# StopIteration in the if clause of a generator expression

Discussion in 'Python' started by Peter Otten, Apr 1, 2005.

1. ### Peter OttenGuest

To confuse a newbies and old hands alike, Bengt Richter wrote:

> Need something more straightforward, e.g., a wrapped one-liner:
>
> >>> def guess(n=3): print ("You're right!", 'No more tries for
> >>> you!!!')[n-1 in

> ... (x for x in xrange(n) for t in [raw_input('Guess my name:
> ')=='Ben']
> ... if not t or iter([]).next())]
> ...
> >>> guess()

To make it a bit clearer, a StopIteration raised in a generator expression
silently terminates that generator:

>>> def stop(): raise StopIteration

....
>>> list(i for i in range(10) if i < 5 or stop())

[0, 1, 2, 3, 4]

In a list comprehension, on the other hand, it is propagated:

>>> [i for i in range(10) if i < 5 or stop()]

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 1, in stop
StopIteration

Is that an intentional difference?

Peter

Peter Otten, Apr 1, 2005

2. ### Carl BanksGuest

Peter Otten wrote:
> To confuse a newbies and old hands alike, Bengt Richter wrote:
>
> > Need something more straightforward, e.g., a wrapped one-liner:
> >
> > >>> def guess(n=3): print ("You're right!", 'No more tries for
> > >>> you!!!')[n-1 in

> > ... (x for x in xrange(n) for t in [raw_input('Guess my name:
> > ')=='Ben']
> > ... if not t or iter([]).next())]
> > ...
> > >>> guess()

>
> To make it a bit clearer, a StopIteration raised in a generator

expression
> silently terminates that generator:
>
> >>> def stop(): raise StopIteration

> ...
> >>> list(i for i in range(10) if i < 5 or stop())

> [0, 1, 2, 3, 4]
>
> In a list comprehension, on the other hand, it is propagated:
>
> >>> [i for i in range(10) if i < 5 or stop()]

> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> File "<stdin>", line 1, in stop
> StopIteration
>
> Is that an intentional difference?

Very interesting. I'm not sure if the designers even considered this
particular subtlety. Why it happens is pretty plain. In the generator
expression case, the generator expression does propogate the
StopIteration, but list() traps it. List comprehensions are internally
treated as a for-loop (kind of), which doesn't trap StopIteration.
Maybe it should.

The list comprehension [ x for x in y ] is currently treated as
equivalent to the following, with byte-code optimizations:

.. _ = []
.. for x in y:
.. _.append(x)

Perhaps it ought to be equivalent to:

.. _ = []
.. try:
.. for x in y:
.. _.append(x)
.. except StopIteration:
.. pass

However, I would guess the Python gods wouldn't approve of this use of
StopIteration, and so would make no sacrifices to get it.
Nevertheless, it seems likely to be how a list comprehension would
behave in Python 3.0, so maybe we should do it.

--
CARL BANKS

Carl Banks, Apr 1, 2005

3. ### Raymond HettingerGuest

[Peter Otten]
> a StopIteration raised in a generator expression
> silently terminates that generator:
>
> >>> def stop(): raise StopIteration

> ...
> >>> list(i for i in range(10) if i < 5 or stop())

> [0, 1, 2, 3, 4]
>
> In a list comprehension, on the other hand, it is propagated:
>
> >>> [i for i in range(10) if i < 5 or stop()]

> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> File "<stdin>", line 1, in stop
> StopIteration
>
> Is that an intentional difference?

I would call it an unfortunate assymmetry -- one the never comes up unless
you're up to no good ;-)

In a way, both behave identically. They both raise StopIteration. In the case
of the generator expression, that StopIteration is intercepted by the enclosing
list() call. That becomes obvious if you write a pure python equivalent for
list:

def lyst(s):
it = iter(s)
result = []
try:
while 1:
result.append(it.next())
except StopIteration: # guess who trapped StopIter
return result

Raymond Hettinger

Raymond Hettinger, Apr 1, 2005
4. ### Bengt RichterGuest

On Fri, 01 Apr 2005 16:34:32 GMT, "Raymond Hettinger" <> wrote:

>[Peter Otten]
>> a StopIteration raised in a generator expression
>> silently terminates that generator:
>>
>> >>> def stop(): raise StopIteration

>> ...
>> >>> list(i for i in range(10) if i < 5 or stop())

>> [0, 1, 2, 3, 4]
>>
>> In a list comprehension, on the other hand, it is propagated:
>>
>> >>> [i for i in range(10) if i < 5 or stop()]

>> Traceback (most recent call last):
>> File "<stdin>", line 1, in ?
>> File "<stdin>", line 1, in stop
>> StopIteration
>>
>> Is that an intentional difference?

