On 2/17/17, Chris Angelico <rosuav@gmail.com> wrote:
Do delayed-expressions have identities or only values? For example:
rand = delayed: random.randrange(10) otherrand = rand assert rand is otherrand # legal? randid = id(rand) # legal? print(rand) # force to concrete value assert any(rand is x for x in range(10)) # CPython int caching assert randid == id(rand) # now what?
Alternatively, once the value becomes concrete, the delayed-expression becomes a trampoline/proxy to the actual value. Its identity remains unchanged, but all attribute lookups would be passed on to the other object. That does mean a permanent performance penalty though - particularly if it's doing it all in Python code rather than some sort of quick C bouncer.
what about: lazy_string = delayed: f"{fnc()}" could it be possible? BTW I was also thinking about something like l-strings (aka lazy f-string) to solve this problem: logger.debug("format {expensive()}") Still not sure if it is a good idea but with something like this: class Lazy_String: ''' this is only quick and dirty test implementation! ''' def __init__(self, string=None): if string is None: self.body = "'None'" else: self.body = compile('f'+repr(string), "<lazy_string>", 'eval') def __sub__(self, string): self.__init__(string) return self def __str__(self): return eval(self.body) def __repr__(self): return self.__str__() lazy = Lazy_String() we could use this: from lazy_string import lazy as l logger.debug(l-"format {expensive()}") # kind of l-string without new syntax