Iteration in Python


Iteration is one of Python's greatest strengths. Once I learned its power, I haven't used an 'i' or a 'j' in ages. Before this summer, I was used to the power of the in operator, but I think that the beauty of Python's iteration lies in list comprehensions and generator expressions.

Consider a simple list comprehension:

post_authors = [ for post in blog_posts()]

The list comprehension allows users to map a function over each element in another list. The new list will contain all of the authors of the post objects returned by the blog_posts() function.

What if I wanted to get only authors of posts written less than three weeks ago?

from datetime import date, timedelta
def is_recent_post(post):
    return < - timedelta(weeks=4)

post_authors = [ for post in blog_posts if is_recent_post(post)]

List comprehensions are a very powerful way to create complex lists while keeping a low syntax overhead.

Now, it's important to think about why lists are created in the first place.
Often, there really isn't a need for the overhead of a mutable list object with all of its elements in memory at once. In fact, much of the time a list is created only to be iterated over once. This is the perfect situation for an iterator. They keep a low memory overhead while retaining a list-like interface for the in keyword. Often, generator expressions are the way to go when there is no need to keep the entire list in memory or creating the list involved expensive operations like file I/O.

def blog_posts():
    return (read_post(file_name) for file_name in file_names)

List comprehensions and generator expressions are two of my favorite features in Python. They have clean syntax and allow for easy manipulation of data in a functional way.