En Sat, 12 Dec 2009 23:43:20 -0300, Ned Deily <n...@acm.org> escribió:
In article <nad-8cdb63.18012412122...@news.gmane.org>,
Ned Deily <n...@acm.org> wrote:
In article
<ec96e1390912121653w56c3dbe3p859a7b979026b...@mail.gmail.com>,
Benjamin Kaplan <benjamin.kap...@case.edu> wrote:
> On Sat, Dec 12, 2009 at 7:15 PM, Tom Machinski
<tom.machin...@gmail.com>
> wrote:
> > >>> def sit(): raise StopIteration()
> > ...
> > >>> [f() for f in (lambda:1, sit, lambda:2)]
> > Traceback (most recent call last):
> > File "<stdin>", line 1, in <module>
> > File "<stdin>", line 1, in sit
> > StopIteration
> > >>> list(f() for f in (lambda:1, sit, lambda:2))
> > [1]
> > In most cases, `list(generator)` works as expected. Thus,
> > `list(<generator expression>)` is generally equivalent to
`[<generator
> > expression>]`.
> Actually, it's list(generator) vs. a list comprehension. I agree that
> it can be confusing, but Python considers them to be two different
> constructs.
I think nobody has addressed the OP arguments (as I understand them).
First, except the obvious outer delimiters (and some corner cases in 2.x,
fixed in Python 3), a list comprehension and a generator expression share
the same syntax: (x for x in some_values) vs [x for x in some_values].
Also, *almost* always, both list(<comprehension>) and [<comprehension>],
when evaluated, yield the same result [1]. *Almost* because StopIteration
is handled differently as the OP discovered: the list comprehension
propagates a StopIteration exception to its caller; the list constructor
swallows the exception and the caller never sees it.
Despite a promise in PEP 289, generator expressions semantics isn't
explained in detail in the language reference. I can't tell if the
difference is intentional, accidental, undocumented behavior, an
implementation accident, a bug, or what...
[1] <comprehension> being a syntactic construct like:
x**2 for x in range(5)
or:
f() for f in [lambda:1, sit, lambda:2]
--
Gabriel Genellina
--
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