type annotation vs working code

[Karsten Hilbert]

Am Sun, Oct 01, 2023 at 09:04:05AM +1300 schrieb dn via Python-list:

> >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

> >Where's the error in my thinking (or code) ?
>
> What is your thinking?
> Specifically, what is the purpose of testing self.already_initialized?

The purpose is to check whether the singleton class has been
... initialized :-)

The line

	self.already_initialized = True

is misleading as to the fact that it doesn't matter at all
what self.already_initialized is set to, as long as is
exists for the next time around.

> 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.

I fail to see how that can differentiate between first-call
and subsequent call.

> Alternately, how about using hasattr()? eg
>
> if hasattr( self.already_initialized, 'attribute_name' ):

That does work. I am using that idiom in other children of
Borg. But that's besides the point. I was wondering why it
does not work the same way with and without the type
annotation.

>         try:
>             self.already_initialized
>
> line is flagged by the assorted linters, etc, in my PyCharm as:
>
> Statement seems to have no effect.

Well, the linter simply cannot see the purpose, which is
test-of-existence.

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

It borders on the illegal, I suppose, as the self-
introspection capabilities of the language are being
leveraged to achieve a legal purpose.

Which seems akin constructs for generating compatibility
between versions.

It seems the answer is being pointed to in Matts response.

It just mightily surprised me.

Karsten
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