>
>I would call it an unfortunate assymmetry -- one the never comes up unless
>you're up to no good ;-)

;-)
>
>In a way, both behave identically. They both raise StopIteration. In the case
>of the generator expression, that StopIteration is intercepted by the enclosing
>list() call. That becomes obvious if you write a pure python equivalent for
>list:
>
> def lyst(s):
> it = iter(s)
> result = []
> try:
> while 1:
> result.append(it.next())
> except StopIteration: # guess who trapped StopIter
> return result
>
>

I assumed that all standard sequence consumers (including list, of course) would intercept
the StopIteration of a sequence given them in the form of a generator expression, so your
lyst example would have an analogue for other sequence consumers as well, right?
I.e., there's not a hidden list(genex) in those others I would hope ;-)

E.g., "in" in my toy exposed more clearly, using Peter's stop:

>>> def show(x): print x,; return x

...
>>> def stop(): raise StopIteration

...
>>> 2 in (x for x in xrange(5) if show(x)<4 or stop())

0 1 2
True
>>> 7 in (x for x in xrange(5) if show(x)<4 or stop())

0 1 2 3 4
False

BTW I notice that this also nicely shortcuts when the 2 is found.

Regards,
Bengt Richter

Bengt Richter, Apr 1, 2005
5. ### Raymond HettingerGuest

> I assumed that all standard sequence consumers (including list, of course)
would intercept
> the StopIteration of a sequence given them in the form of a generator

expression, so your
> lyst example would have an analogue for other sequence consumers as well,

right?
> I.e., there's not a hidden list(genex) in those others I would hope ;-)

Right.

> E.g., "in" in my toy exposed more clearly, using Peter's stop:
>
> >>> def show(x): print x,; return x

> ...
> >>> def stop(): raise StopIteration

> ...
> >>> 2 in (x for x in xrange(5) if show(x)<4 or stop())

> 0 1 2
> True
> >>> 7 in (x for x in xrange(5) if show(x)<4 or stop())

> 0 1 2 3 4
> False
>
> BTW I notice that this also nicely shortcuts when the 2 is found.

That's a fact.

Raymond

Raymond Hettinger, Apr 2, 2005
6. ### jfjGuest

Peter Otten wrote:
> To confuse a newbies and old hands alike, Bengt Richter wrote:

got me for one

>
> To make it a bit clearer, a StopIteration raised in a generator expression
> silently terminates that generator:

*any* exception raised from a generator, terminates the generator

jfj

jfj, Apr 2, 2005
7. ### Peter OttenGuest

Raymond Hettinger wrote:

(quoting Bengt)
>> I assumed that all standard sequence consumers (including list, of
>> course) would intercept the StopIteration of a sequence given them in the
>> form of a generator expression, so your lyst example would have an
>> analogue for other sequence consumers as well, right?
>> I.e., there's not a hidden list(genex) in those others I would hope ;-)

>
> Right.

I see I followed the historical evolvement and saw generator expressions as
a lazy listcomp rather than a cool new way to write a generator. That
turned out to be the road to confusion.

Thanks Carl, thanks Raymond for setting me straight.

> I would call it an unfortunate assymmetry -- one the never comes up unless
> you're up to no good ;-)

Do you see any chance that list comprehensions will be redefined as an
alternative spelling for list(<generator expression>)?

Peter

Peter Otten, Apr 3, 2005
8. ### Peter OttenGuest

jfj wrote:

>> To make it a bit clearer, a StopIteration raised in a generator
>> expression silently terminates that generator:

>
> *any* exception raised from a generator, terminates the generator

Yeah, but StopIteration is the only expected exception and therefore the
only one that client code (nearly) always knows to deal with:

>>> def choke(): raise ValueError

....
>>> list(i for i in range(10) if i < 3 or choke())

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 1, in <generator expression>
File "<stdin>", line 1, in choke
ValueError
>>> [i for i in range(10) if i < 3 or choke()]

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 1, in choke
ValueError

Here you can *not* tell apart list(genexp) and listcomp.

(Of course, as has since been pointed out, the StopIteration is actually
caught in the list constructor, so nothing magic to the example in my
initial post)

Peter

Peter Otten, Apr 3, 2005
9. ### Raymond HettingerGuest

[Peter Otten]
> Do you see any chance that list comprehensions will be redefined as an
> alternative spelling for list(<generator expression>)?

Not likely. It is possible that the latter spelling would make it possible for
Py3.0. eliminate list comps entirely. However, they are very popular and
practical, so my bet is that they will live on.

The more likely change is that in Py3.0 list comps will no longer expose the
loop variable outside the loop.

Raymond Hettinger

Raymond Hettinger, Apr 3, 2005
10. ### Steven BethardGuest

Raymond Hettinger wrote:
> [Peter Otten]
>
>>Do you see any chance that list comprehensions will be redefined as an
>>alternative spelling for list(<generator expression>)?

>
> Not likely. It is possible that the latter spelling would make it possible for
> Py3.0. eliminate list comps entirely. However, they are very popular and
> practical, so my bet is that they will live on.

I suspect you're right, but I certainly wouldn't complain if list comps
disappeared. TOOWTDI and all, and I often find myself alternating
between the two when I can't decide which one seems more Pythonic.
(These days I generally write a listcomp, but I wouldn't put any money

STeVe

Steven Bethard, Apr 3, 2005
11. ### Guest

This is all just making everything far too complicated. What you really
want to do is quite simple:

import itertools
def condition(x): return x < 5

list(itertools.takewhile(condition, (i for i in range(10))))

The 'Stop Iteration In Generator Expression' problem was solved in the
library, prelude, had a series of functions that have found their way
into the itertools toolbox. I highly recommend having a read of the
itertools docs if you want to continue hacking around with generators.

