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Safari | Python Developer's Handbook -> Encapsulation





< BACKMake Note | BookmarkCONTINUE >
152015024128143245168232148039199167010047123209178152124239215162147039203225041225189172

Encapsulation

All Python attributes (variables and methods) are public. Even though you cannot have private attributes in Python, you can use the following two agreements:

  • By convention, attributes preceded with a single underscore (for example, _n) are to be viewed as internal variables, not to be used externally.

  • Attributes starting with double underscores (for example, __n) aren't explicitly exported. They are renamed to _Class__Variablename when byte compiled. Because the name of a class is used as part of the variable name, the attribute __n (when inside a subclass) isn't the same __n variable defined at a base class. This is probably the closest to private that you will get. But, it isn't really a private implementation because when you know the name of the class, you can access the attribute. C++ programmers probably know this as name mangling.

We cannot say that Python supports private attributes because it is still possible to have access to the attributes if you know the class and attributes names. For example, in a class called C, the attribute self.__attr becomes self._C__attr, when exported from the class. Hence, you can access this attribute by referencing it as _C__attr.

					
>>> class Number:
def __init__(self, value):
    self._n = value
    self.__n = value
def __repr__(self):
    return '%s(%s)'% (self.__class__.__name__, self._n)
def add(self, value):
    self._n = self._n + value
def incr(self):
    self._n = self._n + 1


				

Based on the previous class, we will have some interactive examples next.

					
>>> a = Number(20)
>>> a
Number(20)
>>> a.add(4)
>>> a
Number(24)
>>> a.incr()
>>> a
Number(25)
>>> a._n
25
>>> a._n = 30
>>> a
Number(30)
>>> a._Number__n
20

				

The important thing to remember is that nothing in Python is private (unless it is hidden within a C extension type).

To demonstrate that you can use default arguments to help storing the environment variables in a variable from the class namespace, the next example initializes the value of the variable n by using a default argument. The value of n is assigned at the time of defining the function and is stored at the class namespace.

					
>>> v = 10
>>> class C:
def storen(self, n=v):
    return n

>>> objA = C()
>>> objA.storen()
10
>>> v = 20
>>> objB = C()
>>> objB.storen()
10
>>> n = 30
>>> objC = C()
>>> objC.storen()
10

				

Note that the value of n remains constant for all instances of the class C.

The following example shows that it is possible to manipulate the internal attributes of an object by directly accessing the members of a class.

					
>>> class fun:
def __init__(self):
    self.total = None

>>> a = fun()
>>> b = fun()
>>> a.total = 2
>>> b.total = 3
>>> print a, b
2 3

				

In this next example, we hide the a() method definition by preceding it with two underscores. Note that if you later need to access this method (and you don't want to rename it), you must create a reference to the method, as shown in the following example.

					
>>> class C:
def __a(self):
    print "ni!"
b = __a

>>> a = C()
>>> a.b()
ni! 

				


Last updated on 1/30/2002
Python Developer's Handbook, © 2002 Sams Publishing

< BACKMake Note | BookmarkCONTINUE >

Index terms contained in this section

accessing
      private attributes 2nd
attributes
     objects
            changing
     private
            accessing 2nd
changing
      object attributes
class namespaces 2nd
editing
      object attributes
encapsulation
mangling
      name
manipulating
      object attributes
modifying
      object attributes
name mangling
namespaces
      class 2nd
object-oriented programming (OOP)
      encapsulation
objects
      changing attributes
private attributes
      accessing 2nd
programming
     object-oriented (OOP)
            encapsulation

2002, O'Reilly & Associates, Inc.

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0672319942_s.jpg2.91 kB29-01-02 11:29
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