[Python-ideas] PEP 505 (None coalescing operators) thoughts

[Steven D'Aprano]

On Wed, Sep 30, 2015 at 09:41:53AM -0700, Jeff Hardy wrote:

> 'def' is currently short for 'define', which would be too confusing.
> Spelling out 'default' isn't so bad, though:
> 
>     self.x = x default []

Heh, people jest :-)

Oh, you're serious? That just goes to show how different we all are. I 
think that using "default" as an infix operator is hideous.

"default" as a function/verb? Sure, that works fine:

if len(missed_payments) > 5:
    loan.default()

"default" as a variable/noun? Absolutely fine:

default = "something or other"

But "x default z" just doesn't read well: in English, it sounds like you 
are ordering x to cause z to default. Not very many words read naturally 
as a binary infix operator.

Besides, I would expect that making "default" a keyword will break a 
*huge* number of programs. I'm sure that many, many people use it as a 
variable or parameter name.


> And if it's going to be that long anyway, we might as well just put a
> `default` function in the builtins:
> 
>     self.x = default(x, [])

That's useless, as it breaks the short-circuiting property, which is one 
of the critical motives for introducing the null coalescing operator. 
Without the short-circuiting property, there's no point in making it a 
built-in. If all you want is a default function, then just add it as a 
helper function to the top of your code:

def default(obj, alternative):
    if obj is None:
        return alternative
    return obj



> I actually really like 'otherwise', but it's certainly not brief:
> 
>     self.x = x if x is not None else []
>     self.x = x otherwise []

That's almost half the length of the alternative. And if you have 
something more realistic, you save proportionally even less:

    document if document is not None else Document()
    document otherwise Document()


But personally, I have absolutely zero interest in the null coalescing 
operator on its own. I don't object to it specifically, but what really 
interests me are the two (pseudo)operators, null-aware indexing and 
null-aware attribute lookup. How do you extend "otherwise" to work with 
the [] and . operators?



-- 
Steve