Re: [Python-ideas] Fwd: Null coalescing operator

On 2016-09-13 16:27, Rob Cliffe wrote: On 13/09/2016 12:37, Nick Coghlan wrote: On 13 September 2016 at 21:15, Rob Cliffe wrote: On 13/09/2016 04:43, Guido van Rossum wrote: Yeah, that's exactly my point. PEP 463 gives you a shorter way to catch an exception, so

Re: [Python-ideas] Fwd: Null coalescing operator

On 13/09/2016 12:37, Nick Coghlan wrote: On 13 September 2016 at 21:15, Rob Cliffe wrote: On 13/09/2016 04:43, Guido van Rossum wrote: Yeah, that's exactly my point. PEP 463 gives you a shorter way to catch an exception, so it gives you less motivation to find a

Re: [Python-ideas] Fwd: Null coalescing operator

On Tue, Sep 13, 2016, at 08:44, Michel Desmoulin wrote: > You won't see very complex usages, mostly things like: > > val = foo[-1] except IndexError: "bar" > doh = val.attr.other except AttributeError: "default" It occurs to me that if lambda were more lightweight [whether it's the syntax or the

Re: [Python-ideas] Fwd: Null coalescing operator

I doubt very much it will be used for very complexe cases. Just like comprehensions or ternary expressions, they are a good fit for some specific use cases, and people will quickly catch on which one. You rarely see nested comprehensions or ternary expressions while it's possible to do so,

Re: [Python-ideas] Fwd: Null coalescing operator

On 13 September 2016 at 21:15, Rob Cliffe wrote: > On 13/09/2016 04:43, Guido van Rossum wrote: >> Yeah, that's exactly my point. PEP 463 gives you a shorter way to >> catch an exception, so it gives you less motivation to find a way to >> write your code (or define

Re: [Python-ideas] Fwd: Null coalescing operator

On 13/09/2016 04:43, Guido van Rossum wrote: On Mon, Sep 12, 2016 at 5:45 PM, Rob Cliffe wrote: On 12/09/2016 16:37, Guido van Rossum wrote: For the record, I still really don't like PEP 463. We should strive to catch fewer exceptions, not make it easier to catch

Re: [Python-ideas] Fwd: Null coalescing operator

On 13 September 2016 at 07:13, Paul Moore wrote: > If I understand the proposal, f is actually intended to be equivalent to: > > def f(spam): > spam_val = spam() > if spam_val is None: > return None > eggs_val = spam_val.eggs() >

Re: [Python-ideas] Fwd: Null coalescing operator

On 13/09/2016 01:45, Rob Cliffe wrote: On 12/09/2016 16:37, Guido van Rossum wrote: For the record, I still really don't like PEP 463. We should strive to catch fewer exceptions, not make it easier to catch them. Can you please clarify what you are saying in the last sentence? The first

Re: [Python-ideas] Fwd: Null coalescing operator

On 09/12/2016 08:37 AM, Guido van Rossum wrote: For the record, I still really don't like PEP 463. We should strive to catch fewer exceptions, not make it easier to catch them. I certainly agree with the first part, slightly reworded: we should strive to generate fewer exceptions that we

Re: [Python-ideas] Fwd: Null coalescing operator

On 12 September 2016 at 21:47, Eric Snow wrote: > Note that there's a subtle difference here when multiple lookups are > involved. Given: > > def f(spam): > return spam().eggs().ham > > With null-coalescing: > > def f(spam): > return

Re: [Python-ideas] Fwd: Null coalescing operator

On Mon, Sep 12, 2016 at 1:05 AM, Michel Desmoulin wrote: > There is also an alternative to this operator, and it's allowing a > shortcut to do: > > try: > val = do_thing() > except ThingError: > val = "default" > > In the form of: > > val = do_thing() except

Re: [Python-ideas] Fwd: Null coalescing operator

On Tue, Sep 13, 2016 at 12:03 AM, Rob Cliffe wrote: > Assuming you can't break existing code that already traps TypeError, > AttributeError, etc., I don't see how you can do this without > having separated kinds of NoneError which were subclasses of TypeError, >

Re: [Python-ideas] Fwd: Null coalescing operator

On 12/09/2016 08:05, Michel Desmoulin wrote: I messed up my answer and replied to one person instead of the list, so I'll post it again. There is also an alternative to this operator, and it's allowing a shortcut to do: try: val = do_thing() except ThingError: val = "default" In

