Python list subtraction operation – Dev

The best answers to the question “Python list subtraction operation” in the category Dev.

QUESTION:

I want to do something similar to this:

>>> x = [1,2,3,4,5,6,7,8,9,0]  
>>> x  
[1, 2, 3, 4, 5, 6, 7, 8, 9, 0]  
>>> y = [1,3,5,7,9]  
>>> y  
[1, 3, 5, 7, 9]  
>>> y - x   # (should return [2,4,6,8,0])

But this is not supported by python lists
What is the best way of doing it?

ANSWER:

Use set difference

>>> z = list(set(x) - set(y))
>>> z
[0, 8, 2, 4, 6]

Or you might just have x and y be sets so you don’t have to do any conversions.

ANSWER:

Use a list comprehension:

[item for item in x if item not in y]

If you want to use the - infix syntax, you can just do:

class MyList(list):
    def __init__(self, *args):
        super(MyList, self).__init__(args)

    def __sub__(self, other):
        return self.__class__(*[item for item in self if item not in other])

you can then use it like:

x = MyList(1, 2, 3, 4)
y = MyList(2, 5, 2)
z = x - y   

But if you don’t absolutely need list properties (for example, ordering), just use sets as the other answers recommend.

ANSWER:

That is a “set subtraction” operation. Use the set data structure for that.

In Python 2.7:

x = {1,2,3,4,5,6,7,8,9,0}
y = {1,3,5,7,9}
print x - y

Output:

>>> print x - y
set([0, 8, 2, 4, 6])

ANSWER:

if duplicate and ordering items are problem :

[i for i in a if not i in b or b.remove(i)]

a = [1,2,3,3,3,3,4]
b = [1,3]
result: [2, 3, 3, 3, 4]