list comprehensions

[Robert Brewer]

Elaine Jackson wrote:
> List comprehensions don't work the way you intuitively expect 
> them to work.
>
> partitions = lambda n: [[n]]+[[k]+x for x in partitions(n-k) for k in
> range(1,n)]
> 
> As defined above, the function raises an exception when you 
> call it ('k'
> referenced before assignment). For the sake of clarity, here 
> is workable code
> expressing the same intention:
> 
> def partitions(n):
>     reVal=[[n]]
>     for k in range(1,n):
>         for x in partitions(n-k):
>             reVal.append([k]+x)
>     return reVal
> 
> So I guess what I want to ask is: Can somebody explain the 
> semantics of list comprehensions to me?

Try reversing the loops:

>>> partitions = lambda n: [[n]]+[[k]+x for k in range(1,n) for x in
partitions(n-k)]
>>> partitions(3)
[[3], [1, 2], [1, 1, 1], [2, 1]]

When writing combined comprehensions, you write the "for" elements in
the same order they would be in the expanded code.


Robert Brewer
MIS
Amor Ministries
fumanchu at amor.org