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Suppose I have three lists:

list1 --> [a, b, c, d, e, f, g, h]
list2 --> [i, j, k]
list3 --> [l, m, n, o, p]

I wish to generate all combinations where I take five elements from list1, two elements from list2 and three elements from list3.

eg.

a, b, c, d, e, i, j, l, m, n 
a, b, c, d, e, i, j, l, m, o
etc.

I tried to use itertools.combinations.

l1_combinations = itertools.combinations(list1, 5)
l2_combinations = itertools.combinations(list2, 2)
l3_combinations = itertools.combinations(list3, 3)
for l1_iterator in list(l1_combinations):
 for l2_iterator in list(l2_combinations): #added a missing )
 for l3_iterator in list(l3_combinations):
 sample = l1_iterator + l2_iterator + l3_iterator
 print(sample)

But I am getting output with iterations happening only on list3. In all the output, only first five elements from list1 and first two elements from list2 are present. Combinations with other elements from those two lists aren't present.

Can someone help me here and also explain what exactly did i miss ?

As an alternative to regenerating the list of combinations, compute the product of the combinations up front; this also saves you from nesting for loops.

from itertools import combinations, product


list1 = list("abcdefgh")
list2 = list("ijk")
list3 = list("lmnop")

l1 = combinations(list1, 5)
l2 = combinations(list2, 2)
l3 = combinations(list3, 3)
for c1, c2, c3 in product(l1, l2, l3):
 sample = c1 + c2 + c3
 print(sample)

Don't iterate over the same iterator multiple times, after the first time it's exhausted. Iterate over a fresh iterator each time:

for l1_iterator in itertools.combinations(list1, 5):
 for l2_iterator in itertools.combinations(list2, 2):
 for l3_iterator in itertools.combinations(list3, 3):
 sample = l1_iterator + l2_iterator + l3_iterator
 print(sample)

Or make lists of each one in advance to avoid recomputation:

l1_combinations = list(itertools.combinations(list1, 5))
l2_combinations = list(itertools.combinations(list2, 2))
l3_combinations = list(itertools.combinations(list3, 3))
for l1_iterator in l1_combinations:
 for l2_iterator in l2_combinations:
 for l3_iterator in l3_combinations:
 sample = l1_iterator + l2_iterator + l3_iterator
 print(sample)