[Tutor] Stuck on Error

[Steven D'Aprano]

On Sun, Dec 22, 2013 at 04:39:24AM -0500, Keith Winston wrote:

[...]
> You could do this entire piece with a list comprehension in one line, but
> I'm only mentioning it b/c I just learned them. My crude first effort would
> look like this:
> 
> RN = []  # create the array RN
> [RN.append(random.randint(1, 75)) for i in range(5)]  # populate the array

Definitely a crude first effort :-)

I'm afraid you've missed the point of list comprehensions if you write 
them like that. Here is how you should write the above: just call the 
function you want to append with directly, no need for an extra append.

RN = [random.randint(1, 75)) for i in range(5)]

The list comprehension itself creates a new list and populates it. No 
need to pre-create an empty list and then populate it.

What your code ends up doing is:

- create an empty list, RN
- run a list comprehension which builds a temporary 
  list [None, None, None, None, None]
- as a side-effect of building that temp list, populate
  the RN list
- throw away the temp list


The end result probably does twice as much work as needed.

In your list comp, you call RN.append(something) five times. Each call 
to RN.append returns None. That's not obvious from the interactive 
interpreter, because it suppresses printing of None as a convenience, 
but it is true:

py> result = [].append(23)
py> print(result)
None


So your list comp builds a list made up of nothing but None, only to 
throw it away immediately it is done.


-- 
Steven