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. -- _ | Peter J. Holzer| Story must make more sense than reality. |_|_) || | | | h...@hjp.at |-- Charles Stross, "Creative writing __/ | http://www.hjp.at/ | challenge!" signature.asc Description: PGP signature -- https://mail.python.org/mailman/listinfo/python-list
Re: A Single Instance of an Object?
On Tue, 12 Mar 2024 at 08:04, Ivan "Rambius" Ivanov wrote: > > A Singleton is just a global variable. Why do this? Did someone tell > > you "global variables are bad, don't use them"? > > I have bad experience with global variables because it is hard to > track what and when modifies them. I don't consider them bad, but if I > can I avoid them. > If you have a singleton, how will you track "what and when modifies" it? How is it any different from a global? ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: A Single Instance of an Object?
On Mon, Mar 11, 2024 at 5:06 PM dn via Python-list wrote: > > Good question Rambius! > > On 12/03/24 09:53, Ivan "Rambius" Ivanov via Python-list wrote: > > Hello, > > > > I am refactoring some code and I would like to get rid of a global > > variable. Here is the outline: > > > > import subprocess > > > > CACHE = {} > > First thought: don't reinvent-the-wheel, use lru_cache > (https://docs.python.org/3/library/functools.html) > > > > 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 = {} > > > > Change "cache" to be a class-attribute (it is currently an instance. > > Then, code AFTER the definition of Lookup can refer to Lookup.cache, > regardless of instantiation, and code within Lookup can refer to > self.cache as-is... > Thank you for your suggestions. I will research them! Regards rambius -- Tangra Mega Rock: http://www.radiotangra.com -- https://mail.python.org/mailman/listinfo/python-list
Re: A Single Instance of an Object?
On Mon, Mar 11, 2024 at 5:01 PM Chris Angelico via Python-list wrote: > > On Tue, 12 Mar 2024 at 07:54, 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: > > > > ... > > > > I have never done that in Python because I deliberately avoided such > > complicated situations up to now. I know about the Singleton pattern, > > but I have never implemented it in Python and I don't know if it is > > Pythonish. > > > > A Singleton is just a global variable. Why do this? Did someone tell > you "global variables are bad, don't use them"? I have bad experience with global variables because it is hard to track what and when modifies them. I don't consider them bad, but if I can I avoid them. Regards rambius -- Tangra Mega Rock: http://www.radiotangra.com -- https://mail.python.org/mailman/listinfo/python-list
Re: A Single Instance of an Object?
Good question Rambius! On 12/03/24 09:53, Ivan "Rambius" Ivanov via Python-list wrote: Hello, I am refactoring some code and I would like to get rid of a global variable. Here is the outline: import subprocess CACHE = {} First thought: don't reinvent-the-wheel, use lru_cache (https://docs.python.org/3/library/functools.html) 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 = {} Change "cache" to be a class-attribute (it is currently an instance. Then, code AFTER the definition of Lookup can refer to Lookup.cache, regardless of instantiation, and code within Lookup can refer to self.cache as-is... -- Regards, =dn -- https://mail.python.org/mailman/listinfo/python-list
Re: A Single Instance of an Object?
On Tue, 12 Mar 2024 at 07:54, 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: > > ... > > I have never done that in Python because I deliberately avoided such > complicated situations up to now. I know about the Singleton pattern, > but I have never implemented it in Python and I don't know if it is > Pythonish. > A Singleton is just a global variable. Why do this? Did someone tell you "global variables are bad, don't use them"? ChrisA -- https://mail.python.org/mailman/listinfo/python-list
A Single Instance of an Object?
Hello, I am refactoring some code and I would like to get rid of a global variable. Here is the outline: import subprocess CACHE = {} def lookup(key): Runs the command cmd, parses its output, extract's the key's value, caches it and returns it. If the key has already been in the cache, returns its cached value. If the command cmd returns an error, the value is set to None and cached as None.""" if key in CACHE: return 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") CACHE[key] = value return value ... def main(): while True: keys = load_keys() for key in keys: call_function_that_call_function_that_calls_lookup(key) 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 have never done that in Python because I deliberately avoided such complicated situations up to now. I know about the Singleton pattern, but I have never implemented it in Python and I don't know if it is Pythonish. 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? Regards rambius -- Tangra Mega Rock: http://www.radiotangra.com -- https://mail.python.org/mailman/listinfo/python-list