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for k in range(2,n + 2,2):
    print(eval("P_%s"%k))

Ugly, I admit. It's hard to just stick in these things. Niles' solution is better.

for k in range(2,n + 2,2):
    print(eval("P_%s"%k))

Ugly, I admit. It's hard to just stick in these things. Niles' solution is better.

Edit: But if you insist (I note you also used print in your original example, so it was hard to anticipate your next issue)...

sage: n = 5
sage: P = list(var('P_%d' % i) for i in range(2*n + 1))
sage: for k in range(2,n + 2,2):
....:     tem = eval('P_%s' %k)
....:     x.subs(x=tem)
....:     
P_2
P_4
P_6