list comprehensions

[Elaine Jackson]

OK, it's pretty embarassing that I gave up when it wasn't in the contents and
never even thought of the index. And so far everybody's been too polite to point
out that I left the base case out of my recursion. But I stand by my complaint
about unintuitiveness, because I've discovered that you get an error from

x = [(i,j) for i in range(7-j) for j in range(3)]

while

y = [[(i,j) for i in range(7-j)] for j in range(3)]

works fine. In other words, my intuition was that x would be
reduce(list.__add__,y). And in fact I'm still a little confused, because I would
describe the syntax of y as going, as you say, from left to right (ie: a
left-branching tree). I suppose I'm going from the inside out, and you're
talking about going from the outside inward. Or something like that. Anyway,
thanks for your help. Now that I know what to read, I'll read it.

Peace


"Terry Reedy" <tjreedy at udel.edu> wrote in message
news:mailman.434.1081374131.20120.python-list at python.org...
|
| "Elaine Jackson" <elainejackson7355 at home.com> wrote in message
| news:yjZcc.42823$Pk3.9547 at pd7tw1no...
| >
| > List comprehensions don't work the way you intuitively expect them to
| work.
|
| One of my pet posting peeves is when people ascribe their own
| characteristics to others, as when they write 'you' when they really mean,
| or should have meant, 'I'.  But nevermind.
|
| > I realize many people have no intuitions about how list comprehensions
| 'should'
| > work,
|
| List comprehensions are sufficiently 'artificial' that I would advise
| anyone to avoid the temptation to intuit how they work without consulting
| the manual, tutorials, or other written explanations.
|
| The Python Reference Manual Index, section L, entry List..comprehensions
| takes one to subsection 5.2.4 List displays.  The last two sentences
| therein read
|
|  "When a list comprehension is supplied, it consists of a single expression
| followed by at least one for clause and zero or more for or if clauses. In
| this case, the elements of the new list are those that would be produced by
| considering each of the for or if clauses a block, nesting from left to
| right, and evaluating the expression to produce a list element each time
| the innermost block is reached."
|
| OK, takes a couple of readings and maybe some working examples.
|
| > 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).
|
| Of course, you have the two for clauses reversed, as the error message
| might suggest to an experienced Pythoneer.  As the manual specifies, for/if
| clauses nest *left to right*.  To match the double loop below, you want
| instead
|
| [[k]+x for k in range(1,n) for x in partitions(n-k)]
|
| > def partitions(n):
| >     reVal=[[n]]
| >     for k in range(1,n):
| >         for x in partitions(n-k):
| >             reVal.append([k]+x)
| >     return reVal
|
| Terry J. Reedy
|
|
|
|