[Python-ideas] Delayed Execution via Keyword

[David Mertz]

I rather like this at first brush!

On Feb 16, 2017 9:25 PM, "Joseph Hackman" <josephhackman at gmail.com> wrote:

> Howdy All!
>
> This suggestion is inspired by the question on "Efficient debug logging".
>
> I propose a keyword to mark an expression for delayed/lazy execution, for
> the purposes of standardizing such behavior across the language.
>
> The proposed format is:
> delayed: <expr>
> i.e. log.info("info is %s", delayed: expensiveFunction())
>
> Unlike 'lambda' which returns a function (so the receiver must be
> lambda-aware), delayed execution blocks are for all purposes values. The
> first time the value (rather than location) is read, or any method on the
> delayed object is called, the expression is executed and the delayed
> expression is replaced with the result. (Thus, the delayed expression is
> only every evaluated once).
>
> Ideally:
> a = delayed: 1+2
> b = a
> print(a) #adds 1 and 2, prints 3
> # a and b are now both just 3
> print(b) #just prints 3
>
> Mechanically, this would be similar to the following:
>
> class Delayed():
>     def __init__(self, func):
>         self.__func = func
>         self.__executed = False
>         self.__value = None
>
>     def __str__(self):
>         if self.__executed:
>             return self.__value.__str__()
>         self.__value = self.__func()
>         self.__executed = True
>         return self.__value.__str__()
>
>
> def function_print(value):
>     print('function_print')
>     print(value)
>
> def function_return_stuff(value):
>     print('function_return_stuff')
>     return value
>
> function_print(function_return_stuff('no_delay'))
>
> function_print(Delayed(lambda: function_return_stuff('delayed')))
>
> delayed = Delayed(lambda: function_return_stuff('delayed_object'))
> function_print(delayed)
> function_print(delayed)
>
> Unfortunately, due to https://docs.python.org/3/reference/datamodel.html#
> special-lookup , this magic delayed class would need to implement many
> magic methods, as __getattribute__ is not _always_ called.
>
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