Python find list lengths in a sublist

Question

I am trying to find out how to get the length of every list that is held within a particular list. For example:

a = []
a.append([])
a[0].append([1,2,3,4,5])
a[0].append([1,2,3,4])
a[0].append([1,2,3])

I'd like to run a command like:

len(a[0][:]) 

which would output the answer I want which is a list of the lengths [5,4,3]. That command obviously does not work, and neither do a few others that I've tried. Please help!

Answer 1

[len(x) for x in a[0]] ?

>>> a = []
>>> a.append([])
>>> a[0].append([1,2,3,4,5])
>>> a[0].append([1,2,3,4])
>>> a[0].append([1,2,3])
>>> [len(x) for x in a[0]]
[5, 4, 3]

Answer 2

[len(x) for x in a[0]]

Answer 3

This is known as List comprehension (click for more info and a description).

[len(l) for l in a[0]]

Answer 4

map(len, a[0])

Answer 5

def lens(listoflists):
  return [len(x) for x in listoflists]

now, just call lens(a[0]) instead of your desired len(a[0][:]) (you can, if you insist, add that redundant [:], but that's just doing a copy for no purpose whatsoever -- waste not, want not;-).

Answer 6

using the usual "old school" way

t=[]
for item in a[0]:
    t.append(len(item))
print t