Question

Just like I’m allowed to do this:

(EDIT – I’m NOT allowed to do this either, sorry messed up, but anyway I guess my question is more on the lines of how to go about doing parallel assignments in a loop)

for (x,i) in ([0,1,2],[3,4,5]):
  # do something

why am I not allowed to do this in a for each loop (different list length)

 for (x,i) in ([4,6,5,7,8,9],[1,2,3]):
   # do something

I know you get a “ValueError: too many values to unpack”, but why doesn’t the loop stop when i is done with its iteration?

A real use of this would be for something like this that I was trying to do

for (keys,i) in (sorted(dic.keys(),key=custom_sort), range(10)):
  print dic[keys]

where I’m sorting the dictionary as well as printing only the first 10 top results (assume the dictionary has hundreds of keys) – I only wanted the top 10 results.

Since this kind of syntax is not allowed anyway what could be the next best thing to do?

thanks

Answer 1

For this kind of stuff, itertools is your friend.

In your case, itertools.islice() will do the trick:

import itertools
for key in itertools.islice(sorted(dict.keys(), key=custom_sort), 10):
    # Only the first 10 keys are processed.

Alternatively, if you need the index as well, use zip or itertools.izip:

for i, key in zip(range(10), sorted(dict.keys(), key=custom_sort)):
    # i will be 0, 1, ..., 9