Dangerous behavior of list(generator)

[Tom Machinski]

On Wed, Dec 30, 2009 at 4:01 PM, Steven D'Aprano
<steve at remove-this-cybersource.com.au> wrote:
> On Wed, 30 Dec 2009 15:18:11 -0800, Tom Machinski wrote:
>> Bottom line, I'm going to have to remove this pattern from my code:
>>
>>   foo = (foo for foo in foos if foo.bar).next()
>
> 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.

Exactly; this seems innocuous, but if some caller of this code uses it
in a list() constructor, a very subtle and dangerous bug is introduced
- see OP. This is the entire point of this post.

In a large, non-trivial application, you simply cannot afford the
assumption that no caller will ever do that. Even if you have perfect
memory, some of your other developers or library users may not.

As for what's wrong with the "if not any" solution, Benjamin Kaplan's
post hits the nail on its head. This is a bioinformatics application,
so the iterable "foos" tends to be very large, so saving half the
runtime makes a big difference.

  -- Tom