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Symbolic function that sums over variable sequence

How do I define a symbolic function that takes a sum of variables as value?

I have tried the following:

k=var('k')
f(x)=sum(x[k], k, 1, 5)


But I get the following error:

TypeError: unable to convert k to an integer


I want to be able to symbolically differentiate f with regards to e.g. x[3].