Python's "all" and "any" keywords, and what happens when an empty iterable is passed

Hello, world! 👋🏼

In this short post, let me take you through the happy li'l any and all keywords, along with a nice gotcha later!

Already aware of these keywords? Jump to the gotcha then.

The any keyword

For the ones new to Python, both any and all accept an iterable.

A bit about iterables

In case you are wondering what an iterable is, it's just an object that you can iterate over. Common examples include list, tuple, even dict and str (yes, you can iterate over them - try a for loop over them). There's more to iterables; some other day..

any simply returns True if any of the elements in the iterable are truthy. Else, it returns False.

More about truthy

truthy or falsy are simply those objects that evaluate to True or False when considered as boolean. To quickly check this, consider the below snippet:

>> bool(1)
True
>> bool('a')
True
>> bool(' ')
True
>> bool(True)
True
>> bool(0)
False
>> bool('')
False
>> bool(None)
False
>> bool(False)
False

Let's see a few examples:

>>> any([0, 1])
True
>>> any([1, 2])
True
>>> any([None, 1])
True
>>> any([0, 0])
False
>>> any([None, 0])
False
>>> any([False, None, 0])
False

The all keyword

As you might have guessed, all returns True if all the elements in the iterable are truthy, else, False.

Let's see this with the same examples as above:

>>> all([0, 1])    # this returned True for "any"
False
>>> all([1, 2])
True
>>> all([None, 1])
False
>>> all([0, 0])
False
>>> all([None, 0])
False
>>> all([False, None, 0])
False

Notice why only [1, 2] returned True for all..

---

Gotcha

Now that we know what these keywords do, think about what will they return when you pass them an empty iterable?

Think for a while before going ahead..

Let's check:

>>> any([])
False
>>> 
>>> all([])
True
>>> 

So, when we pass an empty list to any, it returns False (may be because an empty list is considered as falsy - quite intuitive). But behold! all returns True for an empty list!! Not very intuitive, eh?!

How could one ever know this? May be, by looking at the help on these functions:

>>> help(any)
Help on built-in function any in module builtins:

any(iterable, /)
    Return True if bool(x) is True for any x in the iterable.

    If the iterable is empty, return False.

>>> help(all)
Help on built-in function all in module builtins:

all(iterable, /)
    Return True if bool(x) is True for all values x in the iterable.

    If the iterable is empty, return True.

Note the last lines in both sections -
any - If the iterable is empty, return False.
all - If the iterable is empty, return True.

As a good practice, and towards becoming better at Python (or any language in general), always consider looking up at the documentation - it gives good insights about how things are implemented in the language.

Happy coding..