Most pythonic and/or performant way to assign a single value to a slice?

Question

I want to assign a single value to a part of a list. Is there a better solution to this than one of the following? Maybe most performant but somehow ugly: ``` >>> l=[0,1,2,3,4,5] >>> for i in range(2,len(l)): l[i] = None >>> l [0, 1, None, None, None, None] ``` Concise (but I don't know if Python recognizes that no rearrangement of the list elements is necesssary): ``` >>> l=[0,1,2,3,4,5] >>> l[2:] = [None]*(len(l)-2) >>> l [0, 1, None, None, None, None] ``` Same caveat like above: ``` >>> l=[0,1,2,3,4,5] >>> l[2:] = [None for _ in range(len(l)-2)] >>> l [0, 1, None, None, None, None] ``` Not sure if using a library for such a trivial task is wise: ``` >>> import itertools >>> l=[0,1,2,3,4,5] >>> l[2:] = itertools.repeat(None,len(l)-2) >>> l [0, 1, None, None, None, None] ``` The problem that I see with the assignment to the slice (vs. the for loop) is that Python maybe tries to prepare for a change in the length of "l". After all, changing the list by inserting a shorter/longer slice…

Accepted Answer

I think there's no straight out of the box feature in Python to do this. I like your second approach, but keep in mind that there's a tradeoff between space and time. This is a very good reading recommended by @user4815162342: Python Patterns - An Optimization Anecdote.

Anyhow, if this is an operation you'll be performing eventually in your code, I think your best option is to wrap it inside a helper function:

```
def setvalues(lst, index=0, value=None):
    for i in range(index, len(lst)):
        lst[i] = value

>>>l=[1,2,3,4,5]
>>>setvalues(l,index=2)
>>>l
>>>[1, 2, None, None, None]
```

This has some advantages:

- The code is refactored inside a function, so easy to modify if you change your mind about how to perform the action.

- You can have several functions that accomplish the same target and therefore can measure their performance.

- You can write tests for them.

- Every other advantage you can get by refactoring :)

Since IMHO there's no straight Python future for this action, this is the best workaround I can imagine.

Hope this helps!

Most pythonic and/or performant way to assign a single value to a slice?

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