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devel / comp.lang.python / Re: typing instance variable

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o Re: typing instance variableDennis Lee Bieber

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Re: typing instance variable

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From: wlfr...@ix.netcom.com (Dennis Lee Bieber)
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Subject: Re: typing instance variable
Date: Fri, 14 May 2021 11:44:13 -0400
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 by: Dennis Lee Bieber - Fri, 14 May 2021 15:44 UTC

On Fri, 14 May 2021 08:28:48 +0200, David Kolovratník
<david@kolovratnik.net> declaimed the following:

>
>python3 myclass.py
>Traceback (most recent call last):
> File "myclass.py", line 1, in <module>
> class MyClass:
>ValueError: 'foo' in __slots__ conflicts with class variable
>
>This is where the way of type declaration of instance variable comes from:
>
>https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html
>class MyClass:
> # You can optionally declare instance variables in the class body
> attr: int
> # This is an instance variable with a default value
> charge_percent: int = 100
>
>Why Python believes foo is class variable?
>

Based upon previous experience -- because it IS. Anything declared
outside of a method is a "class variable" which provides an initial value
to instance variable when used within a method.

https://docs.python.org/3.8/reference/compound_stmts.html#class-definitions
"""
Programmer’s note: Variables defined in the class definition are class
attributes; they are shared by instances. Instance attributes can be set in
a method with self.name = value. Both class and instance attributes are
accessible through the notation “self.name”, and an instance attribute
hides a class attribute with the same name when accessed in this way. Class
attributes can be used as defaults for instance attributes, but using
mutable values there can lead to unexpected results.
"""
https://docs.python.org/3.8/tutorial/classes.html#class-and-instance-variables

--
Wulfraed Dennis Lee Bieber AF6VN
wlfraed@ix.netcom.com http://wlfraed.microdiversity.freeddns.org/

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