On 14Oct2010 10:16, Tony <ton...@ximera.net> wrote:
| I have been using generators for the first time and wanted to check for
| an empty result. Naively I assumed that generators would give
| appopriate boolean values. For example
|
| def xx():
| l = []
| for x in l:
| yield x
|
| y = xx()
| bool(y)
|
|
| I expected the last line to return False but it actually returns True.
| Is there anyway I can enhance my generator or iterator to have the
| desired effect?
The generator is not the same as the values it yields.
What you're doing is like this:
>>> def f():
... return False
...
>>> bool(f)
True
>>> bool(f())
False
In your code, xx() returns a generator object. It is not None, nor any
kind of "false"-ish value. So bool() returns True.
The generator hasn't even _run_ at that point, so nobody has any idea if
iterating over it will return an empty sequence.
What you want is something like this:
values = list(xx())
bool(values)
or more clearly:
gen = xx()
values = list(gen)
bool(values)
You can see here that you actually have to iterate over the generator
before you know if it is (will be) empty.
Try this program:
def lines():
print "opening foo"
for line in open("foo"):
yield line
print "closing foo"
print "get generator"
L = lines()
print "iterate"
text = list(L)
print "done"
For me it does this:
get generator
iterate
opening foo
closing foo
done
You can see there that the generator _body_ doesn't even run until you
start the iteration.
Does this clarify things for you?
Cheers,
--
Cameron Simpson <c...@zip.com.au> DoD#743
http://www.cskk.ezoshosting.com/cs/
Winter is gods' way of telling us to polish.
- Peter Harper <bo...@freenet.carleton.ca> <harp...@algonquinc.on.ca>
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