Regards,
Stephen Thorne

, Apr 3, 2005
12. ### Raymond HettingerGuest

[Steven Bethard]
> and I often find myself alternating
> between the two when I can't decide which one seems more Pythonic.

Both are pythonic.

Use a genexp when you need a generator
and use a listcomp when you need a list.

Raymond Hettinger

Raymond Hettinger, Apr 4, 2005
13. ### Steven BethardGuest

Raymond Hettinger wrote:
> [Steven Bethard]
>
>>and I often find myself alternating
>>between the two when I can't decide which one seems more Pythonic.

>
> Both are pythonic.
>
> Use a genexp when you need a generator
> and use a listcomp when you need a list.

So do I read this right in preferring
[<x> for <y> in <z>]
over
list(<x> for <y> in <z>)
?

STeVe

Steven Bethard, Apr 4, 2005
14. ### Raymond HettingerGuest

> > Use a genexp when you need a generator
> > and use a listcomp when you need a list.

[Steven Bethard]
> So do I read this right in preferring
> [<x> for <y> in <z>]
> over
> list(<x> for <y> in <z>)

Yes!

Raymond

Raymond Hettinger, Apr 5, 2005
15. ### Simon BrunningGuest

On Apr 5, 2005 2:04 AM, Raymond Hettinger <> wrote:
> [Steven Bethard]
> > So do I read this right in preferring
> > [<x> for <y> in <z>]
> > over
> > list(<x> for <y> in <z>)

>
> Yes!

Why? (Serious question. I'm sure that you have a good reason - I just
can't figure out what it is.)

The generator expression has the advantage of not leaking references
into the enclosing namespace. What's advantage of the list comp?

--
Cheers,
Simon B,
,
http://www.brunningonline.net/simon/blog/

Simon Brunning, Apr 5, 2005
16. ### Duncan BoothGuest

Simon Brunning wrote:

> On Apr 5, 2005 2:04 AM, Raymond Hettinger <> wrote:
>> [Steven Bethard]
>> > So do I read this right in preferring
>> > [<x> for <y> in <z>]
>> > over
>> > list(<x> for <y> in <z>)

>>
>> Yes!

>
> Why? (Serious question. I'm sure that you have a good reason - I just
> can't figure out what it is.)
>
> The generator expression has the advantage of not leaking references
> into the enclosing namespace. What's advantage of the list comp?
>

The list comprehension is about 15-20% faster according to timeit.py:

C:\Python24\Lib>..\python.exe timeit.py -s "t = range(1000)" "[ x for x in t]"
10000 loops, best of 3: 116 usec per loop

C:\Python24\Lib>..\python.exe timeit.py -s "t = range(1000)" "list(x for x in t)"
1000 loops, best of 3: 144 usec per loop

C:\Python24\Lib>..\python.exe timeit.py -s "t = range(100000)" "[ x for x in t]"
10 loops, best of 3: 13.9 msec per loop

C:\Python24\Lib>..\python.exe timeit.py -s "t = range(100000)" "list(x for x in t)"
10 loops, best of 3: 16.3 msec per loop

Alternatively you could just regard the list comprehension as having
less clutter on the screen so it may be clearer.

Duncan Booth, Apr 5, 2005
17. ### Raymond HettingerGuest

> > [Steven Bethard]
> > > So do I read this right in preferring
> > > [<x> for <y> in <z>]
> > > over
> > > list(<x> for <y> in <z>)

[Raymond Hettinger]
> > Yes!

[Simon Brunning]
> Why? (Serious question. I'm sure that you have a good reason - I just
> can't figure out what it is.)
>
> The generator expression has the advantage of not leaking references
> into the enclosing namespace. What's advantage of the list comp?

One advantage relates to mental parsing and chunking.
A list comp reads as a single step: "make a list".
The genexp form reads as "make a generator and turn it into a list."

The listcomp form has slightly more economy of expression (it is succinct) and
the brackets are a nice visual cue that may save a neuron or two.

Another advantage is that listcomps are older. They tend to be better
understood already. And, they run on older pythons.

The design rule, "use listcomps to make lists and genexps to make generators",
encourages data centric thinking.With being distracting, it helps maintain an
awareness of whether you're filling memory or
generating elements one-at-a-time.

Internally, there are performance differences favoring listcomps when the
desired output is a list. List comprehensions are interpreted immediately
through syntax rather than a global lookup of the list() builtin. The code for
listcomps does not have to create, switch between, and destroy a separate
stackframe. The compiler creates custom code for list comps that takes
advantage of the new LIST_APPEND opcode.

Partially balancing out all of the above are some small advantages for the
list(somegen) form. It lets you forget about listcomps and it works with other
contructors, deque(somegen) or set(somegen) for example.

To my tastes, the net balance favors using listcomps whenever you need to create
a list.

Raymond Hettinger

Raymond Hettinger, Apr 5, 2005