empty lists vs empty generators

[Jeremy Bowers]

On Mon, 02 May 2005 16:14:57 -0700, Brian Roberts wrote:
> Q1: Is there a better or alternate way to handle this? Q2: Is there a way
> that handles both lists and generators, so I don't have to worry about
> which one I've got?

Are you in control of your generators? You could put a method on them that
tells if there is anything in them by manually implementing the .next()
call.

The other thing you could do is a generator wrapper that can tell for you,
but you'll lose some performance:

class EmptyGeneratorDetector(object):
    """Provides a method you can call to detect an empty
    generator. You should probably name this class something
    shorter.

    Check if the generator is empty after construction by looking at
    the isEmpty property."""

    def __init__(self, generator):
        self.generator = generator

        self.isEmpty = False
        self.givenFirst = False
        try:
            self.firstItem = generator.next()
        except StopIteration:
            self.isEmpty = True

    def next(self):
        if self.isEmpty:
            raise StopIteration

        if not self.givenFirst:
            self.givenFirst = True
            return self.firstItem
        else:
            return self.generator.next()

    def __iter__(self):
        return self
    
In action:

Python 2.3.5 (#1, Mar  3 2005, 17:32:12) 
[GCC 3.4.3  (Gentoo Linux 3.4.3, ssp-3.4.3-0, pie-8.7.6.6)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from genwrap import *
>>> def emptyGenerator():
...     raise StopIteration 
...     yield None
... 
>>> def nonEmptyGenerator():
...     yield 1
...     yield 2
...     yield 3 
... 
>>> e = emptyGenerator()
>>> n = nonEmptyGenerator()
>>> E = EmptyGeneratorDetector(e)
>>> N = EmptyGeneratorDetector(n)
>>> E.isEmpty
True
>>> N.isEmpty
False
>>> for i in E:
...     print i
... 
>>> for i in N: 
...     print i
... 
1
2
3
>>> 

It is tested as much as you see it above :-)

(I recall a lengthy discussion of the best way to create an empty iterator
a while back, and that was not the winner. But it will do for now.)