list + [0], then things work. I find this confusing, but it could be because I'm new to Python.

Why is there a difference in this case? I'm looking for a more existential answer, rather than "list is a reference so += modifies the original"

Am I doing something "un-pythonic" that's causing me to run into the bug?

Answer 1

Not sure what you are after, if I use both list += [0] and list = list + [0] I get the same result.

That said, += mutates the list, while + creates a new list. Try:

def fn_that_uses_a_list(list):
  if (list[-1] < 0):
    print id(list)
    list += [0]
    # list = list + [0]
    print id(list)
  for item in list:
    print(item)

l = [-4, -2]
fn_that_uses_a_list(l)