# [Python] Iterables and Generators (3) - itertools

This post is continued from Iterables and Generators (2) - Flattening a multi-dimensional list.

In this post, I will introduce you to one of the most powerful python libraries, which is called an itertools. Itertools supports various functions that create iterators for efficient looping. This module implements a number of iterator building blocks, and you can see some similar implementations from some functional programming languages, such as Haskell.

## 1. Combinatoric Generators

I feel that Combinatoric generators are frequently used and one of the most useful functions in this module. You can easily see the difference between different functions and can know how to use those by following examples.

### 1-1. combinations()

@returns: all possible combinations, in sorted order, without any repeated elements

import itertools as it
a = ['a', 'b', 'c' ,'d']

print(it.combinations(a, 2))    # <itertools.combinations object at 0x10dfe2ea8>
print(list(it.combinations(a, 2))) # [('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]

for (former, latter) in it.combinations(a, 2):
print(former, latter)


Terminal output:

<itertools.combinations object at 0x107774ea8>
[('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]
a b
a c
a d
b c
b d
c d


### 1-2. permutations()

@returns: all possible orderings with no repeated elements

import itertools as it
a = ['a', 'b', 'c' ,'d']

print(it.permutations(a, 2))
print(list(it.permutations(a, 2)))

for (former, latter) in it.permutations(a, 2):
print(former, latter)


Terminal output:

<itertools.permutations object at 0x10caa9570>
[('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'a'), ('b', 'c'), ('b', 'd'), ('c', 'a'), ('c', 'b'), ('c', 'd'), ('d', 'a'), ('d', 'b'), ('d', 'c')]
a b
a c
a d
b a
b c
b d
c a
c b
c d
d a
d b
d c


### 1-3. product()

@returns: cartesian product equivalent to a nested for-loop

import itertools as it
a = ['a', 'b', 'c' ,'d']
b = ['1', '2', '3' ,'4']

print("product: ", list(it.product(a, b)))



## 2. Iterators terminating on the shortest input sequence

Like I mentioned before, this module is heavily influence by functional programming languages (They are really awesome once you learn it, and my favorite one is Haskell!!).
Functions you will see now are the things that you will see from Haskell. One of reasons why my favorite language is python is because python does not confined to a certain programming paradigm, which means that you can choose whatever programming style you want (imperative, functional or object-oriented) that fits the best in the context.

### 2-1. chain(*iterables)

Used for treating consecutive sequence as a single sequence.
@param: *iterables @description: returns chained iterables in a single sequence object.

import itertools as it
l1 = ['a', 'b', 'c' ,'d']
l2 = ['1', '2', '3' ,'4']

s1 = "abc"
s2 = "def"

t1 = (1, 'a')
t2 = (2, 'b')

chained = it.chain(l1, l2, s1, s2, t1, t2)
print(type(chained))
print(chained)
print(list(chained))


Terminal output:

<class 'itertools.chain'>
<itertools.chain object at 0x104abbe10>
['a', 'b', 'c', 'd', '1', '2', '3', '4', 'a', 'b', 'c', 'd', 'e', 'f', 1, 'a', 2, 'b']


### 2-2. groupby()

You will see very similar functionality in pandas library, sql and of course Haskell too.

[k for k, g in groupby('AAAABBBCCDAABBB')] --> A B C D A B
[list(g) for k, g in groupby('AAAABBBCCD')] --> AAAA BBB CC D


Above is a possible use case of groupby() according to the python document.
However, there is one important point that worth taking a look.

import itertools as it
data = [{'module': 'Comp0147', 'score': "pass"},
{'module': 'Comp0010', 'score': "fail"},
{'module': 'Comp0009', 'score': "pass"},
{'module': 'Comp0008', 'score': "fail"}]

grouped_data = it.groupby(data, key=lambda x: x['score'])
print(grouped_data)    # <itertools.groupby object at 0x107585570>
for key, member in grouped_data:
print('{}: {}'.format(key, list(member)))


This should give you a grouped data that has two groups (pass and fail). However, python does not give you an intended output. Terminal output:

<itertools.groupby object at 0x10dcb3570>
pass: [{'module': 'Comp0147', 'score': 'pass'}]
fail: [{'module': 'Comp0010', 'score': 'fail'}]
pass: [{'module': 'Comp0009', 'score': 'pass'}]
fail: [{'module': 'Comp0008', 'score': 'fail'}]


If you were using SQL, it would’ve worked properly, but in python, you have to first sort the keys before grouping it.

import itertools as it
data = [{'module': 'Comp0147', 'score': "pass"},
{'module': 'Comp0010', 'score': "fail"},
{'module': 'Comp0009', 'score': "pass"},
{'module': 'Comp0008', 'score': "fail"}]

sorted_data = sorted(data, key=lambda x: x['score'])
grouped_data = it.groupby(sorted_data, key=lambda x: x['score'])
print(grouped_data)    # <itertools.groupby object at 0x107585570>
for key, member in grouped_data:
print('{}: {}'.format(key, list(member)))


Terminal output:

<itertools.groupby object at 0x10dcb3c50>
fail: [{'module': 'Comp0010', 'score': 'fail'}, {'module': 'Comp0008', 'score': 'fail'}]
pass: [{'module': 'Comp0147', 'score': 'pass'}, {'module': 'Comp0009', 'score': 'pass'}]


### 2-3. takewhile(predicate, iterable)

@params: predicate, iterable
@description: evaluate predicate(iterable element) and returns iterable element that makes it true.
@beware: This stops at the first instance of an element that the predicate returns false.

### 2-4. dropwhile(predicate, iterable)

@params: predicate, iterable
@description: returns exactly the opposite to what takewhile() does.

import itertools as it
testList = [-1, -4, 2, 4, 7, 3, 5, 6]
def pred(x):
return x<3

takeWhileResult = it.takewhile(lambda x: x<3, testList)
print(list(takeWhileResult))      # [-1, -4, 2]

dropWhileResult = it.dropwhile(pred, testList)  # don't do pred(). This is functional programming.
print(list(dropWhileResult))      # [4, 7, 3, 5, 6]


### 2-5. built-in map(), zip(), filter()

To be clear, map(), zip() and filter() do not belong to itertool module, but rather it is a built-in function. Before python3, there was imap(), izip() and ifilter() that returns an iterator rather than a list, but from python3 those functions have been removed and replaced by built-int map and zip that returns a list.
However, I mention these functions in this article because these two functions are actually using iter() and next() method, and can be really synergistic once used together with itertools module.

(Remember from Iterables and Generators(1)-theory, we learned that we can make iterator object by doing iter(iterable) and this iterator object has next() method by default).

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