Nested list comprehensions in Python

python-nested_list_comprehensions.md

Nested list comprehensions in Python

For some reason or another, I'm always second guessing myself when writing nested list comprehensions. Here's a quick example to clarify what's going behind the scenes:

>>> words = ["foo", "bar", "baz"]
>>> [letter for word in words for letter in word]
['f', 'o', 'o', 'b', 'a', 'r', 'b', 'a', 'z']

In other words, the for loops are nested from left-to-right. If we just wanted to print the values in the above example, we could replace the nested list comprehensions with the following for statements:

>>> for word in words:
...   for letter in word:
...     print letter
... 
f
o
o
b
a
r
b
a
z

Hopefully this will clarify nested list comprehensions for future me.