CODEWITHSUNDEEP
python
Cheshie
Cheshie

Reputation: 2837

can't delete a string from a list

I wrote the following code:

f = copy.deepcopy(features)
if best_att in features:
   f = features.remove(best_att)

where features is a list of strings and best_att is a string. Sometimes the code works fine, but sometimes I get the following problem:

If, for example, features = ['outlook', 'temperature', 'humidity', 'wind'], and best_att = 'outlook'. When debugging, I see that it enters the if. However, when trying to preform the remove, it gets an error: f is NoneType: None (i.e., it couldn't find the string in the list).

Any ideas why this happens?

Upvotes: 1

Views: 73

Answers (3)

BrianO
BrianO

Reputation: 1514

It should be:

f = copy.deepcopy(features)
if best_att in f:
    f.remove(best_att)

or

if best_att in features:
    features.remove(best_att)

It seems from your code that you want to change the original features, so just do that. If you want also want a deepcopy f of it, create that after modifying features.

You got that message because remove returns None: after you assign its return value to f, that's the value of f too. In general, Python builtin methods that mutate state "don't return anything" (they return None) -- they're "all side-effect". The return value is not the mutated object, and it is not a success/failure flag. Some languages draw the distinction between "procedures" and "functions"; these would be "procedures".

If you want to know whether, say, best_att is in features, use features.find(best_att) or features.index(best_att).

Upvotes: 3

Azsgy
Azsgy

Reputation: 3307

In python, lists are mutable, which you are probably aware of, since you are using deepcopy

If you call list.remove on f, it changes f and returns None

>>> f = ["spam", "eggs", "harald"]
>>> print(f.remove("harald"))
None
>>> print(f)
["spam", "eggs"]

Upvotes: 1

Eugene Yarmash
Eugene Yarmash

Reputation: 149776

L.remove returns None, which is then assigned to the original list.
help([].remove) is your friend.

Upvotes: 2

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