[New-bugs-announce] [issue17921] explicit empty check instead of implicit booleaness

[lesmana]

New submission from lesmana:

Python should have a builtin method `isempty()` which calls the special method name `__isempty__()` for explicit emptiness check.

The special method `__isempty__()` should return `True` if the object is "empty" and `False` if the object is "not empty".

Observe emptiness check using implicit booleaness:

    if not somecollection:
      foo()

Note that this code also handles `None`, `0` and `False` silently. If those are possible values then I have to test explicitly:

    if somecollection is not None and not somecollection:
      foo()

Also handling the `0` and `False` cases will make the code really really ugly.

Usually the `None`, `0` or `False` cases only happen in case of a programming error. But if I do not test for them explicitly then they will be handled silently and the error will occur somewhere else in the code.

Compare against explicit emptiness check:

    if not isempty(somecollection):
      foo()

This code will break immediately if somecollection does not have `__isempty__()`. If `None`, `0` or `False` are possible values they will not be handled silently, instead an error will be reported at `isempty()`.

Advantage of explicit emptiness check:

* More explicit and fluently readable code
* No silent implicit booleaness check when code should do emptiness check
* Clearer separation of meaning for the special methods `__len__()` and `__empty__()`

Possible use case for explicit emptiness check: a list with memory effect.

    >>> ml = MemoryEffectList()
    >>> ml.charge()
    >>> ml.discharge()
    >>> isempty(ml)
    True
    >>> len(ml) == 0
    False

----------
components: Interpreter Core
messages: 188617
nosy: lesmana
priority: normal
severity: normal
status: open
title: explicit empty check instead of implicit booleaness
type: enhancement

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17921>
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