Pythonic way to check that the lengths of lots of lists are the same

Question

I have a number of lists that I'm going to use in my program, but I need to be sure that they are all the same length, or I'm going to get problems later on in my code.

What's the best way to do this in Python?

For example, if I have three lists:

a = [1, 2, 3]
b = ['a', 'b']
c = [5, 6, 7]

I could do something like:

l = [len(a), len(b), len(c)]
if max(l) == min(l):
   # They're the same

Is there a better or more Pythonic way to do this?

Answer 1

Assuming you have a non-empty list of lists, e.g.

my_list = [[1, 2, 3], ['a', 'b'], [5, 6, 7]]

you could use

n = len(my_list[0])
if all(len(x) == n for x in my_list):
    # whatever

This will short-circuit, so it will stop checking when the first list with a wrong length is encountered.

Answer 2

len(set(len(x) for x in l)) <= 1

Latter I ended up writing:

def some(x):
    """Replacement for len(set(x)) > 1"""

    if isinstance(x, (set, frozenset)):
       return len(x) > 1

    s = set()
    for e in x:
        s.add(e)
        if len(s) > 1:
            return True
    return False

def lone(x):
    """Replacement for len(set(x)) <= 1"""
    return not some(x)

Which allows the above to be written as:

lone(len(x) for x in l)

This will stop taking the lengths of the lists as soon as it finds a list with a different length.