Trampolined-style (workaround of tail recursion) http://lifegoo.pluskid.org/?p=327

output.txt

calling tramp_fac(n=5, k=1)
bouncer: ('bounce', 'tramp_fac', (4, 5))
calling tramp_fac(n=4, k=5)
bouncer: ('bounce', 'tramp_fac', (3, 20))
calling tramp_fac(n=3, k=20)
bouncer: ('bounce', 'tramp_fac', (2, 60))
calling tramp_fac(n=2, k=60)
bouncer: ('bounce', 'tramp_fac', (1, 120))
calling tramp_fac(n=1, k=120)
bouncer: ('bounce', 'tramp_fac', (0, 120))
calling tramp_fac(n=0, k=120)
bouncer: ('land', 120)
120

tramp.py

def bounce(func, *args):
    return ('bounce', func, args)


def land(value):
    return ('land', value)


def pogo(bouncer):
    ''' Keep bouncing until landing. '''
    while True:
        print 'bouncer:',
        if bouncer[0] == 'land':
            print bouncer
            return bouncer[1]
        else:  # 'bounce'
            print (bouncer[0], bouncer[1].func_name, bouncer[2])
            bouncer = bouncer[1](*bouncer[2])


def tramp_fac(n, k=1):
    print 'calling tramp_fac(n={}, k={})'.format(n, k)
    if n == 0:
        return land(k)
    else:
        return bounce(tramp_fac, n - 1, k * n)


print pogo(tramp_fac(5))