Why is there no explicit emptyness check (for example `is Empty`) in Python

Question

The Zen of Python says "Explicit is better than implicit". Yet the "pythonic" way to check for emptiness is using implicit booleaness:

if not some_sequence:
    some_sequence.fill_sequence()

This will be true if some_sequence is an empty sequence, but also if it is None or 0.

Compare with a theoretical explicit emptiness check:

if some_sequence is Empty:
    some_sequence.fill_sequence()

With some unfavorably chosen variable name the implicit booleaness to check for emptiness gets even more confusing:

if saved:
    mess_up()

Compare with:

if saved is not Empty:
    mess_up()

See also: "Python: What is the best way to check if a list is empty?". I find it ironic that the most voted answer claims that implicit is pythonic.

So is there a higher reason why there is no explicit emptiness check, like for example is Empty in Python?

Answer 1

Polymorphism in if foo: and if not foo: isn't a violation of "implicit vs explicit": it explicitly delegates to the object being checked the task of knowing whether it's true or false. What that means (and how best to check it) obviously does and must depend on the object's type, so the style guide mandates the delegation -- having application-level code arrogantly asserts it knows better than the object would be the height of folly.

Moreover, X is Whatever always, invariably means that X is exactly the same object as Whatever. Making a totally unique exception for Empty or any other specific value of Whatever would be absurd -- hard to imagine a more unPythonic approach. And "being exactly the same object" is obviously transitive -- so you could never any more have distinct empty lists, empty sets, empty dicts... congratulations, you've just designed a completely unusable and useless language, where every empty container crazily "collapses" to a single empty container object (just imagine the fun when somebody tries to mutate an empty container...?!).

Answer 2

The reason why there is no is Empty is astoundingly simple once you understand what the is operator does.

From the python manual:

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object. x is not y yields the inverse truth value.

That means some_sequence is Empty checks whether some_sequence is the same object as Empty. That cannot work the way you suggested.

Consider the following example:

>>> a = []
>>> b = {}

Now let's pretend there is this is Empty construct in python:

>>> a is Empty
True
>>> b is Empty
True

But since the is operator does identity check that means that a and b are identical to Empty. That in turn must mean that a and b are identical, but they are not:

>>> a is b
False

So to answer your question "why is there no is Empty in python?": because is does identity check.

In order to have the is Empty construct you must either hack the is operator to mean something else or create some magical Empty object which somehow detects empty collections and then be identical to them.

Rather than asking why there is no is Empty you should ask why there is no builtin function isempty() which calls the special method __isempty__().

So instead of using implicit booleaness:

if saved:
  mess_up()

we have explicit empty check:

if not isempty(saved):
  mess_up()

where the class of saved has an __isempty__() method implemented to some sane logic.

I find that far better than using implicit booleaness for emptyness check.

Of course you can easily define your own isempty() function:

def isempty(collection):
  try:
    return collection.__isempty__()
  except AttributeError:
    # fall back to implicit booleaness but check for common pitfalls
    if collection is None:
      raise TypeError('None cannot be empty')
    if collection is False:
      raise TypeError('False cannot be empty')
    if collection == 0:
      raise TypeError('0 cannot be empty')
    return bool(collection)

and then define an __isempty__() method which returns a boolean for all your collection classes.

Answer 3

I agree that sometimes if foo: isn't explicit for me when I really want to tell the reader of the code that it's emptiness I'm testing. In those cases, I use if len(foo):. Explicit enough.

I 100% agree with Alex w.r.t is Empty being unpythonic.

Answer 4

There is an explicit emptyness check for iterables in Python. It is spelled not. What's implicit there? not gives True when iterable is empty, and gives False when it is nonempty.

What exactly do you object to? A name? As others have told you, it's certainly better than is Empty. And it's not so ungrammatical: considering how things are usually named in Python, we might imagine a sequence called widgets, containing, surprisingly, some widgets. Then,

if not widgets:

can be read as "if there are no widgets...".

Or do you object the length? Explicit doesn't mean verbose, those are two different concepts. Python does not have addition method, it has + operator, that is completely explicit if you know the type you're applying it to. The same thing with not.

Answer 5

Consider that Lisp has been using () empty list or its symbol NIL quite some years as False and T or anything not NIL as True, but generally computation of Truth already produced some useful result that need not be reproduce if needed. Look also partition method of strings, where middle result works very nicely as while control with the non-empty is True convention.

I try generally avoid using of len as it is most times very expensive in tight loops. It is often worth while to update length value of result in program logic instead of recalculating length.

For me I would prefer Python to have False as () or [] instead of 0, but that is how it is. Then it would be more natural to use not [] as not empty. But now () is not [] is True so you could use:

emptyset = set([])    
if myset == emptyset:

If you want to be explicit of the empty set case (not myset is set([]))

I myself quite like the if not myset as my commenter.

Now came to my mind that maybe this is closest to explicit not_empty:

if any(x in myset for x in myset): print "set is not empty"

and so is empty would be:

if not any(x in myset for x in myset): print "set is empty"