On Sat, 17 Apr 2010 19:55:44 +0400, Alexander wrote:
> Ok, I'll try to explain on the following example. Let's consider class
> MyClass that holds one string and concatenate it with other not defined
> in this class:
[...]
> and with working application:
>
> a = MyClass()
> a.set('key1')
>
> b1 = a.new('value1')
> b2 = a.new('value2')
>
> print b1, "," ,b2 # give 'key1 value1 , key1 value2'
>
> a.set('key2')
>
> print b1, ",", b2 # give 'key2 value1 , key2 value2'
Looking at your design, I can't see any reason for SubClass to be a
subclass of MyClass. It doesn't inherit any behaviour, and the
constructor takes completely different arguments. Why make it a Subclass?
MyClass is dangerous: creating an instance doesn't fully initialise the
instance, it leaves it in a half-initialised state that can cause
exceptions from simple operations like:
instance = MyClass()
print instance
This is very bad design.
Redesigning the pair of classes, I get this:
class MyClass(object):
def __init__(self, key):
self.key = key # No obvious need to make key a private attribute.
def new(self, value):
return AnotherClass(self, value)
class AnotherClass(object):
def __init__(self, obj, value):
self.obj = obj
self.value = value
def __str__(self):
return "%s %s" % (self.obj.key, self.value)
which gives the behaviour you ask for:
>>> a = MyClass('key1')
>>> b1 = a.new('value1')
>>> b2 = a.new('value2')
>>> print b1, "," ,b2
key1 value1 , key1 value2
>>>
>>> a.key = 'key2'
>>> print b1, "," ,b2
key2 value1 , key2 value2
--
Steven
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