Filtering out items from a list using nested list comprehensions in Python

Question

I have two lists. One contains sentences, the other contains words.

I want to have all the sentences, which do NOT contain any of the words from the list of words.

I'm trying to achieve this with list comprehensions. Example:

cleared_sentences = [sentence for sentence in sentences if banned_word for word in words not in sentence]

However, it doesn't seem to be working as I get an error telling me that a variable is used before assignment.

I've tried looking for nested comprehensions and I am sure this must have been asked for but I can not find anything.

How can I achieve this?

Martijn Pieters · Accepted Answer · 2013-07-27 09:27:56Z

3

You got the order mixed up:

[sentence for sentence in sentences for word in words if banned_word not in sentence]

Not that that'll work as that'll list the sentence every time a banned word does show up in the sentence. Take a look at the fully expanded nested loops version:

for sentence in sentences:
    for word in words:
        if banned_word not in sentence:
            result.append(sentence)

Use the any() function to test for banned words instead:

[sentence for sentence in sentences if not any(banned_word in sentence for banned_word in words)]

any() loops over the generator expression only until a True value is found; it'll stop doing work the moment a banned word is found in the sentence. This is more efficient at least.

edited Jul 27, 2013 at 9:27

answered Jul 27, 2013 at 9:22

Martijn Pieters

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1 Comment

Phil Over a year ago

I tried the list comprehension (the latter) and I still get an error. Is there any chance it might need to be .... any(banned_word in sentence for banned_word in words)?

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