Python operator:single star(*)

with Zip

Read a block of pseudo Python code like

comA = zip(lsA, lsB)
comB = zip(*comA)

then made a try to see how * works:

1
2
3
4
5
6
7
8

a = [1, 2, 3]
b = [4, 5, 6]
c = zip(a,b)
for j in c:
print(j)
d = zip(*c)
for i in d:
print(i)

and the output:

(1, 4)
(2, 5)
(3, 6)

There is nothing was outputted by line8: print(i), but it works after removing line4-5:

a = [1, 2, 3]
b = [4, 5, 6]
c = zip(a,b)
d = zip(*c)
for i in d:
print(i)

with the output:

1, 2, 3)
(4, 5, 6)

Just as it was mentioned in the Python doc:

zip() in conjunction with the * operator can be used to unzip a list

But I’m still wondering why the first try failed, submitted a question on Stackoverflow: Why “for” affects another zip object? after my debugging, and got the answer shortly: zip variable empty after first use:

In Python2.x, zip returned a list of tuples

In Python3, a lot of the builtin functions now return iterators rather than lists (map, zip, filter)

List comprehensions: the type of elements is list

Updated @ 25/Aug/2019

a = [[1,2], [3,4], [5, 6, 7]]
c  = [ [*i, sum(i)] for i in a]
print(c)

Output:

[[1, 2, 3], [3, 4, 7], [5, 6, 7, 18]]