On Tue, 2009-02-10 at 12:50 -0800, Josh Dukes wrote:
The thing I don't understand is why a generator that has no iterable
values is different from an empty list. Why shouldn't bool ==
has_value?? Technically a list, a tuple, and a string are also objects
but if they lack values they're evaluated as False. It seems to me that
a generator is an object that intends to replace lists where lazy
evaluation would be more efficent. Here is one place where that's
definitely true.
Just in case it's not perfectly clear: until you call next() there is no
way to know whether the generator will yield any value or not -- and once
it does, it's lost until you explicitely save it.
This generator doesn't yield any value - but you have to wait for a while
if you call .next() on it, until eventually raises StopIteration:
(x for x in xrange(2000000000) if x>100000000000)
En Tue, 10 Feb 2009 19:25:47 -0200, Albert Hopkins
<mar...@letterboxes.org> escribió:
The main reason I'm interested in this is that it improves performance
immensely over boolean evaluation of large lists (as the attached code
shows). It seems to me if I could use find a good use for it in my
experimentation that someone else might also want to do the same thing
in real-world code.
I don't understand what you mean by this. But if you really want to
know if a generator is "non-empty":
def non_empty(virgin_generator):
try:
virgin_generator.next() # note you just lost the first value
return True
except StopIteration:
return False
The only way to get around this is to put all the values of a generator
inside a container (e.g. a list):
For a long generator you may not want to do that, also you may not want to
lose the next element. A variation of your function above is useful in
such cases:
py> def end_of_gen(g):
... """returns (False, next_element) when it exists or (True, None) when
it's
empty"""
... try: return False, g.next()
... except StopIteration: return True, None
...
py> g = (c for c in "Python" if c in "aeiou")
py> eog, c = end_of_gen(g)
py> eog
False
py> c
'o'
py> eog, c = end_of_gen(g)
py> eog
True
py> c
--
Gabriel Genellina
--
http://mail.python.org/mailman/listinfo/python-list
