Useful Functions of Python Itertools Module

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Python itertools module provide memory-efficient solutions for producing complex iterators. It is quite useful in different situations where it provides functionalities for faster programs along with efficient memory usage.

In this post, we will take a look at some basic functions of this module.

  1. zip_longest() fills-up zip list for uneven lengths of lists. It fills the empty spaces in pairs with a given value. Typically, the basic syntax is itertools.zip_longest(*iterables, fillvalue=None).
     mylist = ["a","b","c"]
     index_items = list(zip(range(1,10),mylist))
     print(index_items)
     index_future_items = list(it.zip_longest(range(1,10),mylist))
     print(index_future_items)
     # [(1, 'a'), (2, 'b'), (3, 'c')]
     # [(1, 'a'), (2, 'b'), (3, 'c'), (4, None), (5, None), (6, None), (7, None), (8, None), (9, None)]
    
  2. cycle creates a copy of an iterable and start iterating over the copied version when the previous iterator is exhausted. Typically, the basic syntax isitertools.cycle(iterable). This function is useful for writing the simplest round-robin function in just two lines.
     mylist = ["a","b","c"]
     my_list_cycle = it.cycle(mylist)
     new_list = [next(my_list_cycle) for _ in range(5)]
     print(new_list)
     # ['a', 'b', 'c', 'a', 'b']
    
  3. combinations and permutations provides the mathematical combination and permutation over a iterable. Typically, the syntax are itertools.combinations(iterable, r) and itertools.permutations(iterable, r).
     mylist = ["a","b","c"]
     combination = list(it.combinations(mylist,2))
     permutation = list(it.permutations(mylist,2))
     print(combination)
     print(permutation)
     # [('a', 'b'), ('a', 'c'), ('b', 'c')]
     # [('a', 'b'), ('a', 'c'), ('b', 'a'), ('b', 'c'), ('c', 'a'), ('c', 'b')]
    
  4. Cartesian Product using product(). The syntax is itertools.product(*iterables, repeat=1).
     letters = ["a","b","c"]
     numbers = [0,1,2]
     cart_product_1 = list(it.product(numbers,letters))
     cart_product_2 = list(it.product(numbers,letters, repeat=2))
     print(cart_product_1)
     print(cart_product_2)
     # [(0, 'a'), (0, 'b'), (0, 'c'), (1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b'), (2, 'c')]
     # [(0, 'a', 0, 'a'), (0, 'a', 0, 'b'), (0, 'a', 0, 'c'), (0, 'a', 1, 'a'), (0, 'a', 1, 'b'), (0, 'a', 1, 'c'), (0, 'a', 2, 'a'), (0, 'a', 2, 'b'), (0, 'a', 2, 'c'), (0, 'b', 0, 'a'), (0, 'b', 0, 'b'), (0, 'b', 0, 'c'), (0, 'b', 1, 'a'), (0, 'b', 1, 'b'), (0, 'b', 1, 'c'), (0, 'b', 2, 'a'), (0, 'b', 2, 'b'), (0, 'b', 2, 'c'), (0, 'c', 0, 'a'), (0, 'c', 0, 'b'), (0, 'c', 0, 'c'), (0, 'c', 1, 'a'), (0, 'c', 1, 'b'), (0, 'c', 1, 'c'), (0, 'c', 2, 'a'), (0, 'c', 2, 'b'), (0, 'c', 2, 'c'), (1, 'a', 0, 'a'), (1, 'a', 0, 'b'), (1, 'a', 0, 'c'), (1, 'a', 1, 'a'), (1, 'a', 1, 'b'), (1, 'a', 1, 'c'), (1, 'a', 2, 'a'), (1, 'a', 2, 'b'), (1, 'a', 2, 'c'), (1, 'b', 0, 'a'), (1, 'b', 0, 'b'), (1, 'b', 0, 'c'), (1, 'b', 1, 'a'), (1, 'b', 1, 'b'), (1, 'b', 1, 'c'), (1, 'b', 2, 'a'), (1, 'b', 2, 'b'), (1, 'b', 2, 'c'), (1, 'c', 0, 'a'), (1, 'c', 0, 'b'), (1, 'c', 0, 'c'), (1, 'c', 1, 'a'), (1, 'c', 1, 'b'), (1, 'c', 1, 'c'), (1, 'c', 2, 'a'), (1, 'c', 2, 'b'), (1, 'c', 2, 'c'), (2, 'a', 0, 'a'), (2, 'a', 0, 'b'), (2, 'a', 0, 'c'), (2, 'a', 1, 'a'), (2, 'a', 1, 'b'), (2, 'a', 1, 'c'), (2, 'a', 2, 'a'), (2, 'a', 2, 'b'), (2, 'a', 2, 'c'), (2, 'b', 0, 'a'), (2, 'b', 0, 'b'), (2, 'b', 0, 'c'), (2, 'b', 1, 'a'), (2, 'b', 1, 'b'), (2, 'b', 1, 'c'), (2, 'b', 2, 'a'), (2, 'b', 2, 'b'), (2, 'b', 2, 'c'), (2, 'c', 0, 'a'), (2, 'c', 0, 'b'), (2, 'c', 0, 'c'), (2, 'c', 1, 'a'), (2, 'c', 1, 'b'), (2, 'c', 1, 'c'), (2, 'c', 2, 'a'), (2, 'c', 2, 'b'), (2, 'c', 2, 'c')]
    
  5. chain() makes an iterator that returns sequential iterators from the sequential iterables. The syntax is itertools.chain(*iterables).

     combined = list(it.chain(letters,numbers))
     print(combined)
     # ['a', 'b', 'c', 0, 1, 2]
    
  6. compress() works like a filter. The syntax is itertools.compress(data, selectors).
     mylist = ["a","b","c"]
     my_bool = [True,False,True]
     filtered = list(it.compress(mylist,my_bool))
     print(filtered)
     # ['a', 'c']
    
  7. accumulate() makes an iterator that returns accumulated result of of any binary functions. The syntax is itertools.accumulate(iterable[, func, *, initial=None]).

     import operator as op
     acc = list(it.accumulate(numbers))
     acc_mul = list(it.accumulate(numbers, op.mul))
     acc_sub = list(it.accumulate(numbers, op.sub))
     acc_div = list(it.accumulate(numbers, op.floordiv))
     print(acc)
     print(acc_sub)
     print(acc_mul)
     print(acc_div)
     # [1, 3, 6, 10]
     # [1, -1, -4, -8]
     # [1, 2, 6, 24]
     # [1, 0, 0, 0]
    

    or you can use customized lambda functions

     acc_sub = list(it.accumulate(numbers,
     lambda x, y: y - x))
    
  8. tee() returns $n$ number of independent copies of a single iterable. The standard syntax is itertools.tee(iterable, n=2).

     numbers = [1,2,3,4]
     c1, c2, c3 = it.tee(numbers,3)
     print(list(c1), list(c2), list(c3))
     # [1, 2, 3, 4] [1, 2, 3, 4] [1, 2, 3, 4]
    
  9. islice() returns selected elements from the iterable. Works similar to the range() function. The basic syntax is itertools.islice(iterable, stop) or itertools.islice(iterable, start, stop[, step]).

     my_slice = list(it.islice([1, 2, 3, 4], 1, 3))
     print(my_slice)
     evenlist = list(it.islice(list(range(20)),2,15,2))
     print(evenlist)
     # [2, 3]
     # [2, 4, 6, 8, 10, 12, 14]
    

There are some other useful functions too. I included only the most common ones here. For more details, checkout the Official Documentation.

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