Question

Can’t figure out how to do this in a pretty way :

I have a list of n elements,
I want to access every m elements of the list.

For example : [1, 2, 3, 4, 5] and m = 2 would give
[2, 4]

I can do it simply with a loop, but ins’t there a more “pythonic” way?

Thanks by advance !

EDIT :

Seems like I forgot something.
I want, not only get those values but modify them.
I tried slicing a[::2] = 3, but it doesn’t work. . .

I’m searching for something similar

Answer 1

Slicing syntax does this for you:

>>> my_list = range(10)
>>> my_list
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> my_list[::2]
[0, 2, 4, 6, 8]
>>> my_list[1::2]
[1, 3, 5, 7, 9]

Here’s a way to wrap a list to get the original assignment behavior you wanted, but I’m not sure I’d recommend it:

class AssignableSlice(list):
    def __setitem__(self, i, v):
        if isinstance(i, slice):
            for ii in xrange(*i.indices(len(self))):
                self[ii] = v
        else:
            super(AssignableSlice, self).__setitem__(i, v)

a = AssignableSlice(range(10))
print a
a[::2] = 3
print a
a[1::3] = 99
print a

produces:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[3, 1, 3, 3, 3, 5, 3, 7, 3, 9]
[3, 99, 3, 3, 99, 5, 3, 99, 3, 9]