[Python-ideas] PEP 505: None-aware operators

[Chris Angelico]

On Thu, Jul 26, 2018 at 12:30 PM, David Mertz <mertz at gnosis.cx> wrote:
> Btw. Here's a way of spelling the proposed syntax that gets the semantics
> right:
>
>>>> # pip install coalescing
>>>> NullCoalesce(spam).eggs.bacon

Let's try it.

rosuav at sikorsky:~$ sudo python3 -m pip install coalescing
Collecting coalescing
  Downloading https://files.pythonhosted.org/packages/f3/f4/120f04cc59f9fa8c55c711b67f1c9c34d8a59c34cd69249e6ff61b098987/coalescing-0.1.1.tar.gz
Installing collected packages: coalescing
  Running setup.py install for coalescing ... done
Successfully installed coalescing-0.1.1

rosuav at sikorsky:~$ python3
Python 3.8.0a0 (heads/literal_eval-exception:ddcb2eb331, Feb 21 2018, 04:32:23)
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from coalescing import NullCoalesce
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'coalescing'
>>> from coalesce import NullCoalesce
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/site-packages/coalesce.py", line 56,
in <module>
    import wrapt
ModuleNotFoundError: No module named 'wrapt'

A bit problematic. But after (a) figuring out that your module is
named "coalesce" even though I installed "coalescing" AND (b) going
and separately installing wrapt, and finally (c) doing the import that
you didn't mention, we still have this fundamental problem:

rosuav at sikorsky:~$ python3
Python 3.8.0a0 (heads/literal_eval-exception:ddcb2eb331, Feb 21 2018, 04:32:23)
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from coalesce import NullCoalesce
>>> from types import SimpleNamespace
>>> spam, spam.eggs, spam.eggs.bacon = SimpleNamespace(), SimpleNamespace(), 42
>>> NullCoalesce(spam).eggs.bacon
<NullCoalesce proxy for 42>

That isn't 42. That's a thing that, forever afterwards, will be a
proxy. And look at this:

>>> spam.nil = None
>>> print(NullCoalesce(spam).nil)
<NullCoalesce proxy for None>
>>> print(NullCoalesce(spam).nil.nil)
None
>>> print(NullCoalesce(spam).nil.nil.nil)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'nil'
>>>

Whoooooops.

So, no, this is most definitely NOT equivalent to the proposed semantics.

ChrisA