[Tutor] Dynamically generated classes

[Karim]

Hello Alan,

You mentioned the facade pattern. Could you please provide a basic 
simple implementation
of this pattern in python?

Regards
Karim

On 12/23/2010 10:13 AM, Alan Gauld wrote:
>
> "Rasjid Wilcox" <rasjidw at gmail.com> wrote
>
>> I've been playing with dynamically generated classes.  In particular:
>>
>> class ClassMaker(object):
>>    def __init__(maker_self, name):
>>        maker_self.name = name
>>        #
>>        class Foo(object):
>>            def __init__(self):
>>                self.parent = maker_self
>>            def whoami(self):
>>                print 'I am a Foo instance made by %s' % self.parent.name
>>        maker_self.Foo = Foo
>
> I'd probably just define the classes in a module rather than in the
> init method itself. Then pass the required class to the parent init
> as an argument. But the example code doesn't give enough clue
> as to how these classes will be used to be sure that will suit
> your need.
>
>>>>> a = ClassMaker('Alice')
>>>>> af = a.Foo()
>>>>> ab = a.Bar()
>>>>> af.whoami()
>> I am a Foo instance made by Alice
>>>>> ab.whoami()
>> I am a Bar instance made by Alice
>>>>> a.Foo is b.Foo
>> False
>
>> The actual use case is a system where there are multiple databases of
>> essentially the same form, where a is database A, and b is database B,
>> and Foo and Bar represent tables in both the databases.
>
> Unless you are actually building a database it would be unusual to have
> a class represent a table, a table normally represents a class - ie 
> its not
> a two way relation - or the class uses a table to populate attributes.
> But again we have no clues about what the classes actually do - its
> normally the methods of the class that define its usage...
>
>> the Foo table in database A, and bf would be the Foo table in database
>> B.
>
> That sounds like you may be trying to create a facade pattern?
>
>> My question is: is there a better way?  Based on my playing with the
>> above, it all seems to do what I want.  My only concern is that I've
>> not seen this idiom before, and perhaps there is a simpler or more
>> standard way?
>
> I think it will do what you want but I wouldn't bother putting the class
> definitions inside the init - unless there is a good reason. At the very
> least I would create two factory methods which are called by init.
> That would allow you to reset the classes if needed too.
>
> class A:
>  def __init__(self, name):
>        self.foo = makeFoo()
>        self.bar = makeBar()
>  def makeFoo(self):
>         class Foo: pass
>         return Foo
>  def makeBar(self):
>         class Bar: pass
>         reurn Bar
>
> But I still think its much more flexible to define the classes in a
> separate module:
>
> import foo, bar
>
> class A:
>    def __init__(self,foo,bar):
>         self.foo = foo
>         self.bar = bar
>
> a = A(foo.Foo(A),bar.Bar(A))  # pass A so Foo/Bar know their parent
>
> You can then extend the use of A without changing A by defining
> new derivatives of Foo and Bar (as you use new databases for
> example)
>
> HTH,
>