Why less emphasis on private data?

Paul Boddie paul at boddie.org.uk
Mon Jan 8 12:28:24 CET 2007


Steven D'Aprano wrote:
>
> The truth of the matter is, MyClass.__private is not private at all. It is
> still a public attribute with a slightly unexpected name. In other words,
> if you want to code defensively, you should simply assume that Python has
> no private attributes, and code accordingly.
>
> Problem solved.

Well, it isn't really solved - it's more avoided than anything else.
;-)

Still, if one deconstructs the use of private data in various
programming languages, one can identify the following roles (amongst
others):

 1. The prevention of access to data from program sections
    not belonging to a particular component.
    (The classic "keep out" mechanism.)
 2. The enforcement of distinct namespaces within components.
    (Making sure that subclass attributes and superclass attributes
    can co-exist.)
 3. To support stable storage layouts and binary compatibility.

Most Python adherents don't care too much about #1, and Python isn't
driven by the need for #3, mostly due to the way structures (modules,
classes, objects) are accessed by the virtual machine. However, one
thing which does worry some people is #2, and in a way it's the
forgotten but more significant benefit of private data.

Before I became completely aware of the significance of #2, I remember
using various standard library classes which are meant to be subclassed
and built upon, thinking that if I accidentally re-used an attribute
name then the operation of such classes would be likely to fail in
fairly bizarre ways. Of course, a quick browse of the source code for
sgmllib.SGMLParser informed me of the pitfalls, and I'm sure that
various tools could also be informative without the need to load
sgmllib.py into a text editor, but if I had been fully aware of the
benefits of private attributes and could have been sure that such
attributes had been used (again, a tool might have given such
assurances) then I wouldn't have needed to worry.

So I suppose that to "code accordingly" in the context of your advice
involves a manual inspection of the source code of superclasses or the
usage of additional tools. Yet I suppose that this isn't necessarily
unusual behaviour when working with large systems.

Paul




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