Dangerous behavior of list(generator)

[Benjamin Kaplan]

On Wed, Dec 30, 2009 at 7:01 PM, Steven D'Aprano
<steve at remove-this-cybersource.com.au> wrote:
>
> I don't see why. What's wrong with it? Unless you embed it in a call to
> list, or similar, it will explicitly raise StopIteration as expected.
>
>
>> I used to have that a lot in cases where not finding at least one valid
>> foo is an actual fatal error.
>
> What's wrong with the obvious solution?
>
> if not any(foo for foo in foos if foo.bar):
>    raise ValueError('need at least one valid foo')

That would require 2 iterations through foos- once in the test, once
for the assignment if successful. If foos takes a long time to iterate
through, it might be faster to put a try-except around the original
statement, catch the StopIteration, and raise a ValueError in its
place. Which I agree is much better practice than letting the
StopIteration signal the fatal error.