@ChrisA You, my friend, certainly put the nail in the coffin!
Sent from my iPhone > On Aug 5, 2015, at 5:52 AM, Chris Angelico <ros...@gmail.com> wrote: > >> On Wed, Aug 5, 2015 at 7:01 PM, Marko Rauhamaa <ma...@pacujo.net> wrote: >> Chris Angelico <ros...@gmail.com>: >> >>> You can chain 'for' and 'if' clauses as much as you like, and they >>> behave exactly the way you'd expect. >> >> How do you know what I'd expect? >> >> I wouldn't know what to expect myself. > > A list comprehension can always be unwound into statement form. [1] For > example: > > squares = [n*n for n in range(10)] > > can be unwound into: > > squares = [] > for n in range(10): > squares.append(n*n) > > You simply take the expression at the beginning and put that into the > append() method call, and then the rest become statements. Here's a > filtered version: > > odd_squares = [n*n for n in range(20) if n%2] > > Which becomes: > > odd_squares = [] > for n in range(20): > if n%2: > odd_squares.append(n*n) > > So what would you expect nested 'if' clauses to do? Well, they become > nested 'if' statements: > > primes = [n for n in range(2,24) if n%2 if n%3] > > primes = [] > for n in range(2,24): > if n%2: > if n%3: > primes.append(n) > > What if we have multiple 'for' loops? Same thing! > > ways_to_get_seven = [(a,b) for a in range(1,7) for b in range(1,7) if a+b==7] > > ways_to_get_seven = [] > for a in range(1,7): > for b in range(1,7): > if a+b==7: > ways_to_get_seven.append((a,b)) > > No matter what combination of 'if' and 'for' you use, it can be > unwound like this, and it'll always behave the same way. Not sure what > to expect? Follow the simple rules of unwinding, and then read the > code that way. > > Of course, if you don't know how to predict what the statement form > will do, then you won't understand what the comprehension will do. But > that's not the comprehension's fault. > > peculiar = [a*x*x+b*x+c for a in range(1,10) for b in range(2,30) for > c in range(-3,5) if b*b-4*a*c>=0 if (-b+math.sqrt(b*b-4*a*c))/2/a>0 > for x in (x*.01 for x in range(-500,500))] > > I suppose you could graph that. Or something. But that's just bad > code, don't blame the list comp for that... > > ChrisA > > [1] To be technically correct, this unwinding should be done in a > nested function, which you then call. There are some other minor > technicalities too. But it's pretty much this. > -- > https://mail.python.org/mailman/listinfo/python-list -- https://mail.python.org/mailman/listinfo/python-list
