Creating custom "list" attributes

Question

I am trying to create functions for the property list. How do you create the attribute function for something like this?

list([1,2,3,4,5]).even()

should return:

[2,4]

The method should be easy but I am having trouble associating the function with the predefined object list.

Don't put links to secondary questions that only appear behind a login wall. If you have another question, then ask it. — Adam Smith
– Adam Smith, Commented May 28, 2015 at 22:59
No, i want the function call above to work. Apparently there is a solution but, based on the posts below, i do not think it is possible. The above is one test code. — Kharrid
– Kharrid, Commented May 28, 2015 at 23:07

score 4 · Accepted Answer · 2015-05-28 23:10:07Z

4

You cannot add methods or attributes to any of the built-in objects. This is by design.

Instead, you can create your own list type that is derived from the built-in one:

class MyList(list):
    def even(self):
        return [x for x in self if x % 2 == 0]

Demo:

>>> class MyList(list):
...     def even(self):
...         return [x for x in self if x % 2 == 0]
... 
>>> MyList([1,2,3,4,5]).even()
[2, 4]

For more information, see Classes in the documentation, specifically the section on Inheritance.

edited May 28, 2015 at 23:10

answered May 28, 2015 at 22:56

user2555451

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2 Comments

Padraic Cunningham Over a year ago

Or self[:] = (x for x in self if not x % 2) to change the original list,

user2555451 Over a year ago

@PadraicCunningham - Well, the OP said he wanted to have even() return [2, 4]. If the method mutated the list, then it should probably return None instead to be consistent with list.sort() or list.reverse(). That's why I made it return a new list.

Adam Smith · Accepted Answer · 2015-05-28 22:56:41Z

4

You can't monkeypatch built in objects in Python like you can in Ruby. You'd have to build a new object, inherit list, and put your method on that.

class MyList(list):
    def even(self):
        return [num for num in self if num % 2 == 0]

MyList([1,2,3,4,5]).even()

answered May 28, 2015 at 22:56

Adam Smith

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