Alan Gauld wrote:
> "euoar" <[EMAIL PROTECTED]> wrote in
>> So, in python, you can add methods at run time to an
>> object, and even you can add them to a class at run time?
>
> I'm not sure about adding methods at run time, I've never
> tried it but I think the magic around the self parameter
> might not work. But you can definitely add attributes.
Sure it works:
In [1]: class foo(object): pass
...:
In [2]: f=foo()
In [3]: f.show()
---------------------------------------------------------------------------
<type 'exceptions.AttributeError'> Traceback (most recent call last)
D:\Projects\e3po\<ipython console> in <module>()
<type 'exceptions.AttributeError'>: 'foo' object has no attribute 'show'
In [4]: def show(self): print "Hi, I'm a foo"
...:
In [5]: foo.show=show
In [6]: f.show()
Hi, I'm a foo
More advanced explanation:
The magic around the self parameter is actually built-in to every
function object (or its class, anyway). Functions have __get__() methods
which means they can be used as descriptors. When Python evaluates
f.show(), quite a few steps happen:
- find the value of the show attribute in the class definition. This
finds the show function object.
- the function object has a __get__() method, so call show.__get__(obj)
where obj is the original object being accessed.
- the __get__() method wraps obj and the original function into a new
callable object and returns that.
- finally the temporary callable is actually called (by calling its
__call__() method) and the wrapper adds the self parameter to the
argument list and dispatches to the wrapped (original) function.
The descriptor mechanism is only used for class attributes, not instance
attributes, so if you want to add a method to an individual instance you
have to do a little more work using new.instancemethod:
In [12]: def show2(self): print "I'm still a foo"
....:
The naive approach won't work:
In [14]: f.show2 = show2
In [15]: f.show2()
<type 'exceptions.TypeError'>: show2() takes exactly 1 argument (0 given)
In [17]: import new
In [21]: f.show2 = new.instancemethod(show2, f)
In [22]: f.show2()
I'm still a foo
I hope that makes sense to someone; I had to see it about 10 times
myself before a light bulb went on. (This is number 10 :-)
More info here and in the references:
http://www.python.org/doc/2.2.3/whatsnew/sect-rellinks.html#SECTION000320000000000000000
Kent
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