List question in Python

Question

Alright, so I have a list of 296 data points and four blank spaces. I cannot edit this list of data points. i have another list of 300 data points. I want to multiply the lists together, with python skipping multiplying the data points when a blank space shows up. Here's what the lists look like:

a = [[6], [7], [], [7]]
b = [[100], [200], [300], [400]]

What sort of exception handling would I have to put in? My current code uses

for items in mathList:
 try:
 sumlist = [x * y for x,y in zip(grearp, rex)]
 except:
 print 'No data for',items

Answer 1

Is the length of both lists actually 300 then, with 0's or blank strings for the missing data points? If so, this should come close:

newList = [x[0] * y[0] if x else None for x, y in zip(l1, l2)]

-- Edited --

I realized I didn't review the sample data quite as well as I could've. As the inner list is empty it'll fail a truth test on its own, so just if x. Also, added indexing for the inner lists on x, y.

• miles82: This won't work as he has lists of lists, not lists of integers.
• Tom Collins: It's okay, I just removed the minilists and this worked. Thanks for your help!
• g.d.d.c: @miles82 -> you're correct. I've modified the code slightly. In point of fact though, sometimes it's about demonstrating an approach more than it is providing a copy/paste solution. The question boils down to "how do I conditionally operate within a list comprehensions" which both versions of my code demonstrate.
• g.d.d.c: @Tim - Happy to help! I'm glad it made sense. :)

Answer 2

Think that you can also use something like this(code below will prepare lists to be the same size and then calculates '*' if both values present - otherwise include the only value exist):

from itertools import izip_longest
a = [[6], [7], [], [7]]
b = [[100], [200], [300], [400]]
newList = [[x[0] * y[0]] if x and y else (x or y) for x,y in izip_longest(a,b, fillvalue=[])]