how to sum sums

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I am new to sagemath and I am trying to calculate some formulas using it.

I learned how to write a sum such as

sage: a = [2, 2, 3, 4] 
sage: d = [2, 4, 5, 6] 
sage: def pi(i): 
....:      var("i")
....:      f = sum([a[k]*d[k] for k in range(i + 1)])
....:      return f

However, I am having difficulties to write a sum for pi(i) such as

sage: def Z(a, b): 
....:      var("a b")
....:      l = len(d) - 1 
....:      def pi(i):
....:          var("i")
....:          f = sum([a[k]*d[k] for k in range(i + 1)])
....:          return f
....:      h = sum([pi(i) for i in range(l - 1)])
....:      return h

but I got error. I tried to use

for i in range(l - 1):

also it doesn't work.