reduce with an initial value

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Accepted Answer

The third argument to reduce() is the initial accumulator value. Here the fold starts with 10: 10*2=20, 20*3=60, 60*4=240. Without an initial value, reduce uses the first element as the starting accumulator and begins folding from the second element. Providing an initial value is especially important when the input list might be empty - reduce on an empty list with no initial value raises TypeError, but with an initial value it simply returns that value.