Python zip function with iterator vs iterable [duplicate]

Question

I am having trouble understanding working of the zip() function in python when an iterator is passed in instead of iterable.

Have a look at these two print statements:

string = "ABCDEFGHI"

print(list(zip(*[iter(string)] * 3)))
# Output: [('A', 'B', 'C'), ('D', 'E', 'F'), ('G', 'H', 'I')]

print(list(zip(*[string] * 3)))
# Output: [('A', 'A', 'A'), ('B', 'B', 'B'), ('C', 'C', 'C'), ('D', 'D', 'D'), ('E', 'E', 'E'), ('F', 'F', 'F'), ('G', 'G', 'G'), ('H', 'H', 'H'), ('I', 'I', 'I')]

Can someone explain me the working of zip() in both the cases?

Answer 1

The difference is that for [iter(string)] * 3, zip creates aliases of a single iterator. For [string] * 3, zip creates unique iterators per argument. The shorter output without duplicates is zip exhausting the single aliased iterator.

See what is meaning of [iter(list)]*2 in python? for more details on how [iter(...)] * 2 works and causes potentially unexpected results.

See the canonical answer List of lists changes reflected across sublists unexpectedly if the [...] * 3 aliasing behavior is surprising.

Answer 2

Let's use a clearer example:

a = iter("123456")  # One iterator 
list(zip(a, a, a)) 
# [('1', '2', '3'), ('4', '5', '6')]

vs

a = iter("123456")
b = iter("123456")
c = iter("123456")
list(zip(a, b, c))
# [('1', '1', '1'), ('2', '2', '2'), ('3', '3', '3'), ('4', '4', '4'), ('5', '5', '5'), ('6', '6', '6')]

Obviously in the first example a can only yield 6 elements, and has to yield 3 to zip whenever zip needs to create a value. In contrast, the second example has 18 elements total, and yields them in 6 groups of 3.