docs on for-loop with no __iter__?

Alex Martelli aleaxit at
Tue Sep 7 19:45:15 CEST 2004

Steven Bethard <steven.bethard at> wrote:

> Alex Martelli <aleaxit <at>> writes:
> > It's partly a matter of "look before you leap" versus "easier to ask
> > forgiveness than permission", a conceptual distinction that IS quite a
> > hobby-horse of mine (although "practicality beats purity", mind you.
> Well, in either case, someone is looking before they leap.  If __len__ is
> checked in the protocol, then the protocol (i.e. the Python interpreter) has
> to do the looking.  Otherwise, the programmer has to do the looking to
> determine when to raise the IndexError.

If and only if __len__ is the ONLY sane way to tell, yes.  Such cases
are exceedingly rare in practice, as you point out:

> Hmm...  Though I guess it kinda depends what you do in your __getitem__...
> The example we've been looking at was something like:
> class S:
>     def __len__(self): return 42
>     def __getitem__(self, i):
>         if 0 <= i < len(self):
>             return i
>         raise IndexError, i
> So in this case, the programmer has to "look before they leap" (hence the if
> statement).  But in a more realistic situation, I can see that maybe you could
> just "ask forgivenesss instead of permission":
> class T:
>     def __init__(self, data): = data
>     def __len__(self): return len(
>     def __getitem__(self, i):
>         try:
>             return[i]
>         except (IndexError, KeyError):
>             raise IndexError, i
> No look-ahead here -- assume you'll usually get valid indices and catch the
> exception if you don't.

Right, except you don't have to intercept and reraise IndexError, just
let it propagate (if KeyError is meaningfully possible and excepted then
yes, you do have to catch and translate _that_ one).


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