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From: "Peter J. Holzer" <hjp-python@hjp.at>
Newsgroups: comp.lang.python
Subject: Re: A Single Instance of an Object?
Date: Tue, 12 Mar 2024 00:37:14 +0100
Lines: 109
Message-ID: <mailman.91.1710200583.3452.python-list@python.org>
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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:

=2E..

> 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:
>=20
> class Lookup:
>     def __init__(self):
>         self.cache =3D {}
>=20
>     def lookup(key):
>         if key in self.cache:
>             return self.cache[key]
>=20
>         value =3D None
>=20
>         cmd =3D f"mycmd {key}"
>         proc =3D subprocess(cmd, capture_output=3DTrue, text=3DTrue, chec=
k=3DFalse)
>         if proc.returncode =3D=3D 0:
>             value =3D proc.stdout.strip()
>         else:
>             logger.error("cmd returned error")
>=20
>         self.cache[key] =3D value
>         return value
>=20
> 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.
>=20
> How would you advise me to implement that?

Just add a dict of Lookup objects to your module:

lookups =3D {}

def get_lookup(name):
    if name not in lookups:
        lookups[name] =3D Lookup()
    return lookups[name]

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

import lookup

lo =3D lookup.get_lookup("whatever")

=2E..
v =3D lo.lookup("a key")

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

lo =3D lookup.get_lookup("test1")
=2E..
lo =3D lookup.get_lookup("test2")
=2E..
lo =3D lookup.get_lookup("test3")

        hp

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

--=20
   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp@hjp.at         |    -- Charles Stross, "Creative writing
__/   | http://www.hjp.at/ |       challenge!"

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