what is a singleton? (and python patterns) (intermediate - advanced) anthony explains #188

today I talk about the singleton pattern and a bunch of different ways to implement it in python!

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[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!

[tombraril]

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

[bojandimic]

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

[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

[alsonyang]

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?

[tobb]

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

[dorb]

Awesome video. Great tricks. Thank you

[bhushanmakode]

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!

[splendorman]

great video as always. saved me lots of headaches.

[dorb]

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...

[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.

[talalkalai]

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

[walkdead]

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

[yaron-ehsass]

More patterns and design infrastructure please

[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)