[Python-ideas] PEP 531: Existence checking operators

[Stephan Hoyer]

On Sat, Oct 29, 2016 at 3:53 AM, Steven D'Aprano <steve at pearwood.info>
wrote:

> Hmmm. I see your point, but honestly, None *is* special. Even for
> special objects, None is even more special.


As a contributor to and user of many numerical computing libraries in
Python (e.g., NumPy, pandas, Dask, TensorFlow) I also agree here.

Implicit non-existence for NotImplemented and Ellipsis seem particularly
problematic, because these types are rarely used, and the meaning of these
types intentionally differs from other missing types:
- In NumPy, None is a valid indexing argument, used as a sentinel marker
for "insert a new axis here". Thus x[..., None] means "insert a new axis at
the end."
- Likewise, implicit checks for NotImplemented would be problematic in
arithmetic, because NaN is also a perfectly valid result value for
arithmetic. Especially in this case, checking for "missingness" could look
attractive at first glance to implementors of special methods for
arithmetic but could later lead to subtle bugs.

I'm have more mixed fillings on testing for NaNs. NaNs propagate, so
explicit testing is rarely needed. Also, in numerical computing we usually
work with arrays of NaN, so operator.exists() and all this nice syntax
would not be a substitute for numpy.isnan or pandas.isnull.

On the whole, I do think that adding systematic checks for None to Python
with dedicate syntax would be a win. If making NaN "missing" and allowing
user defined types to be "missing" would help make that happen, then sure,
go ahead, but I see few use cases.
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