Set run vars with each call

Steven D'Aprano steve+comp.lang.python at pearwood.info
Wed Jul 13 02:44:54 CEST 2011


Gnarlodious wrote:

> Question. Is there a special method or easy way to set default values
> with each call to an instance? Any ideas to make it easier? What I
> want to do is have a constantly updating set of values which can be
> overridden. Just thought there was an easy way to set that up.

All the words are in English, but the sentences make no sense :)

Seriously, I don't understand what you mean. "Call to an instance"? Do mean
treating instances as a callable (like a function), or do you mean calling
an arbitrary method?

To make an instance itself callable, define a __call__ method.

What do you mean, "constantly updating set of values that can be
overridden"? Perhaps a simple example might help.

The closest thing I can think of, might be: you want to store a data
attribute in an instance, and use that if the caller doesn't specify
differently. Something like:

class Parrot:
    name = "Polly"
    def speak(self, name=None):
        if name is None:
            name = self.name
        print("%s wants a cracker!" % name)

And in use:

>>> p = Parrot()
>>> p.speak()
Polly wants a cracker!
>>> p.speak("Peter")
Peter wants a cracker!
>>> p.name = "Penelope"
>>> p.speak()
Penelope wants a cracker!



If None is a legitimate value, then you can define your own sentinel to use
instead:

MISSING = object()  # Unique object guaranteed not to be used by the caller.
# (Guarantee void on planet Earth.)

then replace None by MISSING in the code above.

Is this the sort of scenario you are talking about? If not, I'm completely
lost.


-- 
Steven




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