Re: [Python-ideas] Fwd: Null coalescing operator

On 12 September 2016 at 09:05, Michel Desmoulin wrote: > In the form of: > > val = do_thing() except ThingError: "default" > > [...] > > But it also can deal with many common operations in Python without the > need to add more operators or variants: > > val =

Re: [Python-ideas] Fwd: Null coalescing operator

On 09/12/2016 12:05 AM, Michel Desmoulin wrote: There is also an alternative to this operator, and it's allowing a shortcut to do: try: val = do_thing() except ThingError: val = "default" In the form of: val = do_thing() except ThingError: "default" I was debated, and rejected,

Re: [Python-ideas] Fwd: Null coalescing operator

On Sun, Sep 11, 2016 at 6:00 PM, David Mertz wrote: > None if a is None else a.foo This is the crux of the matter to me. It's just too verbose, and the `if` and `else` keywords are lost in the noise of all the other words on the line. Plus the big win when it applies) is

Re: [Python-ideas] Fwd: Null coalescing operator

On Sun, Sep 11, 2016 at 12:44 PM, Daniel Moisset wrote: > Both this discussion, PEP 505, and the one a year ago, tend to mix up 2 > related but separate proposals: I don't think there's that much of a mix-up. PEP 505 clearly describes each proposal separately and even

Re: [Python-ideas] Fwd: Null coalescing operator

Both this discussion, PEP 505, and the one a year ago, tend to mix up 2 related but separate proposals: w (A) Add a None-coalescing operator (like C# a ?? b, what you would write in Python as "a or b" if it didn't have the falsy gotcha) (B) Add some None-aware navigation operators ( The "?.",

Re: [Python-ideas] Fwd: Null coalescing operator

On Sat, Sep 10, 2016 at 6:02 PM, David Mertz wrote: > What I was getting at with "essentially" was that it would *do the same > thing* that an AttributeError does. That is, if `x.foo` can't be evaluated > (i.e. x doesn't have an attribute 'foo'), then access is informally "an >

Re: [Python-ideas] Fwd: Null coalescing operator

On Sat, Sep 10, 2016, at 19:38, David Mertz wrote: > x2 = x?.foo > > x3 = x?.bar?.baz?[x2] A. if you're doing three different things with x, why are you using this instead of wrapping it in an if statement? B. Under some of the proposals, unless x.bar might be None independently of x

Re: [Python-ideas] Fwd: Null coalescing operator

How much of the time is a branch of the None check a single fallback value or attribute access versus how often a suite of statements within the not-None branch? I definitely check for None very often also. I'm curious what the breakdown is in code I work with. On Sep 10, 2016 7:10 PM, "Guido

Re: [Python-ideas] Fwd: Null coalescing operator

To the contrary. I read and write code that performs explicit checks for None all the time. Catching AttributeError is often a code smell or at least a measure of last resort. On Sat, Sep 10, 2016 at 6:46 PM, David Mertz wrote: > Ok, I have been thinking of the behavior too

Re: [Python-ideas] Fwd: Null coalescing operator

Ok, I have been thinking of the behavior too broadly. I realize now that `x?.foo` might still simply raise an AttributeError if x is neither None nor a thing with a foo attribute. The class I wrote is definitely too aggressive for the behavior described. On the other hand, by being narrower in

Re: [Python-ideas] Fwd: Null coalescing operator

On Sat, Sep 10, 2016 at 5:23 PM, Guido van Rossum wrote: > No. PEP 505 actually solves the problem without ever catching > AttributeError. Please read it. > I read it again (I did a year ago, but reviewed it now). I hadn't been thinking that the *mechanism* of a new

Re: [Python-ideas] Fwd: Null coalescing operator

No. PEP 505 actually solves the problem without ever catching AttributeError. Please read it. On Sat, Sep 10, 2016 at 5:15 PM, David Mertz wrote: > On Sep 10, 2016 4:45 PM, "Guido van Rossum" wrote: >> >> There seems to be a major misunderstanding here. A

Re: [Python-ideas] Fwd: Null coalescing operator

There seems to be a major misunderstanding here. A None-coalescing operator is not for catching AttributeError, it's a shortcut similar to "a or b" except that it checks for "a is None" rather than bool(a). On Sat, Sep 10, 2016 at 4:38 PM, David Mertz wrote: > Sorry, I sent this

[Python-ideas] Fwd: Null coalescing operator

Sorry, I sent this accidentally as private reply, then tried to fix it on phone. The latter produced horrible formatting. Please just read this version. On Sat, Sep 10, 2016 at 4:10 PM, Guido van Rossum wrote: > So you're offering `NoneCoalesce(x).bar` as less-ugly