Python. List comprehensions with not in and or
Question:
I have a list of strings that I want to filter:
A = ['enc_1', 'enc_2', 'enc_lag', 'lag_1', 'lag_2', 'price', 'price_std']
If I need strings that contain EITHER ‘enc’ OR ‘lag’, I can do the following:
[_ for _ in A if ('enc' in _) or ('lag' in _)]
Output: ['enc_1', 'enc_2', 'enc_lag', 'lag_1', 'lag_2']
Everything is fine. However, if I need strings that contain NEITHER ‘enc’ NOR ‘lag’, a seemingly obvious solution doesn’t work:
[_ for _ in A if ('enc' not in _) or ('lag' not in _)]
Output: ['enc_1', 'enc_2', 'lag_1', 'lag_2', 'price', 'price_std']
Judging by the result, I would expect an expression with AND to produce such an output (‘enc_lag’ would be removed), but for whatever reason OR does it instead. I am starting deeply questioning my understanding of OR and AND operators… Any help is appreciated!
Answers:
What you actually want is and here. If the element must contain neither 'enc' nor 'lag', then it must not contain 'enc' AND must not contain 'lag'.
[_ for _ in A if ('enc' not in _) and ('lag' not in _)]
Alternatively, by applying De Morgan’s law, we have:
[_ for _ in A if not (('enc' in _) or ('lag' in _))]
You just need an AND instead. You’re checking if every value:
- contains enc OR
- contains lag
So, every value that has enc in it (true) doesn’t have lag (false). which means your condition comes back true.
From what I understand you want it to return [‘price’, ‘price_std’] in the not statement.
In your case take for example ‘enc_1’, (‘enc’ not in _) = False but then you say OR (‘log’ not in _) it would be True. If False or True = True. so enc_1 is True.
solution: [_ for _ in A if (‘enc’ not in _ and ‘lag’ not in _)]
I have a list of strings that I want to filter:
A = ['enc_1', 'enc_2', 'enc_lag', 'lag_1', 'lag_2', 'price', 'price_std']
If I need strings that contain EITHER ‘enc’ OR ‘lag’, I can do the following:
[_ for _ in A if ('enc' in _) or ('lag' in _)]
Output: ['enc_1', 'enc_2', 'enc_lag', 'lag_1', 'lag_2']
Everything is fine. However, if I need strings that contain NEITHER ‘enc’ NOR ‘lag’, a seemingly obvious solution doesn’t work:
[_ for _ in A if ('enc' not in _) or ('lag' not in _)]
Output: ['enc_1', 'enc_2', 'lag_1', 'lag_2', 'price', 'price_std']
Judging by the result, I would expect an expression with AND to produce such an output (‘enc_lag’ would be removed), but for whatever reason OR does it instead. I am starting deeply questioning my understanding of OR and AND operators… Any help is appreciated!
What you actually want is and here. If the element must contain neither 'enc' nor 'lag', then it must not contain 'enc' AND must not contain 'lag'.
[_ for _ in A if ('enc' not in _) and ('lag' not in _)]
Alternatively, by applying De Morgan’s law, we have:
[_ for _ in A if not (('enc' in _) or ('lag' in _))]
You just need an AND instead. You’re checking if every value:
- contains enc OR
- contains lag
So, every value that has enc in it (true) doesn’t have lag (false). which means your condition comes back true.
From what I understand you want it to return [‘price’, ‘price_std’] in the not statement.
In your case take for example ‘enc_1’, (‘enc’ not in _) = False but then you say OR (‘log’ not in _) it would be True. If False or True = True. so enc_1 is True.
solution: [_ for _ in A if (‘enc’ not in _ and ‘lag’ not in _)]