[ Python 常見問題 ] All possible permutations of a set of lists in Python

Source From Here
Question
In Python I have a list of n lists, each with a variable number of elements. How can I create a single list containing all the possible permutations:

For example

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1. [ [ a, b, c], [d], [e, f] ]  

I want:

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1. [ [a, d, e] , [a, d, f], [b, d, e], [b, d, f], [c, d, e], [c, d, f] ]  

Note I don't know n in advance. I thought itertools.product would be the right approach but it requires me to know the number of arguments in advance.

How-To
You don't need to know n in advance to use itertools.product. e.g.:

>>> import itertools
>>> s=[ [ 'a', 'b', 'c'], ['d'], ['e', 'f'] ]
>>> list(itertools.product(*s))
[('a', 'd', 'e'), ('a', 'd', 'f'), ('b', 'd', 'e'), ('b', 'd', 'f'), ('c', 'd', 'e'), ('c', 'd', 'f')]

From HackerRank, this question "Maximiize it" is a good example to leverage above introduction:

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1. from itertools import product  
2. from functools import reduce  
3.   
4. N, M = list(map(lambda e: int(e), input().split()))  
5. nlist = []  
6. for i in range(N):  
7.     n = list(map(lambda e: int(e), input().split()))  
8.     nlist.append(n[1:])  
9.   
10. nset = set()  
11. max_val = 0  
12. for t in product(*nlist):  
13.     val = reduce(lambda s, e: s + pow(e, 2), t, 0) % M  
14.     if val > max_val:  
15.         max_val =val  
16.   
17. print(max_val)