How to replace constructor with factory method

[rogeeff]

Hi,

Just wonder if it's possible in Python.

what I want is to tweak an existing Python class A with no
constructor, so that A() results in fuctory method call?

so o = A() instead being equivalent to:

s = object()
A.__init__(s)
o = s

becomes:

o = my_factory_function( A )

Thanks,

Gennadiy

 

[Chris Rebert]

Hi,

Just wonder if it's possible in Python.

what I want is to tweak an existing Python class A with no
constructor, so that A() results in fuctory method call?

so o = A() instead being equivalent to:

s = object()
A.__init__(s)
o = s

becomes:

o = my_factory_function( A )

I believe you'd need to override __new__ for that.
Docs on __new__: http://docs.python.org/reference/datamodel.html#object.__new__

Cheers,
Chris

 

[Luis Zarrabeitia]

so o = A() instead being equivalent to:

s = object()
A.__init__(s)
o = s

Actually, it would be more like this:

s = A.__new__(A)
if isinstance(s,A):
A.__init__(s)
o = s

o = my_factory_function( A )

You could tweak:
*) A's constructor (__new__) [keep in mind the 'insinstance' part]
*) A's initializer (changing the type, and so on... ugly, fragile and
dangerous, don't ever do it!)
*) A's metaclass (the instantiation happens in the metaclass' __call__ method,
you could rewrite it to suit your needs, as in [1]
*) or, just use a method named A instead of the A class (as the
multiprocessing.Queue function does)

I would use the 4th option. The fact that python classes are callable allows
you to switch from instantiation to function calling without having to change
the user's code.


[1] http://trucosos.crv.matcom.uh.cu/snippets/95/