Still one of the oldest #Python oddities I've ever seen 🐍🤔

>>> a = ([],)
>>> a[0] += ["what"]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> a
(['what'],)

Note that an exception was raised but the operation also worked! 😳 #pythonoddity

@treyhunner Ha! That's a good one, it really illustrates how `+=` is just syntactic sugar over the `+` and the `=`

@chris @treyhunner @s_gruppetta

It's weirder than that!

`a[0] = a[0] + ["what"]` reports the error but doesn't have the side effect.

`a[0] += ["what"]` reports the error and DOES have the side effect!

I'm shocked both that the failed assignment has a side effect and that these do not behave the same way. Can anyone explain?

@peterdrake @chris @treyhunner Trey is the master at these things, but here's my attempt.

In the first one, the expression after the `=` is executed first; this is fine as you're adding one list to another (using `list.__add__()`). This creates a new object. Then, there's an attempt to assign this new object as the first item in `a` which fails since `a` is a tuple.

In the second, the assignment and addition are attempted together using the `__iadd__()` dunder method, rather than `__add__()`. No new object is created.

Here's a different example which may shed some light:

`>>> b = [2]`
`>>> c = [3]`
`>>> id(b)`
`4407498688`
`>>> b = b + c`
`>>>`id(b)`
`4407508864`

`b` has a different id after the assignment, so it's a different object to the original `b`

now, in a new session:

`>>> b = [2]`
`>>> c = [3]`
`>>> id(b)`
`4394855808`
`>>> b += c`
`>>> id(b)`
`4394855808`

`b` is now the same object as it was before (same id)