replace mothod for only one object but not for a class

**Miki** · Oct 14 '08, 06:25 PM

Re: replace mothod for only one object but not for a class

from functools import partial

class Point:
def __init__(self, x, y):
self.x, self.y = x, y

def show(self, n):
for i in range(n):
print "Point: (%s, %s)" % (self.x, self.y)

def new_method(obj, func):
def method(*args, **kw):
return func(obj, *args, **kw)

return method

p1 = Point(1, 2)
p2 = Point(3, 4)

def show(self, n):
print "Not a Point: %s-%s" % (self.x, self.y)
p2.show = partial(show, p2)

p1.show(3)
p2.show(3)

HTH,
--
Miki <miki.tebeka@gm ail.com>

PythonWise

http://pythonwise.blogspot.com

If it won't be simple, it simply won't be. [Hire me, source code]

**George Sakkis** · Oct 14 '08, 06:25 PM

Re: replace mothod for only one object but not for a class

On Oct 14, 1:50 pm, hofer <bla...@dungeon .dewrote:

Hi,
>
I have multiple objects all belonging to the same class
(which I didn't implement and whose code I don't want to modify)
>
Now I'd like to change one method for one object only (after it has
been created) without adding any overhead
to the call of the other object's methods.
>
Is this possible?
>
Example
##### This is NOT what I'd like to do
##### as it overwrites the method for all objects of this class
o1 = aclass()
o2 = aclass()
# call original method
o1.method()
o2.method()
# overwrite the method for the entire class
aclass.method = mymethod
o1.method() # now new method
o2.method() # now new method
>
####### What doesn't work, but what I'd like to do
o1 = aclass()
o2 = aclass()
# call original method
o1.method()
o2.method()
# overwrite the method for the entire class
o1.method = mymethod
o1.method() # now new method
o2.method() # still old method
>
thanks for any pointers.

Please post the actual code that doesn't work. The following works as
expected:

>>class A(object):

... def foo(self): return 'Original'
...

>>a = A()
>>b = A()
>>a.foo()

'Original'

>>b.foo()

'Original'

>>b.foo = lambda: 'Modified'
>>a.foo()

'Original'

>>b.foo()

'Modified'

HTH,
George

**Jason Scheirer** · Oct 14 '08, 06:45 PM

Re: replace mothod for only one object but not for a class

On Oct 14, 11:20 am, George Sakkis <george.sak...@ gmail.comwrote:

On Oct 14, 1:50 pm, hofer <bla...@dungeon .dewrote:
>
>
>

Hi,

>

I have multiple objects all belonging to the same class
(which I didn't implement and whose code I don't want to modify)

>

Now I'd like to change one method for one object only (after it has
been created) without adding any overhead
to the call of the other object's methods.

>

Is this possible?

>

Example
##### This is NOT what I'd like to do
##### as it overwrites the method for all objects of this class
o1 = aclass()
o2 = aclass()
# call original method
o1.method()
o2.method()
# overwrite the method for the entire class
aclass.method = mymethod
o1.method() # now new method
o2.method() # now new method

>

####### What doesn't work, but what I'd like to do
o1 = aclass()
o2 = aclass()
# call original method
o1.method()
o2.method()
# overwrite the method for the entire class
o1.method = mymethod
o1.method() # now new method
o2.method() # still old method

>

thanks for any pointers.

>
Please post the actual code that doesn't work. The following works as
expected:
>
>>class A(object):
... def foo(self): return 'Original'
...
>>a = A()
>>b = A()
>>a.foo()
'Original'
>>b.foo()
'Original'
>>b.foo = lambda: 'Modified'
>>a.foo()
'Original'
>>b.foo()
'Modified'
>
HTH,
George

What you're doing is called monkeypatching. I consider it dangerous,
but to each his own. Python's metaprogramming facilities can handle
it.

