Slicing a list using a variable, in Python

问题

Given a list

a = range(10)

You can slice it using statements such as

a[1]
a[2:4]

However, I want to do this based on a variable set elsewhere in the code. I can easily do this for the first one

i = 1
a[i]

But how do I do this for the other one? I've tried indexing with a list:

i = [2, 3, 4]
a[i]

But that doesn't work. I've also tried using a string:

i = "2:4"
a[i]

But that doesn't work either.

Is this possible?

回答1:

that's what slice() is for:

a = range(10)
s = slice(2,4)
print a[s]

That's the same as using a[2:4].

回答2:

Why does it have to be a single variable? Just use two variables:

i, j = 2, 4
a[i:j]

If it really needs to be a single variable you could use a tuple.

回答3:

>>> a=range(10)
>>> i=[2,3,4]

>>> a[i[0]:i[-1]]
range(2, 4)

>>> list(a[i[0]:i[-1]])
[2, 3]

回答4:

With the assignments below you are still using the same type of slicing operations you show, but now with variables for the values.

a = range(10)
i = 2
j = 4

then

print a[i:j]
[2, 3]

来源：https://stackoverflow.com/questions/10573485/slicing-a-list-using-a-variable-in-python