Python assignment operator differs from non assignment

Question

I have face this weird behavior I can not find explications about.

MWE:

l = [1]
l += {\'a\': 2}
l
[1, \'a\']
l + {\'B\': 3}
Traceback (most recent ca         


        

Answer 1

l += ... is actually calling object.__iadd__(self, other) and modifies the object in-place when l is mutable

The reason (as @DeepSpace explains in his comment) is that when you do l += {'a': 2} the operation updates l in place only and only if l is mutable. On the other hand, the operation l + {'a': 2} is not done in place resulting into list + dictionary -> TypeError.

---

(see here)

---

l = [1]
l = l.__iadd__({'a': 2})
l
#[1, 'a']

is not the same as + that calls object.__add__(self, other)

l + {'B': 3}

TypeError: can only concatenate list (not "dict") to list