Independent Iterators in List Comprehension

In Python, list comprehensions allow multiple iteration loops. Consider the following example:

[(x, y) for x in a for y in b]

where a and b are sequences. This comprehension creates pairs of elements from the Cartesian product of a and b.

Can Iterators Be Dependent?

Can one iterator in a list comprehension refer to another? The answer is yes, and the following code demonstrates it:

[x for x in a for a in b]

In this comprehension, the outer loop iterator a becomes the iterator for the inner loop. This effectively flattens a nested list.

Example

If we have a nested list a:

a = [[1, 2], [3, 4]]

The following list comprehension would flatten it into a single list:

[x for x in a for a in b]

Result:

[1, 2, 3, 4]

Alternative Solution Using Generators

In the provided Python code, the text is stored as sentences, and the task is to extract a single list of words. Here's how you can achieve this using a generator:

text = (("Hi", "Steve!"), ("What's", "up?"))
gen = (word for sentence in text for word in sentence)

The gen variable now yields the flattened list of words:

for word in gen:
    print(word)

Output:

Hi
Steve!
What's
up?