On 12/13/2009 11:33 PM, exar...@twistedmatrix.com wrote:
But if you mistakenly don't catch it, and you're trying to debug your
code to find this mistake, you probably won't be aided in this pursuit
by the exception-swallowing behavior of generator expressions.
As I remember, it was the call to list that swalled the exception, not
the generator expression. List() takes an iterable as arg and stopping
on StopIteration is what it does and how it knows to stop and return the
new list.
The behavior of list comprehensions is pretty good. The behavior of
constructing a list out of a generator expression isn't as good.
I think you are confused. A generator expression is a shorthand for a
def statement that defines a generator function followed by a call to
the generator function to get a generator followed by deletion of the
function. When you call list() to make a list, it constructs the list
from the generator, not from the expression itself. List has no idea
that you used a generator expression or even that it was passed a
generator. Leaving error checks out, it operates something like
def list(it):
res = []
it = iter(it)
for item in it: # stops whenever it raises StopIteration
res.append(item)
return res
The
behavior which is more desirable is for a StopIteration raised out of
the `expression` part of a `generator_expression` to not be treated
identically to the way a StopIteration raised out of the `genexpr_for`
part is.
It is not. StopIteration in for part stops the for loop in the
generator. StopIteration in the expression part stops the loop in the
list() call (sooner than it would have been otherwise). When the
generator raises StopIteration, list() has no idea what statement within
the body raised it. It MUST stop.
This could provide behavior roughly equivalent to the behavior
of a list comprehension.
Impossible. The only serious option for consistency is to special case
list comps to also trap StopIteration raised in the expression part, but
the devs decided not to do this as doing do is arguably a bug.
Terry Jan Reedy
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