Exceptions as a Control Structure

John J. Lee jjl at pobox.com
Tue Aug 10 22:50:54 CEST 2004


[Peter Hansen]
> Exceptions are also used by some as a mechanism for
> returning information from subroutines, though it's
> very likely the information will still be considered
> "exceptional" in some way (think of it as an out-of-band
> mechanism for returning special info).

[Olivier Parisy]
> Do you have some example of this in the standard library ?

Not sure if this is what Peter had in mind, but grep for 'ValueError'.


[Peter]
> While some people object on stylistic grounds (or even
> performance ones, in some cases) to such things, you
> will likely find that exceptions are thrown around
> by Python programmers more readily than you are used to.

[Olivier Parisy]
> That's what I wanted to know. OK.

Control-structure-y uses (esp. within a single function) are much
rarer than the ordinary use: a function signalling an exceptional
condition to its callers.  This ordinary use is certainly used more
extensively in idiomatic Python code than is the case in many other
languagesa.

StopIteration is used a lot, I guess, but it's rare that you're
actually concious of that fact.


John



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