Replacing selected elements in a list in Python

Question

I have a list: mylist = [0, 0, 0, 0, 0]

I only want to replace selected elements, say the first, second, and fourth by a common number, A = 100.

One way to do this:

mylist[:2] = [A]*2
mylist[3] = A
mylist
[100, 100, 0, 100, 0]

I am looking for a one-liner, or an easier method to do this. A more general and flexible answer is preferable.

Answer 1

Especially since you're replacing a sizable chunk of the list, I'd do this immutably:

mylist = [100 if i in (0, 1, 3) else e for i, e in enumerate(mylist)]

It's intentional in Python that making a new list is a one-liner, while mutating a list requires an explicit loop. Usually, if you don't know which one you want, you want the new list. (In some cases it's slower or more complicated, or you've got some other code that has a reference to the same list and needs to see it mutated, or whatever, which is why that's "usually" rather than "always".)

If you want to do this more than once, I'd wrap it up in a function, as Volatility suggests:

def elements_replaced(lst, new_element, indices):
    return [new_element if i in indices else e for i, e in enumerate(lst)]

I personally would probably make it a generator so it yields an iteration instead of returning a list, even if I'm never going to need that, just because I'm stupid that way. But if you actually do need it:

myiter = (100 if i in (0, 1, 3) else e for i, e in enumerate(mylist))

Or:

def elements_replaced(lst, new_element, indices):
    for i, e in enumerate(lst):
        if i in indices:
            yield new_element
        else:
            yield e