python - Star * operator on left vs right side of an assignment statement

Question

This questions stems from PEP 448 -- Additional Unpacking Generalizations and is present in Python 3.5 as far as I'm aware (and not back-ported to 2.x). Specifically, in the section Disadvantages, the following is noted:

Whilst *elements, = iterable causes elements to be a list, elements = *iterable, causes elements to be a tuple. The reason for this may confuse people unfamiliar with the construct.

Which does indeed hold, for iterable = [1, 2, 3, 4], the first case yields a list:

>>> *elements, = iterable
>>> elements
[1, 2, 3, 4]

While for the second case a tuple is created:

>>> elements = *iterable,
>>> elements
(1, 2, 3, 4)

Being unfamiliar with the concept, I am confused. Can anyone explain this behavior? Does the starred expression act differently depending on the side it is on?

Answer 1

The difference between these two cases are explained when also taking into consideration the initial PEP for extended unpacking: PEP 3132 -- Extended iterable unpacking.

In the Abstract for that PEP we can see that:

This PEP proposes a change to iterable unpacking syntax, allowing to specify a "catch-all" name which will be assigned a list of all items not assigned to a "regular" name.

^{(emphasis mine)}

So in the first case, after executing:

*elements, = iterable

elements is always going to be a list containing all the items in the iterable.

Even though it seems similar in both cases, the * in this case (left-side) means: catch everything that isn't assigned to a name and assign it to the starred expression. It works in a similar fashion as *args and **kwargs do in function definitions.

def spam(*args, **kwargs): 
    """ args and kwargs group positional and keywords respectively """

The second case (right-side) is somewhat different. Here we don't have the * working in a "catch everything" way as much as we have it working as it usually does in function calls. It expands the contents of the iterable it is attached to. So, the statement:

elements = *iterable, 

can be viewed as:

elements = 1, 2, 3, 4, 

which is another way for a tuple to be initialized.

Do note, a list can be created by simple using elements = [*iterable] which will unpack the contents of iterable in [] and result in an assignments of the form elements = [1, 2, 3, 4].