Building periodic sequences from finite-support sequences Given a discrete-time finite-support signal x[n]

$$x[n] = \left\{ {\begin{array}{*{20}{l}}
{{{( - 1)}^n}n}&amp;{{\rm{ }}n = 1,2,3}\\
0&amp;{{\rm{otherwise}}}
\end{array}} \right.$$

And consider also its periodic repetition is 

$$y[n] = \sum\nolimits_{k =  - \infty }^\infty  {x[n + 7k]} $$

How we can generate the y[n] in Matlab or Python?

Here is piece of code to generate and plot x[n]. But for Y[n] I have no idea!

% finite support signal
n = -5:1:8;
x = zeros(size(n));
x( find(n==1 | n==2 | n==3) ) = [(-1)^1*1 (-1)^2*2 (-1)^3*3];
stem(n, x, 'filled')
Thanks to @Florian's comments, here is sample solution

clc, clear
n = 0:1:7*10^6-1;
x = zeros(size(n));
x( find(n==1 | n==2 | n==3) ) = [(-1)^1*1 (-1)^2*2 (-1)^3*3];
y = x(mod(n,7)+1);
y = sum(reshape(y,7,[]),2)'


For large $k$s we have for example: y = 0    -1000000     2000000    -3000000           0           0           0.