The lambda approach can leak, I've had garbage collection issues when
shuffling around bound methods (my objects weren't being collected
when expected, basically). The partial approach is cool, too, but it
won't reliably work with things like __getattr__ and __setattr__,
which I also learned the hard way. The probable best way of going
about it is to define a new class that implements your one method, and
use some on-the-fly magic to create a NEW class that inherits from
both your monkeypatch class and your current instance's class:

In [2]: class main_implementa tion(object):
...: def a(self):
...: print 'a'
...: def b(self, argb='b'):
...: print 'B says %r' % argb
...:
...:

In [6]: class new_implementat ion_of_a(object ):
...: def a(self):
...: print "This is a new implementation of A"
...:
...:

In [7]: mymain = main_implementa tion()

In [8]: mymain.a()
a

In [9]: mymain.__class_ _ = type(mymain.__c lass__.__name__ +'MODIFIED',
(new_implementa tion_of_a, mymain.__class_ _), {})

In [10]: mymain
Out[10]: <__main__.main_ implementationM ODIFIED object at 0x0137EA50>

In [11]: mymain.a()
This is a new implementation of A

In [12]: mymain.b()
B says 'b'

The magic here occurs when you create a new class on-the-fly using the
type() built-in function and assign it to the instance's __class__
attribute.

Good luck.

**Christian Heimes** · Oct 14 '08, 08:05 PM

Re: replace mothod for only one object but not for a class

Bruno Desthuilliers wrote:

If the class is a new-style one [1], it just requires invoking the
descriptor protocol by yourself to get a bound method, ie:

Another note about new style classes:
You can NOT overwrite most magic methods (__*__) on the instance. Most
magic methods are only looked up on the class object.

Christian

**George Sakkis** · Oct 14 '08, 09:05 PM

Re: replace mothod for only one object but not for a class

On Oct 14, 12:28 pm, Bruno Desthuilliers
<bdesth.quelque ch...@free.quel quepart.frwrote :

George Sakkis a écrit :
>
>
>

On Oct 14, 1:50 pm, hofer <bla...@dungeon .dewrote:

Hi,

>

I have multiple objects all belonging to the same class
(which I didn't implement and whose code I don't want to modify)

>

Now I'd like to change one method for one object only (after it has
been created) without adding any overhead
to the call of the other object's methods.

>

Is this possible?

>

Example
##### This is NOT what I'd like to do
##### as it overwrites the method for all objects of this class
o1 = aclass()
o2 = aclass()
# call original method
o1.method()
o2.method()
# overwrite the method for the entire class
aclass.method = mymethod
o1.method() # now new method
o2.method() # now new method

>

####### What doesn't work, but what I'd like to do
o1 = aclass()
o2 = aclass()
# call original method
o1.method()
o2.method()
# overwrite the method for the entire class
o1.method = mymethod
o1.method() # now new method
o2.method() # still old method

>

thanks for any pointers.

>

Please post the actual code that doesn't work. The following works as
expected:

>

>>class A(object):

... def foo(self): return 'Original'
...

>>a = A()
>>b = A()
>>a.foo()

'Original'

>>b.foo()

'Original'

>>b.foo = lambda: 'Modified'
>>a.foo()

'Original'

>>b.foo()

'Modified'

>
Except that b.foo is a plain function, not a method:
>

>>class A(object):

... def foo(self): return "original called on %s" % self
...

>>a = A()
>>b = A()
>>a.foo()

'original called on <__main__.A object at 0xb7c0c56c>'

>>b.foo()

'original called on <__main__.A object at 0xb7c0c4ec>'

>>b.foo = lambda: "modified called on %s" % self
>>b.foo

>>b.foo()

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 1, in <lambda>
NameError: global name 'self' is not defined

>>b.foo = lambda self: "modified called on %s" % self
>>b.foo()

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: <lambda>() takes exactly 1 argument (0 given)

>>>

You're right of course; that's what you get with minimal testing ;)
Still it works with a small modification, binding self to b as default
argument:

b.foo = lambda self=b: "modified called on %s" % self

The functools.parti al() solution shown before is the generalization of
this for more than one arguments. So I'm wondering, except for the
callable's exact type (bound method vs function vs functools.parti al
object) is there any other difference and a reason to prefer one over
the other ? If not, functools.parti al() seems more readable than
calling explicitly the descriptor.

