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

[George Leslie-Waksman]

I am rather fond of the idea of null-coalescing, at the very least, for
mutable default values:

def foo(a=None):
    a ??= []
    ...

but I worry about the code messes we will run into with some of the other
options.

Woe be unto anyone forced to understand the behavior of:

thing?.attr?[key]?.subattr ?? 127

What if we added the Elvis operator "?:" for null coalescing and left the
rest for future consideration


On Wed, Jul 18, 2018 at 10:49 PM Tim Peters <tim.peters at gmail.com> wrote:

> [Steve Dower <steve.dower at python.org>]
>
>> ...
>
> * The "``None``-aware attribute access" operator ``?.`` evaluates the
>> complete expression if the left hand side evaluates to a value that is
>> not
>> ``None``
>>
>
> And if the LHS does evaluate to `None` ...?  I'll assume the result is
> also `None` then.
>
>
>> ...
>
>
>>  From ``inspect.py``::
>>
>>      for base in object.__bases__:
>>          for name in getattr(base, "__abstractmethods__", ()):
>>              value = getattr(object, name, None)
>>              if getattr(value, "__isabstractmethod__", False):
>>                  return True
>>
>> After updating to use the ``?.`` operator (and deliberately not
>> converting to use ``any()``)::
>>
>>      for base in object.__bases__:
>>          for name in base?.__abstractmethods__ ?? ():
>>              if object?.name?.__isabstractmethod__:
>>                  return True
>>
>
> I got lost on the `for` here.  The part following `in`:
>
>     for name in getattr(base, "__abstractmethods__", ()):
>
> looks in `base` (regardless of whether `base` is `None`) for an attribute
> named "_abstractmethods__"..  If such an attribute exists, the value of
> the attribute is returned (`None` or not).  Else an AttributeError is
> swallowed and `()` is returned.  It's hard to see how
>
>
>          for name in base?.__abstractmethods__ ?? ():
>
> does the same.  If `base` itself is `None`, I guess it returns `()`, or
> if  `base` has an "_abstractmethods__" attribute then the value of that
> attribute is returned - unless its value is None, in which case `()` is
> again returned.  But if `base` is not `None` and the attribute does not
> exist, doesn't this raise AttributeError?  The later "Exception-aware
> operators" section seemed to explicitly reject the idea that `?.` and `?[]`
> would suppress AttributeError and/or TypeError.
>
> In short, the original getattr() didn't care at all whether `base` was
> `None`, or whether the value of its "__abstractmethods__" attribute was
> `None`, but cared a whole lot about whether that attribute exists.  I just
> can't see how the updated code matches that in any of those respects.
>
> Ignoring that and pressing on, I suffer the same kind of confusions on the
> `if` part.  What am I missing?  For example, do these operators swallow
> exceptions after all?
>
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