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<mailman.372.1696102506.23016.python-list@python.org> 2827d2db
@REPLY:
<d5e8dbff-8b43-4867-10bb-76ab6f6dbaf2@wichmann.us> d518f851
@REPLYADDR Mats Wichmann <mats@wichmann.us>
@REPLYTO 2:5075/128 Mats Wichmann
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@RFC-References: <ZRhwWN29i3mskxEL@hermes.hilbert.loc>
<d5e8dbff-8b43-4867-10bb-76ab6f6dbaf2@wichmann.us>
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@TID: FIDOGATE-5.12-ge4e8b94
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


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