what is a singleton? (and python patterns) (intermediate - advanced) anthony explains

today I talk about the singleton pattern and a bunch of different ways to implement it in python!
- typing Final: • python typing: Final /...
- decorators: • python @decorators - (...
- lru_cache: • python: functools.lru_...
playlist: • anthony explains
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I won't ask for subscriptions / likes / comments in videos but it really helps the channel. If you have any suggestions or things you'd like to see please comment below!

[@chukwunta]

I really enjoy your content. Your love for programming is very evident and it's contagious. Thank you so so much.

[@UriYakir]

Watching you code just leaves me at awe... Truly an inspiration!

[@bojandimic3914]

I am very much enjoying your videos. Keep up the good work Anthony!

[@tombra4ril]

You are a rare gem. I have encountered such problems before, thanks alot for this video. Love from Nigeria.

[@anthonywritescode]

glad it was helfpul!

[@patskanteam]

Here's how i would do it:
from weakref import WeakValueDictionary
class Singleton(type):
_instances = WeakValueDictionary()
def __call__(cls, *args, **kwargs):
try:
return type(cls)._instances[cls]
except KeyError:
obj = type(cls)._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
return obj
@classmethod
def cache_size(mcs):
return len(mcs._instances)
class Foo(metaclass=Singleton):
def __init__(self):
self.x = 1
def do_stuff(self):
return self.x + 1
obj1 = Foo()
obj2 = Foo()
print(obj1 is obj2) # True
print(f"Objects in cache: {Singleton.cache_size()}") # 1
del obj1
del obj2
print(f"Objects in cache: {Singleton.cache_size()}") # 0

[@alexandrugheorghe5610]

What does "mcs" stands for?

[@princeroshan4105]

​@@alexandrugheorghe5610metaclass

[@dorb1337]

Awesome video. Great tricks. Thank you

[@splendorman7922]

great video as always. saved me lots of headaches.

[@bhushanmakode7643]

Sir plz share more about design patterns in python

Well if this wasn't a really great commentary of python's data model!
More patterns/idioms, please!

[@goodclover]

mind=pancake
This is some confusing stuff.
Seeing you write that metaclass taught me alot about how pythons classes work behind-the-scenes, I'm gonna do some more research on that.
I like python for it's "hackability", you can break the language right down and do all sorts of magic with it's internals.

[@sadhlife]

exactly :D python lets you hook into its internals pretty easily when needed

[@yaronshamul2959]

More patterns and design infrastructure please

[@walkdead94]

Singleton is so cursed that at the end it broke anthony scene switcher! hahah

[@alsonyang230]

Thanks for all the great content, I have always been enjoying taking them with my breakfast haha.
I have a question on something could be quite trivial that is not even the main point of this video. At 16:48, you rewrite your `if` statement into `try except`, why did you choose `try` over `if` in this particular case?
I have always been confused about when I should use `try` and when `if`, and a lot of times I find myself using `if` without the need of using `try`, and I can argue that a lot of times I could have used `try` instead. What is your general rule of picking one if the situation allows you to use either?

[@anthonywritescode]

in that case it's for performance -- make the common path fast. the pattern is called EAFP

[@alsonyang230]

@@anthonywritescode Thanks for your reply!

[@tobb10001]

Is there a difference between type(self) and self.__class__?

[@anthonywritescode]

heheheh th-cam.com/video/6rAIttnm3Fs/w-d-xo.html

[@talalkalai8748]

what about a practical, real-life-code-problem-solving, design patterns series?

[@anthonywritescode]

I've done those in the past, they get really poor reception on youtube. my twitch streams are basically that though so maybe check those out!

[@dorb1337]

I wonder - How do you come up and understand the internals of python so good? What do you do rather than read and write a lot of code. It's been a year since I watched this video last and I still can't make up a singleton code from my mind even though I work as software engineer.How do you know when to use cls.inst and when super().__new__(cls) - it feel to me that I could memorize the idea and code from your videos, but I would never know how to use cls for dunder functions such as __new__ or less familiar and used ones like it in order to solve the singleton design pattern of other ideas...

[@anthonywritescode]

mostly just trying a lot of things

[@MichalPlichta]

For me singelton or global state is indication that i made something wrong with architecture ot thought process, i avoid it with any cost! But grate video!

[@kvelez]

class Singleton:
__instance = None

def __new__(cls):
if Singleton.__instance == None:
Singleton.__instance = super().__new__(cls)
return Singleton.__instance
database = Singleton()
database.x = 8
print(database.x)
database2 = Singleton()
print(database2.x)

[@kuijaye]

Apparently There's a guy named Anthony Singleton in one of the videos on youtube :)