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Re: A Single Instance of an Object?

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From: hjp-python@hjp.at (Peter J. Holzer)
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Subject: Re: A Single Instance of an Object?
Date: Tue, 12 Mar 2024 00:37:14 +0100
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by: Peter J. Holzer - Mon, 11 Mar 2024 23:37 UTC

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On 2024-03-11 16:53:00 -0400, Ivan "Rambius" Ivanov via Python-list wrote:
> I am refactoring some code and I would like to get rid of a global
> variable. Here is the outline:

...

> The global cache variable made unit testing of the lookup(key) method
> clumsy, because I have to clean it after each unit test. I refactored
> it as:
>
> class Lookup:
> def __init__(self):
> self.cache = {}
>
> def lookup(key):
> if key in self.cache:
> return self.cache[key]
>
> value = None
>
> cmd = f"mycmd {key}"
> proc = subprocess(cmd, capture_output=True, text=True, check=False)
> if proc.returncode == 0:
> value = proc.stdout.strip()
> else:
> logger.error("cmd returned error")
>
> self.cache[key] = value
> return value
>
> Now it is easier to unit test, and the cache is not global. However, I
> cannot instantiate Lookup inside the while- or for- loops in main(),
> because the cache should be only one. I need to ensure there is only
> one instance of Lookup - this is why I made it a global variable, so
> that it is accessible to all functions in that script and the one that
> actually needs it is 4 levels down in the call stack.
[...]
> I am looking for the same behaviour as logging.getLogger(name).
> logging.getLogger("myname") will always return the same object no
> matter where it is called as long as the name argument is the same.
>
> How would you advise me to implement that?

Just add a dict of Lookup objects to your module:

lookups = {}

def get_lookup(name):
if name not in lookups:
lookups[name] = Lookup()
return lookups[name]

Then (assuming your module is also called "lookup", in all other modules
do

import lookup

lo = lookup.get_lookup("whatever")

...
v = lo.lookup("a key")

In your test cases where you need many different lookup tables use

lo = lookup.get_lookup("test1")
...
lo = lookup.get_lookup("test2")
...
lo = lookup.get_lookup("test3")

PS: You don't have to put that in a separate module but I think it's a
lot cleaner that way.

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The meek are contesting the will.

devel / comp.lang.python / Re: A Single Instance of an Object?

Subject	Author
Re: A Single Instance of an Object?	Peter J. Holzer