reduce((lambda xyx )[ ] reduce((lambda xyx )[ ] import in xnot in at each stepreduce passes the current sum or productalong with the next item from the listto the passed-in lambda function by defaultthe first item in the sequence initializes the starting value to illustratehere' the for loop equivalent to the first of these callswith the addition hardcoded inside the loopl [ , , , res [ for in [ :]res res res coding your own version of reduce is actually fairly straightforward the following function emulates most of the built-in' behavior and helps demystify its operation in generaldef myreduce(functionsequence)tally sequence[ for next in sequence[ :]tally function(tallynextreturn tally myreduce((lambda xyx )[ ] myreduce((lambda xyx )[ ] the built-in reduce also allows an optional third argument placed before the items in the sequence to serve as default result when the sequence is emptybut we'll leave this extension as suggested exercise if this coding technique has sparked your interestyou might also be interested in the standard library operator modulewhich provides functions that correspond to builtin expressions and so comes in handy for some uses of functional tools (see python' library manual for more details on this module)import operatorfunctools functools reduce(operator add[ ]function-based functools reduce((lambda xyx )[ ] togethermapfilterand reduce support powerful functional programming techniques as mentionedmany observers would also extend the functional programming toolset in python to include nested function scope closures ( factory functionsand the anonymous function lambda--both discussed earlier--as well as generators and comprehensionstopics we will return to in the next functional programming tools