Shane Hathaway wrote:
> I'm interested in seeing a good way to write tail calls in Python. Some
> algorithms are more readable when expressed using tail recursion.
About ten seconds after I wrote the previous message, I realized two things:
- It's easy to write "return Return" instead of "raise Return". So
"raise TailCall" is probably better.
- I can write a complete implementation of this idea with nothing but a
simple decorator. Check it out!
Shane
class TailCall(Exception):
def __init__(self, f, *args, **kwargs):
self.f = f
self.args = args
self.kwargs = kwargs
def has_tail_calls(f):
def tail_call_wrapper(*args, **kwargs):
try:
return f(*args, **kwargs)
except TailCall, e:
return e.f(*e.args, **e.kwargs)
tail_call_wrapper.__doc__ = f.__doc__
return tail_call_wrapper
@has_tail_calls
def fact2(n, v=1):
"""
>>> fact2(1)
1
>>> fact2(2)
2
>>> fact2(3)
6
>>> fact2(4)
24
>>> fact2(20)
2432902008176640000L
"""
if not n:
return v
else:
raise TailCall(fact2, n - 1, v * n)
if __name__ == '__main__':
import doctest
doctest.testmod()
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