[Python-Dev] Proper tail recursion

[Michael Chermside]

I wrote:
> The recursion limit (the value of it anyhow) is an implementation detail.

Martin replies:
> No, it is not. In standard Python, the program
>
> def rec(n):
>    rec(n-1)
>
> n=1
> while 1:
>    try:
>      rec(n)
>    except RuntimeError:
>      break
> print "The recursion limit is", n
>
> will terminate. In the modified Python, it will not terminate.
> It is a change in behaviour, and thus a language feature.

First, I'm going to make a slight modification to your program, since I
want a version that actually does display the recursion limit. I tried
the following program instead:

    >>> def rec(n):
    ...     print n
    ...     rec(n+1)
    ...
    >>> rec(1)

Running this in Jython 2.1 (a well-known implementation of the Python
language) produced the numbers from 1 to 888, then immediately exited.

Running it on CPython 2.3 produced the numbers from 1 to 999, then a
traceback that looked like this:
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 3, in rec
      File "<stdin>", line 3, in rec
      File "<stdin>", line 3, in rec
      File "<stdin>", line 3, in rec
      File "<stdin>", line 3, in rec
      File "<stdin>", line 3, in rec
  [... etc for a total of about 1000 lines ...]
      File "<stdin>", line 3, in rec
      File "<stdin>", line 3, in rec
      File "<stdin>", line 3, in rec
      File "<stdin>", line 3, in rec
    RuntimeError: maximum recursion depth exceeded

It then returned me to the interactive prompt.

I presume that since the behavior was different, CPython must not
actually be a correct implementation of the Python language, right?

Not-all-differences-are-language-differences lly yours,

Michael Chermside