Combining multiple for loops in Python

Question

Let's say, we have and list of objects in variable called "articles", each object has a member "tags" (which is simple list).

Expected output: all tags in all articles, joined in a single list.

In multiple lines, solution would be:

arr = []
for article in articles:
    for tag in article.tags:
       arr.append(tag)

Now, how can we write this in single line instead of 4?

This syntax is invalid:

arr = [tag for tag in article.tags for article in articles]

Thanks!

Answer 1

It's invalid because article doesn't exist by the time the first loop is parsed.

arr = [tag for article in articles for tag in article.tags]