Testing for empty iterators?

[Roy Smith]

In the old days, if I wanted to return a sequence of items, I'd return a
list, and loop over it like this:

for thing in getListOfThings ():
do something

With iterators, I'm doing:

for thing in getThingIterato r ():
do something.

Now I need to test to see if the iterator is empty. Actually, it's a
unit test where I want to assert that it is empty. In the old days, I
would have done:

assertEquals (getListOfThing s (), [])

but I don't see any clean way to do this with an iterator. The best I
can come up with is something like:

flag = False
for thing in getThingIterato r ():
flag = True
break
assertEquals (flag, False)

Is that really the only way to do it?

[Roy Smith]

Re: Testing for empty iterators?

I wrote:
[color=blue]
> flag = False
> for thing in getThingIterato r ():
> flag = True
> break
> assertEquals (flag, False)
>
> Is that really the only way to do it?[/color]

Oh, never mind, I got it...

assertEquals (list (getThingIterat or ()), [])

[Paul McGuire]

Re: Testing for empty iterators?

"Roy Smith" <roy@panix.co m> wrote in message
news:roy-03678F.14110203 072004@reader2. panix.com...[color=blue]
> I wrote:
>[color=green]
> > flag = False
> > for thing in getThingIterato r ():
> > flag = True
> > break
> > assertEquals (flag, False)
> >
> > Is that really the only way to do it?[/color]
>
> Oh, never mind, I got it...
>
> assertEquals (list (getThingIterat or ()), [])[/color]

In general, I'd say your first approach was better, or something like:

try:
getThingIterato r.next()
assert False, "getThingIterat or pointed to non-empty iterable"
except StopIteration:
pass # no need to assert, by getting here, we know that getThingIterato r
pointed to an empty iterable

Or if you'd prefer a more typical-looking assert:

expectedNullIte m = null
try:
expectedNullIte m = getThingIterato r.next()
except StopIteration:
pass
assert expectedNullIte m==null, "getThingIterat or was supposed to be empty"


Suppose your iterator, through some bug in your code, pointed to a list of
100,000 database records, instead of an empty list as you expected. Making
a list from this iterator could be very time-consuming, when all you really
needed to know was that the iterator pointed to at least one element.

-- Paul

[Roy Smith]

Re: Testing for empty iterators?

"Paul McGuire" <ptmcg@austin.s topthespam_rr.c om> wrote:[color=blue]
> Suppose your iterator, through some bug in your code, pointed to a list of
> 100,000 database records, instead of an empty list as you expected. Making
> a list from this iterator could be very time-consuming, when all you really
> needed to know was that the iterator pointed to at least one element.[/color]

I see your point, but this is a unit test. Even more so than normally,
in a unit test I think clarity of code is more important that
efficiency. And in this case, it's only inefficient if it fails the
test, which should never happen :-)

[Michele Simionato]

Re: Testing for empty iterators?

Roy Smith <roy@panix.co m> wrote in message news:<roy-0D409A.14060003 072004@reader2. panix.com>...[color=blue]
> In the old days, if I wanted to return a sequence of items, I'd return a
> list, and loop over it like this:
>
> for thing in getListOfThings ():
> do something
>
> With iterators, I'm doing:
>
> for thing in getThingIterato r ():
> do something.
>
> Now I need to test to see if the iterator is empty. Actually, it's a
> unit test where I want to assert that it is empty. In the old days, I
> would have done:
>
> assertEquals (getListOfThing s (), [])
>
> but I don't see any clean way to do this with an iterator. The best I
> can come up with is something like:
>
> flag = False
> for thing in getThingIterato r ():
> flag = True
> break
> assertEquals (flag, False)
>
> Is that really the only way to do it?[/color]

You may want to check this thread:

http://groups.google.it/groups?q=empty+iterators+group:comp.lang.python.*&hl=it&lr=&ie=UTF-8&group=comp.lang.python.*&selm=2259b0e2.0307210626.11a1bbf1%40posting.google.com&rnum=1

Michele Simionato

[Peter Otten]

Re: Testing for empty iterators?

Roy Smith wrote:
[color=blue]
> Now I need to test to see if the iterator is empty. Actually, it's a
> unit test where I want to assert that it is empty. In the old days, I
> would have done:
>
> assertEquals (getListOfThing s (), [])
>
> but I don't see any clean way to do this with an iterator. The best I[/color]

[and in a follow-up]
[color=blue]
> Oh, never mind, I got it...
>
> assertEquals (list (getThingIterat or ()), [])[/color]

An alternative would be

self.assertRais es(StopIteratio n, getThingIterato r().next)

which is a bit stricter as it will choke if getThingIterato r() returns a
list instead of an iterator.

Peter

[Paul Rubin]

Re: Testing for empty iterators?

Peter Otten <__peter__@web. de> writes:[color=blue]
> An alternative would be
>
> self.assertRais es(StopIteratio n, getThingIterato r().next)[/color]

Note that all those methods "consume" at least one element of the
iterator. You need something like ungetc for iterators (untested code):

class wrapper:
def __init__(self, iterator):
self.iterator = iterator
self.pushback = []
def __next__(self):
if self.pushback:
return self.pushback.p op(0)
def is_empty(self):
if self.pushback:
return False
try:
self.pushback.a ppend(self.iter ator.next())
return False
except StopIteration:
return True

Maybe something like that should go into itertools, if it's not
already there. Or the is_empty operation could be built into iterators.

[Larry Bates]

Re: Testing for empty iterators?

Normally I add a __len__ method to my class
that contains an iterator if I need to keep
track of length/empty status. Basically it
is something like:

class getListOfThing:
def __init__(self):
self.ListOfThin gs=[]
self.next_index =0
#
# Code to build ListOfThings can be inserted here
# or in some other method.
#
return

def __iter__(self):
return self

def next(self):
#
# Try to get the next Thing
#
try: Thing=self.List OfThings[self.next_index]
except IndexError:
self.next_index =0
raise StopIteration
#
# Increment the index pointer for the next call
#
self.next_index +=1
return Thing

def __len__(self):
return len(self.ListOf Things)
#
# Where ListOfThings is an attribute
# (list) that is defined in __init__
# and appended to in some way.
#

if __name__=="__ma in__":

T=getListOfThin gs()
print "Length of ListOfThings=", len(T)
for Thing in T:
# do something with Thing

But most of the time the fact that the iterator
doesn't return enything, so the loop is skipped
automatically is sufficient. I think you can do
this same thing with a yield, but this method works
and I continue to use it a lot.

HTH,
Larry Bates
Syscon, Inc.

"Roy Smith" <roy@panix.co m> wrote in message
news:roy-0D409A.14060003 072004@reader2. panix.com...[color=blue]
> In the old days, if I wanted to return a sequence of items, I'd return a
> list, and loop over it like this:
>
> for thing in getListOfThings ():
> do something
>
> With iterators, I'm doing:
>
> for thing in getThingIterato r ():
> do something.
>
> Now I need to test to see if the iterator is empty. Actually, it's a
> unit test where I want to assert that it is empty. In the old days, I
> would have done:
>
> assertEquals (getListOfThing s (), [])
>
> but I don't see any clean way to do this with an iterator. The best I
> can come up with is something like:
>
> flag = False
> for thing in getThingIterato r ():
> flag = True
> break
> assertEquals (flag, False)
>
> Is that really the only way to do it?[/color]