[Python-ideas] Another use case for the 'lazy' (aka 'delayed') keyword

[Mark E. Haase]

There are plenty of PEP-505 threads in the archives, plus the PEP itself. I
was only pointing out that the use case in thread might also be solved with
an existing proposal*, so I won't derail this thread any further.

*There are actually several proposals for short-circuit evaluation,
including PEP-531 and PEP-532.

On Tue, Feb 28, 2017 at 10:29 AM, Joseph Hackman <josephhackman at gmail.com>
wrote:

> I like null coalesce too. :)
>
> I know that BDFL has said no to ? Before, but was that for the if-then
> shorthand only?
>
> Perhaps submit a new thread to this list so people can discuss/find?
>
> -Joseph
>
> On Feb 28, 2017, at 10:21 AM, Mark E. Haase <mehaase at gmail.com> wrote:
>
> This could be solved with a null-coalescing operator, e.g. PEP-505.
>
>    >>> val = conf.get('setting_name') ?? load_from_db('setting_name')
>
> The right side isn't evaluated unless the left side is None. It's similar
> to this:
>
>    >>> val = conf.get('setting_name') or load_from_db('setting_name')
>
> Except that using "or" would result in any false-y value (0, "", [], etc.)
> being overridden by the result of `load_from_db(...)`.
>
> I'm not strongly opposed to "lazy", but I think null-coalescing is easier
> to reason about.
>
>
> On Tue, Feb 28, 2017 at 7:04 AM, Michel Desmoulin <
> desmoulinmichel at gmail.com> wrote:
>
>> The debate on the 'lazy' keyword seems to have settled, but I don't know
>> if somebody is trying to write a PEP about it.
>>
>> Anyway, I was doing something like this the other day:
>>
>> conf.get('setting_name', load_from_db('setting_name'))
>>
>> And then realized I could save a query not doing the load_from_db() call
>> most of the time.
>>
>> But dict.get didn't accept callable so I couldn't do:
>>
>> conf.get('setting_name', lambda key: load_from_db('setting_name'))
>>
>> Which is standard practice in a lot of popular Python libs on Pypi.
>>
>> Instead I did:
>>
>> val = conf.get('setting_name')
>> if val is None:
>>     val = load_from_db('setting_name')
>>
>> Which is way more verbose. It also has a bug if None is a valid
>> configuration value or if I expect my code to be thread safe.
>>
>> It was not a problem for me, but in that case one would even have to do:
>>
>> try:
>>     val = conf['setting_name']
>> except KeyError:
>>     val = load_from_db('setting_name')
>>
>> Which is even more verbose.
>>
>> I was going to suggest to python-ideas to update the dict.get()
>> signature to accept a callable, but it would break compatibility with
>> many code actually getting a callable.
>>
>> We could do it for Python 4, but it's far way.
>>
>> Instead, I think it's a good example of were 'lazy' could help. You
>> can't get simpler than:
>>
>> conf.get('setting_name', lazy load_from_db('setting_name'))
>>
>>
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