Protecting instance variables

[Nikolaus Rath]

Hello,

I am really surprised that I am asking this question on the mailing
list, but I really couldn't find it on python.org/doc.

Why is there no proper way to protect an instance variable from access
in derived classes?

I can perfectly understand the philosophy behind not protecting them
from access in external code ("protection by convention"), but isn't
it a major design flaw that when designing a derived class I first
have to study the base classes source code? Otherwise I may always
accidentally overwrite an instance variable used by the base class...


Best,

-Nikolaus

--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C

[Diez B. Roggisch]

Re: Protecting instance variables

Nikolaus Rath schrieb:

Hello,
>
I am really surprised that I am asking this question on the mailing
list, but I really couldn't find it on python.org/doc.
>
Why is there no proper way to protect an instance variable from access
in derived classes?
>
I can perfectly understand the philosophy behind not protecting them
from access in external code ("protection by convention"), but isn't
it a major design flaw that when designing a derived class I first
have to study the base classes source code? Otherwise I may always
accidentally overwrite an instance variable used by the base class...

Here we go again...

http://groups.google.com/group/comp.lang.python/browse_thread/thread/188467d724b48b32/

To directly answer your question: that's what the __ (double underscore)
name mangling is for.

Diez

[Nikolaus Rath]

Re: Protecting instance variables

Hi,

Sorry for replying so late. Your MUA apparently messes up the
References:, so I saw you reply only now and by coincidence.

"Diez B. Roggisch" <deets@nospam.w eb.dewrites:

Nikolaus Rath schrieb:

>Hello,
>>
>I am really surprised that I am asking this question on the mailing
>list, but I really couldn't find it on python.org/doc.
>>
>Why is there no proper way to protect an instance variable from access
>in derived classes?
>>
>I can perfectly understand the philosophy behind not protecting them
>from access in external code ("protection by convention"), but isn't
>it a major design flaw that when designing a derived class I first
>have to study the base classes source code? Otherwise I may always
>accidentally overwrite an instance variable used by the base class...

>
Here we go again...
>

http://groups.google.com/group/comp.lang.python/browse_thread/thread/188467d724b48b32/

>
To directly answer your question: that's what the __ (double
underscore) name mangling is for.

I understand that it is desirable not to completely hide instance
variables. But it seems silly to me that I should generally prefix
almost all my instance variables with two underscores.

I am not so much concerned about data hiding, but about not
accidentally overwriting a variable of the class I'm inheriting from.
And, unless I misunderstood something, this is only possible if I'm
prefixing them with __.

How is this problem solved in practice? I probably don't have a
representative sample, but in the libraries that I have been using so
far, there were a lot of undocumented (in the sense of: not being part
of the public API) instance variables not prefixed with __. I have
therefore started to first grep the source of all base classes
whenever I introduce a new variable in my derived class. Is that
really the way it's supposed to be? What if one of the base classes
introduces a new variable at a later point?


Best,

-Nikolaus

--
»It is not worth an intelligent man's time to be in the majority.
By definition, there are already enough people to do that.«
-J.H. Hardy

PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6 02CF A9AD B7F8 AE4E 425C