Double Iteration in List Comprehension

Question

In Python you can have multiple iterators in a list comprehension, like

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

for some suitable sequences a and b. I'm aware of the nested loop semantics of Python's list comprehensions.

My question is: Can one iterator in the comprehension refer to the other? In other words: Could I have something like this:

[x for x in a for a in b] 

where the current value of the outer loop is the iterator of the inner?

As an example, if I have a nested list:

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

what would the list comprehension expression be to achieve this result:

[1,2,3,4] 

?? (Please only list comprehension answers, since this is what I want to find out).

Answer 1

Suppose you have a text full of sentences and you want an array of words.

# Without list comprehension list_of_words = [] for sentence in text:     for word in sentence:        list_of_words.append(word) return list_of_words 

I like to think of list comprehension as stretching code horizontally.

Try breaking it up into:

# List Comprehension  [word for sentence in text for word in sentence] 

Example:

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

This also works for generators

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