On Feb 10, 7:58 pm, Steven D'Aprano <ste...@remove.this.cybersource.com.au> wrote: > I sometimes write recursive functions like this simple factorial: > > def fact(n): > if n < 0: raise ValueError > if n = 0: return 1 > return fact(n-1)*n > > At the risk of premature optimization, I wonder if there is an idiom for > avoiding the unnecessary test for n <= 0 in the subsequent recursive > calls? For the sake of the argument, let's pretend the test is expensive > and I have a good reason for wanting to avoid it on subsequent calls :) > > I've done this: > > def _fact(n): > if n = 0: return 1 > return _fact(n-1)*n > > def fact(n): > if n < 0: raise ValueError > return _fact(n) > > but that's ugly. What else can I do? > > -- > Steven
Build a list of function calls, and just replace the base case with a terminating call. >>> def f( n ): ... def rec( i ): ... return i* funcs[ i- 1 ]( i- 1 ) ... def base( i ): ... return 1 ... funcs= [ rec ]* n ... funcs[ 0 ]= base ... return rec( n ) ... >>> f( 5 ) 120 >>> f( 6 ) 720 >>> f( 1 ) 1 -- http://mail.python.org/mailman/listinfo/python-list
