What python idioms for private, protected and public?

[Michael Ekstrand]

On Thursday 29 September 2005 09:08, Michael Schneider wrote:
> Design Intent:
>
> 1) mark an object as dirty in a setter (anytime the object is
> changed, the dirty flag is set without requiring a user to set the
> dirty flag

2 ways: wrap every attribute that is to be set in a property object (in 
new-style classes), and set it in the setter there.

Or override __setattr__ to set the dirty flag.

Or, if you want overkill, but a potentially cleaner interface in some 
cases, a metaclass could also be contrived to accomplish this (crud, 
you can do _anything_ with a metaclass).

> 2) enforce value constraints (even if just during debugging)

Python's property objects

def _get_foo(self, foo):
    return self.__foo
def _set_foo(self, foo, val):
    if self.__is_valid_foo(val):
        self.__foo = foo
    else:
        raise ValueError("Invalid foo")
foo = property(_get_foo, _set_foo, doc="Foo property")

> 3) lazy init, don't bring the data in until needed

Define __getattr__. Or use a metaclass (but I tend to think of 
metaclasses as a solution to too many things - see previous thread 
about synchronization for details).

> 4) adding debug info

A metaclass to do tracing, whatever - they let you modify the nature of 
your class greatly on the fly.

> 5) .... more here????

Overriding magic methods (esp. getattr/setattr), various decorators 
(ether existing or custom for your purposes), metaclasses, etc.

> I usually violate private when adding an aspect to a class, and
> I don't want this code in every class.  Example,  persistence.

Persistence can be done with subclassing, with a base class that sets up 
persistence (I did this in PHP4 once, an ugly object model if I've ever 
seen one). Metaclasses might be cleaner.

> I really like the C# properties, you can access data with a data
> accessor, but add functionality is triggered when accessing the data.

Python has these - use property objects. property() is documented under 
Built-In Functions

> I like the data access syntax, better then the set/get functions.  I
> need the functionality to achieve the design goals above, so i use
> function, but the are ugly, especially in math code.

Yes, get/set is highly ugly IMHO. I have to do Java for one of my 
classes now... I frequently want to gag over the forced usage of such 
things.

> It would be easy for me to say "Add public and private to python so I
> can code the way that I am used to".  What are some python
> alternatives to achieve the design intents specified above above?

See above :-). I think you'll find that you can accomplish most or all 
of your purposes, and then a few you may not have thought of yet.

- Michael