True enough. I remember the prior discussion, but didn't look up the PEP number. I know it's hard to revisit these ideas, but occasionally it works, especially if a nicer spelling is found (Barry proposed some a bit different than my ideas)

On Nov 29, 2017 9:55 AM, "Eric V. Smith" <eric@trueblade.com> wrote:

On Nov 29, 2017, at 12:40 PM, David Mertz <mertz@gnosis.cx> wrote:

I like much of the thinking in Random's approach. But I still think None isn't quite special enough to warrant it's own syntax.

However, his '(or None: name.strip()[4:].upper())' makes me realize that what is being asked in all the '?(', '?.', '?[' syntax ideas is a kind of ternary expression.  Except the ternary isn't based on whether a predicate holds, but rather on whether an exception occurs (AttributeError, KeyError, TypeError).  And the fallback in the ternary is always None rather than being general.

I think we could generalize this to get something both more Pythonic and more flexible.  E.g.:

    val = name.strip()[4:].upper() except None

This would just be catching all errors, which is perhaps too broad.  But it *would* allow a fallback other than None:

    val = name.strip()[4:].upper() except -1

I think some syntax could be possible to only "catch" some exceptions and let others propagate.  Maybe:

    val = name.strip()[4:].upper() except (AttributeError, KeyError): -1

I don't really like throwing a colon in an expression though.  Perhaps some other word or symbol could work instead.  How does this read:

    val = name.strip()[4:].upper() except -1 in (AttributeError, KeyError)

Where the 'in' clause at the end would be optional, and default to 'Exception'.

I'll note that what this idea DOES NOT get us is:

  val = timeout ?? local_timeout ?? global_timeout

Those values that are "possibly None" don't raise exceptions, so they wouldn't apply to this syntax.

See the rejected PEP 463 for Exception catching expressions. 

Eric. 


Yours, David...


On Wed, Nov 29, 2017 at 9:03 AM, Random832 <random832@fastmail.com> wrote:
On Tue, Nov 28, 2017, at 15:31, Raymond Hettinger wrote:
>
> > I also cc python-dev to see if anybody here is strongly in favor or against this inclusion.
>
> Put me down for a strong -1.   The proposal would occasionally save a few
> keystokes but comes at the expense of giving Python a more Perlish look
> and a more arcane feel.
>
> One of the things I like about Python is that I can walk non-programmers
> through the code and explain what it does.  The examples in PEP 505 look
> like a step in the wrong direction.  They don't "look like Python" and
> make me feel like I have to decrypt the code to figure-out what it does.
>
>     timeout ?? local_timeout ?? global_timeout
>     'foo' in (None ?? ['foo', 'bar'])
>     requested_quantity ?? default_quantity * price
>     name?.strip()[4:].upper()
>     user?.first_name.upper()

Since we're looking at different syntax for the ?? operator, I have a
suggestion for the ?. operator - and related ?[] and ?() that appeared
in some of the proposals. How about this approach?

Something like (or None: ...) as a syntax block in which any operation
[lexically within the expression, not within e.g. called functions, so
it's different from simply catching AttributeError etc, even if that
could be limited to only catching when the operand is None] on None that
is not valid for None will yield None instead.

This isn't *entirely* equivalent, but offers finer control.

v = name?.strip()[4:].upper() under the old proposal would be more or
less equivalent to:

v = name.strip()[4:].upper() if name is not None else None

Whereas, you could get the same result with:
(or None: name.strip()[4:].upper())

Though that would technically be equivalent to these steps:
v = name.strip if name is not None else None
v = v() if v """""
v = v[4:] """""""
v = v.upper """""""
v = v() """""""

The compiler could optimize this case since it knows none of the
operations are valid on None. This has the advantage of being explicit
about what scope the modified rules apply to, rather than simply
implicitly being "to the end of the chain of dot/bracket/call operators"

It could also be extended to apply, without any additional syntax, to
binary operators (result is None if either operand is None) (or None: a
+ b), for example, could return None if either a or b is none.

[I think I proposed this before with the syntax ?(...), the (or None:
...) is just an idea to make it look more like Python.]
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
https://mail.python.org/mailman/listinfo/python-dev
Unsubscribe: https://mail.python.org/mailman/options/python-dev/mertz%40gnosis.cx



--
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.