help with generators

[Steven Bethard]

George Sakkis wrote:
> "Steven Bethard" wrote:
>
>>py> def bin(n):
>>...     s = []
>>...     def bin(n):
>>...         if n == 0:
>>...             yield s
>>...         else:
>>...             s.append(0)
>>...             for s1 in bin(n - 1):
>>...                 yield s1
>>...             s.pop()
>>...             s.append(1)
>>...             for s1 in bin(n - 1):
>>...                 yield s1
>>...             s.pop()
>>...     return bin(n)
>>...
[snip]
> A caveat of the implementation above: it yields the same instance (s),
> which is unsafe if the loop variable is modified (e.g. in the "for s in
> bin(n)" loop, s should not be modified). Moreover, each yielded value
> should be 'consumed' and then discarded; attempting to store it (as in
> list(bin(n))) references only the last yielded value.

Yup.  However, this was the most direct translation of the OP's original 
function (which also only had a single list).  Since the question was 
about how generators worked, I figured the most direct translation would 
probably be the most useful response.

> Here's a shorter, clear and safe implementation:
> 
> def bin2(n):
>     if n:
>         for tail in bin2(n-1):
>             yield [0] + tail
>             yield [1] + tail
>     else:
>         yield []

This is definitely the way I would have written it.

STeVe