How to frame two for loops in list comprehension python

python list for-loop list-comprehension
Shiva Krishna Bavandla picture Shiva Krishna Bavandla · Aug 31, 2013 · Viewed 109.2k times · Source

I have two lists as below

tags = [u'man', u'you', u'are', u'awesome']
entries = [[u'man', u'thats'],[ u'right',u'awesome']]

I want to extract entries from entries when they are in tags:

result = []

for tag in tags:
    for entry in entries:
        if tag in entry:
            result.extend(entry)

How can I write the two loops as a single line list comprehension?

Answer

Rohit Jain picture Rohit Jain · Aug 31, 2013

The best way to remember this is that the order of for loop inside the list comprehension is based on the order in which they appear in traditional loop approach. Outer most loop comes first, and then the inner loops subsequently.

So, the equivalent list comprehension would be:

[entry for tag in tags for entry in entries if tag in entry]

In general, if-else statement comes before the first for loop, and if you have just an if statement, it will come at the end. For e.g, if you would like to add an empty list, if tag is not in entry, you would do it like this:

[entry if tag in entry else [] for tag in tags for entry in entries]

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