Fix Python – Does Python optimize tail recursion?

Question

Asked By – Jordan Mack

I have the following piece of code which fails with the following error:

RuntimeError: maximum recursion depth exceeded

I attempted to rewrite this to allow for tail recursion optimization (TCO). I believe that this code should have been successful if a TCO had taken place.

def trisum(n, csum):
    if n == 0:
        return csum
    else:
        return trisum(n - 1, csum + n)

print(trisum(1000, 0))

Should I conclude that Python does not do any type of TCO, or do I just need to define it differently?

Now we will see solution for issue: Does Python optimize tail recursion?


Answer

No, and it never will since Guido van Rossum prefers to be able to have proper tracebacks:

Tail Recursion Elimination (2009-04-22)

Final Words on Tail Calls (2009-04-27)

You can manually eliminate the recursion with a transformation like this:

>>> def trisum(n, csum):
...     while True:                     # Change recursion to a while loop
...         if n == 0:
...             return csum
...         n, csum = n - 1, csum + n   # Update parameters instead of tail recursion

>>> trisum(1000,0)
500500

This question is answered By – John La Rooy

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0