need help on need help on generator...

[Craig Ringer]

On Fri, 2005-01-21 at 16:05 +0100, Francis Girard wrote:
> I recently read David Mertz (IBM DeveloperWorks) about generators and
> got excited about using lazy constructs in my Python programming.

Speaking of totally great articles, and indirectly to lazyness (though
not lazyily evaluated constructs), I was really impressed by this
fantastic article on metaclasses:

    http://gnosis.cx/publish/programming/metaclass_1.html
    http://gnosis.cx/publish/programming/metaclass_2.html

which shows that they're really just not that hard. That saved me an
IMMENSE amount of utterly tedious coding just recently.

> But besides the fact that generators are either produced with the new
> "yield" reserved word or by defining the __new__ method in a class
> definition, I don't know much about them.

They can also be created with a generator expression under Python 2.4. A
generator expression works much like a list comprehension, but returns a
generator instead of a list, and is evaluated lazily. (It also doesn't
pollute the outside namespace with its working variables).

>>> print [ x for x in range(1,10)]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> print ( x for x in xrange(1,10) )
<generator object at 0x401e40ac>
>>> print list(( x for x in xrange(1,10) ))
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Not the use of xrange above for efficiency in the generator expressions.
These examples are trivial and pointless, but hopefully get the point
across.

> In particular, I don't know what Python constructs does generate a
> generator.

As far as I know, functions that use yield, and generator expressions. I
was unaware of the ability to create them using a class with a __new__
method, and need to check that out - I can imagine situations in which
it might be rather handy.

I'm not sure how many Python built-in functions and library modules
return generators for things.

> I know this is now the case for reading lines in a file or with the
> new "iterator" package. But what else ? Does Craig Ringer answer mean
> that list comprehensions are lazy ?

Nope, but generator expressions are, and they're pretty similar.

> Where can I find a comprehensive list of all the lazy constructions
> built in Python ? (I think that to easily distinguish lazy from strict
> constructs is an absolute programmer need -- otherwise you always end
> up wondering when is it that code is actually executed like in
> Haskell).

I'm afraid I can't help you with that. I tend to take the view that side
effects in lazily executed code are a bad plan, and use lazy execution
for things where there is no reason to care when the code is executed.

--
Craig Ringer