Encapsulation in Python

[Rick Johnson]

(NOTE: My apologies to the group if this same message was sent twice. From my end, it appears to have been lost, so i'm sending again. Thanks)

On Thursday, March 10, 2016 at 1:36:48 PM UTC-6, Mark Lawrence wrote:
> On 10/03/2016 14:57, Dan Strohl via Python-list wrote:
> >> I've been studying Object Oriented Theory using Java. Theoretically, all
> >> attributes should be private, meaning no one except the methods itself can
> >> access the attribute;
> >>
> >> public class Foo {
> >>      private int bar;
> >>      ...
> >
> 
> For the benefit of any newbies/lurkers I'll just point out
> that this might well be valid Java, but...
> 
> > class person:
> >       age = 21
> >       name = 'Cool Dude'
> >
> ...this gives you class attributes, so the age is always
> 21 and the name is always 'Cool Dude'.  So you can vary
> the age and name you'd need:-
> 
> class person():
>      def __init__(self, age, name):
>          self.age = age
>          self.name = name

Mark, you've made some absurd claims in the past, but are
you *REALLY* expecting this group of highly intelligent
people to believe, that class attributes are read-only? 

############################################################
# BEGIN INTERACTIVE SESSION
############################################################
PY> class Foo(object):
...     age=21
...     name = 'cool dude'
...     
PY> Foo.name
cool dude
PY> Foo.age
21
PY> Foo.age = 25
PY> Foo.age
25
############################################################
# END INTERACTIVE SESSION
############################################################

And furthermore, do you realize that the "new code" you
posted, creates instance level variables, *NOT* class level
variables?

############################################################
# BEGIN INTERACTIVE SESSION
############################################################
PY> class Foo(object):
...     def __init__(self, name, age):
...         self.name = name
...         self.age = age
...         
PY> Foo.name
AttributeError: type object 'Foo' has no attribute 'name'

PY> Foo.age
AttributeError: type object 'Foo' has no attribute 'age'
############################################################
# END INTERACTIVE SESSION
############################################################

Of course, "name" and "age" would be more appropriate when
defined as instance variables, since both are attributes
specific to each person. A class level variable should only
be used for an attribute that is shared by *ALL* instances
of the class, and one that is *NOT* specific to any one of
them.

For instance, something highly general like keeping a tally
of the total number of "Persons" created, or the total
number of persons with red hair, or with green eyes. These
are the more generalized attributes of *ALL* persons, that
are not specific to any *ONE* person.

By offering example code that attempts to initialize class
level variables in the constructor, you have exposed yourself
as not understanding what a class level variable *IS*. Mark, we
*DON'T* initialize class level variables, no, we assign them
to the "class object". Class level variables are intended to
be used as "shared state" between multiple instances, not
"initialized" by each instance.

But don't feel bad. Most folks would be surprised at the
high percentage of "seasoned programmers" who do not
understand the difference between class level, and instance
level variables. I've have personally witnessed someone say
aloud: "Class variables are useless, always use instance
variables instead".

    Oh my! o_O

There are two types of variables you can create relative to
a class: instance level variables, and class level
variables. In python, both of them can be read and written at
will, however, creating them and accessing them is slightly
different.

PS: Ignorance of the unknown produces fear, and fear can
produce unwarranted hatred. I have suspected for many years
that those in the Python community who are constantly bad-
mouthing OOP, are displaying an emotional reaction rooted in
the psychological distress of ignorance and fear, and your 
posting here has provided evidence of that distress.