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

[Mark E. Haase]

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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