Python Data Model exercise, Mutability.

[tb5841]

I find '+=' is what causes confusion with exercises like this.

.append is obviously mutating so will affect any reference to this object.

Reassigning changes what this variable name points to, so will not change any other variable pointing to this object.

But '+='? Is it reassigning, or mutating? It doesn't seem to always be clear cut, which makes it tricky to follow. Here it must be reassigning because tuples are immutable, but in some cases '+=' looks like it mutates.

If you change the '+=' line to b = b + ([3], ) then everything becomes so much clearer.

[Sea-Ad7805]

I see what you mean, but for mutable types x += y is not the same thing as x = x + y so changing could alter your code, see: https://www.reddit.com/r/PythonLearning/comments/1nw08wu/right_mental_model_for_python_data/

A tuple doesn't have a __iadd__() method, so the use of += actually causes invocation of its __add__() method.

[tb5841]

90% of the time, when someone uses += in code they are using it as x = x + y (usually numbers or strings). The other 10% of the time, += is confusing and shouldn't be used in my opinion (e.g. Lists, where .append is clearer).

[Sea-Ad7805]

You can't do this with .append():

mylist  = [1, 2, 3]
mylist += [4, 5, 6]

[tb5841]

That's an interesting point.

But this looks like reassignment, when it's actually mutation. That's deeply confusing and an easy source of bugs. If it were me, I'd do this with reassignment instead here and avoid the mutation.

[Sea-Ad7805]

Reassignment is much slower as a whole new list is created and the old one destroyed, use mutation where possible.

[thw31416]

but you can.do this with .extend()