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

M.-A. Lemburg mal at egenix.com
Tue Feb 28 12:30:22 EST 2017

On 28.02.2017 17:54, M.-A. Lemburg wrote:
> On 28.02.2017 17:35, David Mertz wrote:
>> Clearly there is SOME semantics that is consistent and coherent since many
>> languages have either a lazy or eager declaration syntax, with different
>> defaults between languages but both being built in.  There are a lot of
>> ways that Python isn't Haskell, obviously.  But both Scheme and OCaml are
>> eager by default with a lazy declaration (and Haskell or Miranda have an
>> eager declaration correspondingly).
>> It might be worth looking at their semantics in the PEP.
> Scheme, for example, uses an explicit approach to turning
> a promise into a value:
> http://www.shido.info/lisp/scheme_lazy_e.html
> This makes a lot of sense, but you can already have the
> same in Python using generators.

Here's an example similar to OCaml's lazy evaluation, which
uses a simple lazy proxy object.

import sys
import time

### OCaml like lazy evaluation

class Lazy:
    def __init__(self, code, frame):
        self.code = code
        self.globals = frame.f_globals
        self.locals = frame.f_locals
    def force(self):
        return eval(self.code, self.globals, self.locals)

def lazy(code):
    return Lazy(code, sys._getframe(1))


def log(level, b, c):
    if level > 100:
    if isinstance(c, Lazy):
        c = c.force()
    print ('%04i: %s' % (level, b % c))

def expensive(x):
    return x

value = 1
log(1000, 'Hello %i', lazy("expensive(value)"))
log(10, 'Error %i', lazy("expensive(value)"))


Everything is nice and explicitly defined in the example.
You can see where the deferred evaluation is requested
and where it's eventually run. There are no surprises.

Marc-Andre Lemburg

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