It seems the conversation has confused two related concepts:

1) The default bool() implementation (Truthiness) -- this is what the OP
said was recommended by PEP 8: "For sequences, (strings, lists, tuples),
use the fact that empty sequences are false:" -- there is some debate about
that whether this is a good recommendation or not, but i don't think that's
the OPs point. Rather:

2) That there is no standard way to explicitly test containers for
"emptiness" - - there is only length, with the assumption that
len(something) == 0 or not len(something) is a good way to test for
emptiness.

I still don't see why length is not perfectly adequate, but i wonder if
there are any  "containers" i.e.things that could be empty, in the std lib
that don't support length.

Looking in the ABCs, a Container is something that supports `in`, and a
Sized is something that supports len()-- so in theory, there could be a
Container that does not have a length. Are there any in the std lib?

Perhaps the ABCs are instructive in another way here -- if we were to add a
__empty__ or some such dunder, what ABC would require it? Container? or yet
another one-method ABC?

-CHB


On Tue, Aug 24, 2021 at 8:39 PM David Mertz, Ph.D. <david.me...@gmail.com>
wrote:

> I wanted to do a survey of various "aggregates" in Python to see if any
> stand out as making the usual `if stuff: ...` troublesome.  I wrote a
> little script at
> https://github.com/DavidMertz/LanguagePractice/blob/main/python/aggregates.py
> .
>
> I'm being deliberately vague about an "aggregate."  It might not be a
> collection strictly speaking, but it is something that might seems to
> "contain" values in some sense.  Basically, I think that standard library
> and built-in stuff behaves per PEP8.  Some other libraries go their own
> way.  I throw in a linked-list implementation I found on PyPI.  I've never
> used it beyond this script; but per what it is, it cannot implement `len()`
> on O(1) (well, it *could* if it does extra bookkeeping; but then it's kinda
> a different thing).
>
> In NumPy land, the np.empty vs. np.zeros case is another oddball.  On my
> system, my memory happened to have some prior value that wasn't zero; that
> could vary between runs, in principle.
>
> These are the results:
>
> Expr: '' | Value: ''
>   Truth: False | Length: 0
> Expr: list() | Value: []
>   Truth: False | Length: 0
> Expr: tuple() | Value: ()
>   Truth: False | Length: 0
> Expr: dict() | Value: {}
>   Truth: False | Length: 0
> Expr: set() | Value: set()
>   Truth: False | Length: 0
> Expr: bytearray() | Value: bytearray(b'')
>   Truth: False | Length: 0
> Expr: bytearray(1) | Value: bytearray(b'\x00')
>   Truth: True | Length: 1
> Expr: bytearray([0]) | Value: bytearray(b'\x00')
>   Truth: True | Length: 1
> Expr: array.array('i') | Value: array('i')
>   Truth: False | Length: 0
> Expr: array.array('i', []) | Value: array('i')
>   Truth: False | Length: 0
> Expr: Nothing() | Value: EmptyNamedTuple()
>   Truth: False | Length: 0
> Expr: deque() | Value: deque([])
>   Truth: False | Length: 0
> Expr: deque([]) | Value: deque([])
>   Truth: False | Length: 0
> Expr: ChainMap() | Value: ChainMap({})
>   Truth: False | Length: 0
> Expr: queue.Queue() | Value: <queue.Queue object at 0x7f0940dd2190>
>   Truth: True | Length: No length
> Expr: asyncio.Queue() | Value: <Queue at 0x7f0940dd2190 maxsize=0>
>   Truth: True | Length: No length
> Expr: multiprocessing.Queue() | Value: <multiprocessing.queues.Queue
> object at 0x7f0940dd2190>
>   Truth: True | Length: No length
> Expr: np.ndarray(1,) | Value: array([5.e-324])
>   Truth: True | Length: 1
> Expr: np.ndarray((1,0)) | Value: array([], shape=(1, 0), dtype=float64)
>   Truth: False | Length: 1
> Expr: np.empty((1,)) | Value: array([5.e-324])
>   Truth: True | Length: 1
> Expr: np.zeros((1,)) | Value: array([0.])
>   Truth: False | Length: 1
> Expr: np.zeros((2,)) | Value: array([0., 0.])
>   Truth: No Truthiness | Length: 2
> Expr: np.ones((1,)) | Value: array([1.])
>   Truth: True | Length: 1
> Expr: np.ones((2,)) | Value: array([1., 1.])
>   Truth: No Truthiness | Length: 2
> Expr: pd.Series() | Value: Series([], dtype: float64)
>   Truth: No Truthiness | Length: 0
> Expr: pd.DataFrame() | Value: Empty DataFrame
>   Truth: No Truthiness | Length: 0
> Expr: xr.DataArray() | Value: <xarray.DataArray ()>
>   Truth: True | Length: No length
> Expr: linkedadt.LinkedList() | Value: <linkedadt.LinkedList object at
> 0x7f08d8d77f40>
>   Truth: False | Length: 0
>
>
>
> --
> Keeping medicines from the bloodstreams of the sick; food
> from the bellies of the hungry; books from the hands of the
> uneducated; technology from the underdeveloped; and putting
> advocates of freedom in prisons.  Intellectual property is
> to the 21st century what the slave trade was to the 16th.
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-- 
Christopher Barker, PhD (Chris)

Python Language Consulting
  - Teaching
  - Scientific Software Development
  - Desktop GUI and Web Development
  - wxPython, numpy, scipy, Cython
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