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Regarding your main problem, you should not use a symbolic expression but a Python function:

sage: exp = lambda r : prod(1-2**(-i) for i in range(1,r-1))

Or, equivalently:

sage: def exp(r): 
....:     return prod(1-2**(-i) for i in range(1,r-1))

Then you can do:

sage: a = exp(12345)

which is a rational number with huge numerator and denominator, but you can get an approximation with:

sage: RDF(a)
0.2887880950866024

Regarding your side question, i did not see anything like that in Sage source code, so i guess you have to define such a symbolic function yourself.