[Python-Dev] What's the status of PEP 505: None-aware operators? (original) (raw)

David Mertz mertz at gnosis.cx
Wed Nov 29 12:40:21 EST 2017

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.

Yours, David...

On Wed, Nov 29, 2017 at 9:03 AM, Random832 <random832 at 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 ?? localtimeout ?? globaltimeout > 'foo' in (None ?? ['foo', 'bar']) > requestedquantity ?? defaultquantity * price > name?.strip()[4:].upper() > user?.firstname.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.]

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