class factory example needed (long)

[Gary Ruben]

I have a class factory problem. You could say that my main problem is
that I don't understand class factories.
My specific problem:
I have a class with several methods I've defined.
To this class I want to add a number of other class methods where the
method names are taken from a list.
For each list member, I want to build a method having the list member
name so that I can override another method of the same name for objects
which are instances of this class. Finally, I want my class factory
which creates the method to call the overridden method.

I'm sure my description is confusing, so I'll try to illustrate it.
import Numeric
# The Numeric module contains a whole lot of methods which operate on
certain number types

class foo:
""" this is a number type class
"""
def __init__(self, value):
self.value = float(value)
def __str__(self):

[Gary Ruben]

Thanks for the very helpful reply Rainer,
I thought I couldn't use setattr because I need the syntactic sugar
sqrt(f) to work, but with your example code Numeric.sqrt(f) does in fact
work correctly. I also need to do a bit more than may be possible with a
simple lambda function, but I should be able to sort it out from here
with the info you provided,
thanks again,
Gary

Rainer Mansfeld wrote:
<snip>

Hi Gary,

you want your 'class factory' to change the methods of Numeric, so that
they accept foo objects and return foo objects?
I've not the slightest idea how to achieve that.

If OTOH you want your foo class to have sqrt, arccos, etc. methods
without defining them explicitly, I think you're looking for something
like:

. import Numeric
.
. class Foo(object):
. def __init__(self, value):
. self.value = float(value)
. for u in ['sqrt', 'cos', 'tan']:
. setattr(self, u, lambda uf=getattr(Nume ric, u):
. uf(self.value + 42.0))

>>> f = Foo(7)
>>> f.sqrt()

7.0

HTH
Rainer

[Gary Ruben]

OK, I've managed to get this to work with Rainer's method, but I
realised it is not the best way to do it, since the methods are being
added by the constructor, i.e. they are instance methods. This means
that every time a foo object is created, a whole lot of code is being
run. It would be better to do the same thing with class 'static'
methods, if this is possible, so that the methods are created just once.
Is this possible?
Gary

Rainer Mansfeld wrote:
<snip>

If OTOH you want your foo class to have sqrt, arccos, etc. methods
without defining them explicitly, I think you're looking for something
like:

. import Numeric
.
. class Foo(object):
. def __init__(self, value):
. self.value = float(value)
. for u in ['sqrt', 'cos', 'tan']:
. setattr(self, u, lambda uf=getattr(Nume ric, u):
. uf(self.value + 42.0))

>>> f = Foo(7)
>>> f.sqrt()

7.0

HTH
Rainer

[Steven Bethard]

Gary Ruben wrote:

OK, I've managed to get this to work with Rainer's method, but I
realised it is not the best way to do it, since the methods are being
added by the constructor, i.e. they are instance methods. This means
that every time a foo object is created, a whole lot of code is being
run. It would be better to do the same thing with class 'static'
methods, if this is possible, so that the methods are created just once.
Is this possible?

See my comment on the recipe at:

http://aspn.activestate.com/ASPN/Coo.../Recipe/389793

STeVe

[Kent Johnson]

Gary Ruben wrote:

OK, I've managed to get this to work with Rainer's method, but I
realised it is not the best way to do it, since the methods are being
added by the constructor, i.e. they are instance methods. This means
that every time a foo object is created, a whole lot of code is being
run. It would be better to do the same thing with class 'static'
methods, if this is possible, so that the methods are created just once.
Is this possible?

Here is a solution using a metaclass:

import Numeric

class MetaFoo(type):
def __init__(cls, name, bases, d):
super(MetaFoo, cls).__init__(c ls, name, bases, d)
for u in ['sqrt', 'cos', 'tan']:
setattr(cls, u, lambda self, uf=getattr(Nume ric, u): uf(self.value + 42.0))
class Foo(object):
__metaclass__ = MetaFoo

def __init__(self, value):
self.value = float(value)

f = Foo(7)
print f.sqrt()
Kent

[Gary Ruben]

Thanks Steven and Kent, both of your suggestions look good to me. I'll
try both out and pick one.
Gary

Gary Ruben wrote:

OK, I've managed to get this to work with Rainer's method, but I
realised it is not the best way to do it, since the methods are being
added by the constructor, i.e. they are instance methods. This means
that every time a foo object is created, a whole lot of code is being
run. It would be better to do the same thing with class 'static'
methods, if this is possible, so that the methods are created just once.
Is this possible?
Gary