Re: A Single Instance of an Object?

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")

        hp

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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