Nested loop limit?

Michael Hudson mwh at
Fri Jul 9 15:49:17 CEST 2004

Peter Hansen <peter at> writes:

> Nelson Minar wrote:
> > Why does CPython have a limit of 21 nested blocks?
> > I'm never going to write code this deeply nested by hand, but I
> > could
> > imagine writing a program that does. It's also sort of a weird limit.
> Is the fact that the limit is actually 20 less weird?  (The 21 was
> where "n" was after it raised an exception, not the last legal
> amount of indentation.)
> And I think the answer is probably something like "CPython,
> being written in C, tends to have certain hard-coded limits
> much like C programs always do, rather than being nice and
> flexible like programs written with more sophisticated languages
> like, say, Python."  :-)

Spot on.  This has to do with the 'blockstack', very much an internal
detail to Python's implementation.  We'd like to get rid of it (*not*
because we want to let people write code with more than 20 nested for
loops :-) but it's not especially easy (finally: blocks are the
biggest problem).


  I really hope there's a catastrophic bug in some future e-mail
  program where if you try and send an attachment it cancels your
  ISP account, deletes your harddrive, and pisses in your coffee
                                                         -- Adam Rixey

More information about the Python-list mailing list