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
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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

def equalSizes(*args):
    """
    # This should pass. It has two lists.. both of length 2
    >>> equalSizes([1,1] , [1,1])
    True

    # This should pass, It has three lists.. all of length 3
    >>> equalSizes([1,1,1] , [1,1,1], [1,1,1])
    True

    # This should pass, It has three lists.. all of length 2
    >>> equalSizes([1,1] , [1,1], [1,1])
    True

    # This should FAIL. It has three list.. one of which is different that the other
    >>> equalSizes([1,1,] , [1,1,1], [1,1,1])
    False
    """
    len0 = len(args[0])
    return all(len(x) == len0 for x in args[1:])

To test it save it to a file so.py and run it like this:

$ python -m doctest so.py -v
Trying:
    equalSizes([1,1] , [1,1])
Expecting:
    True
ok
Trying:
    equalSizes([1,1,1] , [1,1,1], [1,1,1])
Expecting:
    True
ok
Trying:
    equalSizes([1,1] , [1,1], [1,1])
Expecting:
    True
ok
Trying:
    equalSizes([1,1,] , [1,1,1], [1,1,1])
Expecting:
    False
ok

Answer 3

If you want a little more data in failure cases, you could do:

myList1 = [ [1,1] , [1,1]]
lens = set(itertools.imap(len, myList1))
return len(lens) == 1
# if you have lists of varying length, at least you can get stats about what the different lengths are

Answer 4

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