Also, Pam, my Python is awful, but how about just

print do_something()

? It seems to me that it’s not so much about scope as it is that in the first example, the do_something() function just never gets called.


>>> def gen():
... yield 1
... yield 2
... yield 3
...
>>> type(gen)

>>> g.next()
1
>>> g.next()
2
>>> g.next()
3
>>> g.next()
Traceback (most recent call last):
File "", line 1, in
StopIteration


Basically, gen is just a function. It’s the yield statements that let python know it’s a generator. g is the actual generator object created from calling gen().

Calling g.next() the first time returns the first value that’s “yielded”. The second time, the function just picks up where it left off, so g.next() returns 2, etc. Until it drops off the end, at which point it raises a StopIteration exception. This is kind how iterators work behind the scenes. Generators make it easier to write iterators without defining a class and the next, iter methods.

You don’t usually use a generator in that way. Instead you would use it like an iterator, like this:


>>> for num in gen():
... print num
...
1
2
3


Anyway, to your post … showlinks() returns a generator object, and you could call next() or just iterate through it with a for loop.

When you create a list (links = list(showlinks())), it’s basically iterated through the entire generator and stuffed the values in that list.

Each element of the list is actually a an instance of the Link class from the mechanize package (twill is using mechanize for its browsing facilities):


>>> link = links[1]
>>> link
Link(base_url='http://del.icio.us', url='https://secure.del.icio.us/login', text='login',
tag='a', attrs=[('href', 'https://secure.del.icio.us/login')])
>>> type(link)

>>> dir(link)
['__cmp__', '__doc__', '__init__', '__module__', '__repr__', 'absolute_url', 'attrs', 'bas
e_url', 'tag', 'text', 'url']
>>> link.url
'https://secure.del.icio.us/login'
>>> link.text
'login'
>>> link.absolute_url
'https://secure.del.icio.us/login'
>>> link.tag
'a'


Hope that makes sense.

I think the idea was somewhat more half-baked than you might be giving me credit for.

1. None ==> /20484

Where the number is dynamically generated. I am not sure how to follow that link.

Variable scope was a hard thing for me to grasp when I first started programming. When I started working in PHP, it made things worse… It’s a difficult thing to explain to someone with very little programming experience, but a very valuable thing to understand, especially when you have a great language like python.

Paul

(That said, I’m sure there are good reasons to do it on occasion, but my usual reason would be, “I can’t figure out a better way,” which is not good enough.)