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<mailman.373.1696104258.23016.python-list@python.org> 4cf5710e
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<ecc36615-1844-455f-b701-2bd41d98e9e1@DancesWithMice.info> ba82e524
@REPLYADDR dn <PythonList@DancesWithMice.info>
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<ecc36615-1844-455f-b701-2bd41d98e9e1@DancesWithMice.info>
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@TID: FIDOGATE-5.12-ge4e8b94
On 01/10/2023 08.00, 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.
> 
> Output:
> 
> WorkingSingleton :
> initializing
> 42
> 
> FailingSingleton :
> already initialized<====================== Huh ?
> Traceback (most recent call last):
 > File
"/home/ncq/Projekte/gm/git/gnumed/gnumed/client/testing/test-singleton.py", line 48, in 
>     print(s.special_value)
>           ^^^^^^^^^^^^^^^
> AttributeError: `FailingSingleton` object has no attribute `special_value`
> 
> 
> Where`s the error in my thinking (or code) ?

What is your thinking?
Specifically, what is the purpose of testing self.already_initialized?

Isn`t it generally regarded as `best practice` to declare (and define a 
value for) all attributes in __init__()? (or equivalent) In which case, 
it will (presumably) be defined as False; and the try-except reworded to 
an if-else.

Alternately, how about using hasattr()? eg

if hasattr( self.already_initialized, `attribute_name` ):
   # attribute is defined, etc


As the code current stands, the:

         try:
             self.already_initialized

line is flagged by the assorted linters, etc, in my PyCharm as:

Statement seems to have no effect.
Unresolved attribute reference `already_initialized` for class 
`WorkingSingleton`.

but:

             self.already_initialized:bool

passes without comment (see @Mats` response).


Question: is it a legal expression (without the typing)?

-- 
Regards,
=dn
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