type annotation vs working code

[Mats Wichmann]

On 9/30/23 13:00, Karsten Hilbert via Python-list wrote:
> A type annotation isn't supposed to change what code does,
> or so I thought:
> 
> #------------------------------------------------------------
> class Borg:
> 	_instances:dict = {}
> 
> 	def __new__(cls, *args, **kargs):
> 		# look up subclass instance cache
> 		if Borg._instances.get(cls) is None:
> 			Borg._instances[cls] = object.__new__(cls)
> 		return Borg._instances[cls]
> 
> 
> class WorkingSingleton(Borg):
> 
> 	def __init__(self):
> 		print(self.__class__.__name__, ':')
> 		try:
> 			self.already_initialized
> 			print('already initialized')
> 			return
> 
> 		except AttributeError:
> 			print('initializing')
> 
> 		self.already_initialized = True
> 		self.special_value = 42
> 
> 
> class FailingSingleton(Borg):
> 
> 	def __init__(self):
> 		print(self.__class__.__name__, ':')
> 		try:
> 			self.already_initialized:bool
> 			print('already initialized')
> 			return
> 
> 		except AttributeError:
> 			print('initializing')
> 
> 		self.already_initialized = True
> 		self.special_value = 42
> 
> s = WorkingSingleton()
> print(s.special_value)
> 
> s = FailingSingleton()
> print(s.special_value)
> 
> #------------------------------------------------------------
> 
> Notice how Working* and Failing differ in the type annotation
> of self.already_initialized only.

What happens here is in the second case, the line is just recorded as a 
variable annotation, and is not evaluated as a reference, as you're 
expecting to happen, so it just goes right to the print call without 
raising the exception.  You could change your initializer like this:

     def __init__(self):
         print(self.__class__.__name__, ':')
         self.already_initialized: bool
         try:
             self.already_initialized
             print('already initialized')
             return

The syntax description is here:

https://peps.python.org/pep-0526/#global-and-local-variable-annotations