I create a list as:
>>> seq = [1, 2, 3, 4]
Now i assign the variables as follows:
>>> a, b, c, d, *e = seq
>>> print(a, b, c, d, e)
I get output as:
>>> 1 2 3 4 []
Now i change the sequence in which i assign variables as:
>>> a, b, *e, c, d = seq
>>> print(a, b, c, d, e)
I get output as:
>>> 1, 2, 3, 4, []
So my question is Why *e variable above is always assigned an empty list regardless of where it appears?
In the first case
a, b, c, d, *e = seq
since the sequence has only four elements, a
, b
, c
and d
get all of them and the rest of them will be stored in e
. As nothing is left in the sequence, e
is an empty list.
In the second case,
a, b, *e, c, d = seq
First two elements will be assigned to a
and b
respectively. But then we have two elements after the *e
. So, the last two elements will be assigned to them. Now, there is nothing left in the seq
. That is why it again gets an empty list.
It was a design choice, according to PEP 3132 (with my bold):
A tuple (or list) on the left side of a simple assignment (unpacking is not defined for augmented assignment) may contain at most one expression prepended with a single asterisk (which is henceforth called a "starred" expression, while the other expressions in the list are called "mandatory"). This designates a subexpression that will be assigned a list of all items from the iterable being unpacked that are not assigned to any of the mandatory expressions , or an empty list if there are no such items.
Indeed, the first example in the PEP illustrates your point:
>>> a, *b, c = range(5)
>>> a
0
>>> c
4
>>> b
[1, 2, 3]
a, b, *e, c, d = [1, 2, 3, 4]
*e
says "what's left put into e
". Because you have 4 elements, empty sequence is put into e
.
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