Python 2 and Python 3 built-in all() function not behaving equally

Question

The following code:

a = None
b = None
all([a, b, a > b]) # Returns False in python 2 but TypeError in python 3

Error in python 3:

TypeError: '>' not supported between instances of 'NoneType' and 'NoneType'

It seems like the python 2 version is short circuiting but the python 3 version is not. Is this the case? Why is this happening? Is it a bug? Should I report it?

I've tested this code in Python 2.7.17, 3.6.9 and 3.8.2

Answer 1

Short circuiting is irrelevant. The whole list is evaluated before all is executed.

Python 2 is more permissive with its comparisons. In Python 2 you can use < between strings and ints, lots of different types of objects, and None , without an error. In Python 3, the rules were tightened up so you can only use < in cases where it has a clear meaning.

If the functionality you need is

a and b and a > b

then I suggest you use that. For that expression, a > b will not be evaluated if a or b is None .

Answer 2

Not a bug, it's intended.

Python 3 is meant to be an improvement over Python 2. And logically this should give you an error because you can't compare if nothing is greater than something because, well, it's nothing!

There's also no point in using Python 2, unless you have some legacy code that requires is (but even then I'd try and move to Python 3) as support for Python 2 stopped 01/01/2020