Re: Reusing object methods?
Axel Straschil wrote: Hello! Why not: class A: def a_lengthy_method(self, params): # do some work depending only on data in self and params class B(A): pass ? Lg, AXEL. As a rough guess, I think the original poster was wondering how to include *one* specific method from class A into B, without including all the methods of A. Jp Calderone's suggestion of defining a special Mixin class seems to be the cleanest implementation. E.g. class MyMixin: Defines only a single method. It may be a debug call, or a replacement for the classes' string representation. Etc. def a_lengthy_method(self,params): # do some work class A(MyMixin): def other_lengthy_procedures(self, params): pass class B(MyMixin): pass The advantage of this is that you can define a normal inheritance tree for a variety of classes, and then specifically override a single (or group) of methods by placing the MyMixin class at the front of the inheritance call. (The book Programming Python uses this a LOT in the Tkinter section.) E.g. class C: def well_now_what_do_we_do(self): # stuff def a_lengthy_method(self,params): # This does the wrong stuff. class D(MyMixin, C): def __init__(self): # blahblahblah Now class D has the correct a_lengthy_method, inherited from MyMixin, as well as all the other methods from class C, and the methods defined in it's own class statement. Joal -- http://mail.python.org/mailman/listinfo/python-list
Re: Reusing object methods?
If you need just a single method once, don't use a mixin; just do B.meth = A.meth.im_func. Do not underestimate the burden of mixins on people reading your code (at least, for me it is burden, since I have to draw the inheritance tree in my mind, look at which methods are involved, wonder about the resolution order etc.) Michele Simionato -- http://mail.python.org/mailman/listinfo/python-list
Re: Reusing object methods?
Hello! Why not: class A: def a_lengthy_method(self, params): # do some work depending only on data in self and params class B(A): pass ? Lg, AXEL. -- http://mail.python.org/mailman/listinfo/python-list
Re: Reusing object methods?
On Fri, 29 Apr 2005 19:45:54 +0200, Ivan Voras [EMAIL PROTECTED] wrote: Can this be done: (this example doesn't work) class A: def a_lengthy_method(self, params): # do some work depending only on data in self and params class B: def __init__(self): self.a_lengthy_method = A.a_lengthy_method # I know that data in self of class B object is compatible # with that of class A object I want to 'reuse' the same method from class A in class B, and without introducing a common ancestor for both of them - is this possible in an elegant, straightforward way? This is what sub-classing is for. If it makes you feel better, call it mixing in instead. You can share this method between two classes without inheritance (by defining a free function and then ...), but using a common base class really is just the right way to do it. Jp -- http://mail.python.org/mailman/listinfo/python-list
Re: Reusing object methods?
Jp Calderone wrote: I want to 'reuse' the same method from class A in class B, and without introducing a common ancestor for both of them - is this possible in an elegant, straightforward way? This is what sub-classing is for. If it makes you feel better, call it mixing in instead. You can share this method between two classes without inheritance (by defining a free function and then ...), but using a common base class really is just the right way to do it. Ok. -- http://mail.python.org/mailman/listinfo/python-list
Re: Reusing object methods?
Ivan Voras wrote: I want to 'reuse' the same method from class A in class B, and without introducing a common ancestor for both of them - is this possible in an elegant, straightforward way? import new class A: ... def foo(self): ... print Hello, %s! % self.__class__.__name__ ... class B: ... def __init__(self): ... self.foo = new.instancemethod(A.foo.im_func, self, B) ... A().foo() Hello, A! B().foo() Hello, B! -- http://mail.python.org/mailman/listinfo/python-list
Re: Reusing object methods?
Ivan Voras wrote: I want to 'reuse' the same method from class A in class B, and without introducing a common ancestor for both of them - is this possible in an elegant, straightforward way? Straightforward, if not elegant: def method(self, params): ... print self, params ... class A: ... method = method ... class B: ... method = method ... A().method(42) __main__.A instance at 0x402aadac 42 B().method(42) __main__.B instance at 0x402aadac 42 Peter -- http://mail.python.org/mailman/listinfo/python-list
Re: Reusing object methods?
On Fri, 29 Apr 2005 19:45:54 +0200, Ivan Voras [EMAIL PROTECTED] wrote:
Can this be done: (this example doesn't work)
class A:
def a_lengthy_method(self, params):
# do some work depending only on data in self and params
class B:
def __init__(self):
self.a_lengthy_method = A.a_lengthy_method
# I know that data in self of class B object is compatible
# with that of class A object
I want to 'reuse' the same method from class A in class B, and without
introducing a common ancestor for both of them - is this possible in an
elegant, straightforward way?
Your design intent is not quite clear to me, since you are trying to
capture the A method from B's instance initialization. If it is to be
a method of B, why not put the code in the class body of B instead
of in B.__init__, whose concern is B instances, not the B class (at least
normally).
OTOH, if you really want an instance-method as an attribute of a B instance,
see
class B below. Such a method is not shared by different instances, though the
example here doesn't demonstrate that, since they would all be clone
instance-methods.
The C and D examples provide a shared method to their instances.
Here are some ways to do what you might be wanting to do ;-)
class A:
...def a_lengthy(self, params): return self, params
...
class B:
...def __init__(self):
...self.a_lengthy = A.a_lengthy.im_func.__get__(self, type(self))
...
class C:
...a_lengthy = A.a_lengthy.im_func
...
class D:
...a_lengthy = A.__dict__['a_lengthy']
...
a = A()
b = B()
c = C()
d = D()
a.a_lengthy('actual param')
(__main__.A instance at 0x02EF628C, 'actual param')
b.a_lengthy('actual param')
(__main__.B instance at 0x02EF618C, 'actual param')
c.a_lengthy('actual param')
(__main__.C instance at 0x02EF62EC, 'actual param')
d.a_lengthy('actual param')
(__main__.D instance at 0x02EF16CC, 'actual param')
vars(a)
{}
vars(b)
{'a_lengthy': bound method instance.a_lengthy of __main__.B instance at
0x02EF618C}
vars(c)
{}
vars(d)
{}
Note that class B borrows the A method and makes an instance-method -- i.e.,
a bound method bound to the instance and set as an attribute of that instance
(which is why it shows up in vars(b)).
This mechanism could make customized instance-methods for different instances,
if you wanted to, since the class B is not modified by making variations.
A method is just a class variable whose value is a function, until it is
accessed as an attribute of either the class or an instance. What makes
it a method is the attribute-retrieval mechanism. So what you want to do
is possible, but A.a_lengthy_method is not the function, it is an unbound
method (because it was retrieved as an attribute of the class as opposed
to via an instance. For an ordinary method, you can get the function via
the im_func attribute, but if you want to borrow something trickier, you
may want to try the way class D does it.
For new-style classes, you will run across some differences.
Regards,
Bengt Richter
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