The reduce built-in function takes the following arguments:

def reduce(
  <list>, # List to reduce
  <function>( # Reducing function
      acc=<any, mandatory>, # Accumulator
      val=<any, mandatory>, # Current value in iterator
      idx=<dec, optional>, # Current index, 0=first value
      idx_to_end=<dec, optional>, # Current index from end, 0=last value
  ) -> <any>,
  <any>, # Default value
):

And returns the final value of the accumulator after executing over every value in the list.

def red_fn(acc=0, val=0):
  return acc + val
return reduce([1,4,7,10], red_fn, 0) # 22

def red_fn(acc='', val='', idx=0, idx_to_end=0):
  if idx_to_end == 0:
    return acc + val['name'] + ' (' + str(idx+1) + ')'
  return acc + val['name'] + ' (' + str(idx+1) + '), '

return reduce([{"name": "John"}, {"name": "Bob"}, {"name": "Charles"}],
  red_fn, "") # 'John (1), Bob (2), Charles (3)'