can't delete a string from a list

Question

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 (ie, it couldn't find the string in the list).

Any ideas why this happens?

Answer 1

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) .

Answer 2

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

Answer 3

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"]