Can List Comprehensions Have Dependent Iterators?

Independent Iterators in List Comprehension

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

<code class="python">[(x, y) for x in a for y in b]</code>

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:

<code class="python">[x for x in a for a in b]</code>

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:

<code class="python">a = [[1, 2], [3, 4]]</code>

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

<code class="python">[x for x in a for a in b]</code>

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:

<code class="python">text = ((&quot;Hi&quot;, &quot;Steve!&quot;), (&quot;What's&quot;, &quot;up?&quot;))
gen = (word for sentence in text for word in sentence)</code>

The gen variable now yields the flattened list of words:

<code class="python">for word in gen:
    print(word)</code>

Output:

Hi
Steve!
What's
up?

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