Python: Removing entries from ordered list, that are not in unordered list

Question

I have two lists:

ordered = ['salat', 'baguette', 'burger', 'pizza']
unordered = ['pizza', 'burger']

Now I want to remove all entries from the ordered list, that are not in the unordered list while preserving the ordering.

How can I do this?

Answer 1

ordered = [item for item in ordered if item in unordered]

This method creates a new list based on the old ones using Python's list comprehension.

For large amounts of data, turning the unordered list into a set first, as people suggested in comments, makes a huge difference in performance, eg:

unordered = set(unordered)

---

Benchmark!

ordered : 5000 items, unordered : 1000 items
0.09561s without set
0.00042s with set

For 10/2 items the time is almost the same, so it's good to always use a set, no matter what the data size is.

Answer 2

Better use a set for testing membership, like this:

ordered = ['salat', 'baguette', 'burger', 'pizza']
unordered = ['pizza', 'burger']

unord = set(unordered)
ordered = [e for e in ordered if e in unord]

Answer 3

Something like this:

ordered = list(filter(lambda x: x not in unordered, ordered))

The list function is unnecessary if using Python < 3.