Checking to see if a list of lists has equal sized lists

Question

I need to validate if my list of list has equally sized lists in python

myList1 = [ [1,1] , [1,1]] // This should pass. It has two lists.. both of length 2
myList2 = [ [1,1,1] , [1,1,1], [1,1,1]] // This should pass, It has three lists.. all of length 3
myList3 = [ [1,1] , [1,1], [1,1]] // This should pass, It has three lists.. all of length 2
myList4 = [ [1,1,] , [1,1,1], [1,1,1]] // This should FAIL. It has three list.. one of which is different that the other 

I could write a loop to iterate over the list and check the size of each sub-list. Is there a more pythonic way to achieve the result.

Answer 1

all(len(i) == len(myList[0]) for i in myList)

To avoid incurring the overhead of len(myList[0]) for each item, you can store it in a variable

len_first = len(myList[0]) if myList else None
all(len(i) == len_first for i in myList)

If you also want to be able to see why they aren't all equal

from itertools import groupby
groupby(sorted(myList, key=len), key=len)

Will group the lists by the lengths so you can easily see the odd one out

Answer 2

You could try:

test = lambda x: len(set(map(len, x))) == 1

test(myList1) # True
test(myList4) # False

Basically, you get the length of each list and make a set from those lengths, if it contains a single element then each list has the same length