Useful modules and examples from Python's itertools library.

python_itertools.py

# python_itertools.py highlights useful functions in python's itertools module
# Influenced by  Raymond Chandler III pyvideo.org/video/2275 (Super Advanced Python Concepts)
# Author: Aaron Phalen | Twitter: @aaron_phalen | Email: [email protected]
from itertools import chain, combinations, permutations, compress, count, cycle, groupby

# 1.  Itertools Chain Example
letters = ["a", "b", "c", "d"]
numbers = [1, 2, 3]
both = chain(letters, numbers)

# Two methods for iterating through itertools chain generator-like expression.
# Useful for reducing memory footprint when working with large data sets.

# Method 1: for loop
for b in both:
    print b,
# Output: a, b, c, d, 1, 2, 3

# Method 2: while loop 
while True:
    try:
        elem = both.next()
        print elem,
    except StopIteration:
        break
# Output: a, b, c, d, 1, 2, 3      

# Note: must re-populate both if trying to run both method 1 and 2 above, due to generator-like behavior.
      
# 2. Itertools Combinations Example
letters = ["a", "b", "c"]
print list(combinations(letters, 2))
# Output: [('a', 'b'), ('a', 'c'), ('b', 'c')]

# Note: Itertools combinations returns all input list combinations of n-element tuple.
# As given as an example above, letters as input list and 2 as n-element tuple.
# Combinations does not care about result order and will return unique combinations.

# 3. Itertools Permutations Example
numbers = [1, 2, 3]
print list(permutations(num_list, 2))
# Output: [(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)]

# Note: Itertools permutations cares about order and will return all permutations.

# 4. Itertools Compress Example
candidates = ["Obama", "Trump", "Clinton" "Mcafee"]
win_list = [1, 1, 0, 0]
print list(compress(candidates, win_list))
#Ouput: ["Obama", "Trump"]

# Note: Itertools compress is very similar in function to filter in Python.
# Obama, Trump in output because of incumbent and  Trump's likeliness to win.

# 5. Itertools Count Example
c = count(1, 5) # itertools count generator-like object returns 1 to infinity by 5
for _ in xrange(0, 3):
  print c.next(),
# Output: 1, 6, 11

# 6. Itertools Cycle Example
colors = cycle(["red", "green", "blue", "yellow"])

for _ in xrange(0, 6):
    print colors.next(),
    
# Output: red, green, blue, yellow, red

# Note: Cycle will infinitely iterate though an iterable in an orderly fashion and 
# return to start index to repeat.

# 7. Itertools Group By
animals = [("dog", "dachshund"), ("dog", "poodle"), ("insect", "ant")]
for key, group in groupby(animals, key=lambda x: x[0]):
    print key
    for g in group:
        print g[0]
        
# Output: dog
#         dachshund
#         poodle
#
#         insect
#         ant