Different behaviour in list comps and generator expressions

Wolfgang Maier wolfgang.maier at biologie.uni-freiburg.de
Sat Nov 8 22:31:23 CET 2014


On 08.11.2014 02:50, Steven D'Aprano wrote:
> The following list comprehension and generator expression are almost, but
> not quite, the same:
>
> [expr for x in iterable]
>
> list(expr for x in iterable)
>
>
> The difference is in the handling of StopIteration raised inside the expr.
> Generator expressions consume them and halt, while comprehensions allow
> them to leak out.

This is not the right description of what's happening. It is not the 
generator expression that consumes it, but the list constructor and, of 
course, list can't tell at which level in the inside code StopIteration 
got raised.
So, yes this had me confused some times, too, but it is really not 
surprising.

> A simple example:
>
> iterable = [iter([])]
> list(next(x) for x in iterable)
> => returns []
>
> But:
>
> [next(x) for x in iterable]
> => raises StopIteration
>

Yes, but the equivalent to list(next(x) for x in iterable) is:

l = []
for item in (next(x) for x in iterable):
     l.append(item)

=> returns [] because the for consumes the StopIteration

this, on the other hand raises StopIteration

l = []
for item in [next(x) for x in iterable]:
     l.append(item)

because the error gets raised already when for causes iter() to be 
called on the list comprehension.

>
> Has anyone come across this difference in the wild? Was it a problem? Do you
> rely on that difference, or is it a nuisance? Has it caused difficulty in
> debugging code?
>
> If you had to keep one behaviour, which would you keep?
>

The point is: there is no difference in the behavior of comprehensions 
and generator expressions, it is just how you access them:
for anything that follows the iterator protocol, a comprehension (since 
it is evaluated immediately) raises during the iter() phase, while a 
generator expression (which gets evaluated lazily) raises during the 
next() phase where it gets swallowed.

So in your example you would have to use the silly

list([next(x) for x in iterable])

if you want the error to get raised.

I agree this is all rather non-intuitive, but how would you change it ?





More information about the Python-list mailing list