George

**Bruno Desthuilliers** · Oct 15 '08, 08:45 AM

Re: replace mothod for only one object but not for a class

George Sakkis a écrit :
(snip)

You're right of course; that's what you get with minimal testing ;)
Still it works with a small modification, binding self to b as default
argument:
>
b.foo = lambda self=b: "modified called on %s" % self

Ok, now with a real use case : use a named function instead of a lambda,
and define the function before having access to b !-)

The functools.parti al() solution shown before is the generalization of
this for more than one arguments. So I'm wondering, except for the
callable's exact type (bound method vs function vs functools.parti al
object) is there any other difference and a reason to prefer one over
the other ?

Consistency ? Possibly performances (NB : not benchmarked - might be
worth a little homework to check this out) ?

If not, functools.parti al() seems more readable than
calling explicitly the descriptor.

Perhaps is it just me starting to getting senile and resistant to new
ideas, but while it does indeed work, I see the partial() solution as a
WTF. Invoking function.__get_ _ *is* the obvious[1] way to get a bound
method from a function.

[1] for anyone that know how Python turns functions into methods, that is...

But indeed, methods are specialized partial applications, and your
solution is perfectly valid.

**Bruno Desthuilliers** · Oct 15 '08, 08:55 AM

Re: replace mothod for only one object but not for a class

Bruno Desthuilliers a écrit :

hofer a écrit :

>Hi,
>>
>I have multiple objects all belonging to the same class
> (which I didn't implement and whose code I don't want to modify)
>>
>Now I'd like to change one method for one object only (after it has
>been created) without adding any overhead
>to the call of the other object's methods.
>>
>>
>Is this possible?

>
Yes.
>
If the class is a new-style one [1], it just requires invoking the
descriptor protocol by yourself to get a bound method, ie:
>

(snip)

If it's a classic class, you can get by using either new.instancemet hod
or types.MethodTyp e:

Either I'm starting to get old or I need some vacations... Manually
invoking function.__get_ _(obj, type(obj)) also work with classic
classes, of course.

(snip)

**hofer** · Oct 15 '08, 09:25 AM

Re: replace mothod for only one object but not for a class

On Oct 14, 7:50 pm, hofer <bla...@dungeon .dewrote:

Hi,
>
I have multiple objects all belonging to the same class
(which I didn't implement and whose code I don't want to modify)
>
Now I'd like to change one method for one object only (after it has
been created) without adding any overhead
to the call of the other object's methods.
>
Is this possible?
>
Example
##### This is NOT what I'd like to do
##### as it overwrites the method for all objects of this class
o1 = aclass()
o2 = aclass()
# call original method
o1.method()
o2.method()
# overwrite the method for the entire class
aclass.method = mymethod
o1.method() # now new method
o2.method() # now new method
>
####### What doesn't work, but what I'd like to do
o1 = aclass()
o2 = aclass()
# call original method
o1.method()
o2.method()
# overwrite the method for the entire class
o1.method = mymethod
o1.method() # now new method
o2.method() # still old method
>
thanks for any pointers.
>
P.S. I guess, that there is a computer science term for what I try to
achieve.
If anybody knew it I would be interested to learn it as well.

Thanks a lot this works.
Though I must admint, that even after reading the doc I don't really
understand why.

**hofer** · Oct 15 '08, 10:05 AM

Re: replace mothod for only one object but not for a class

Hi

hofer a écrit :

I have multiple objects all belonging to the same class
(which I didn't implement and whose code I don't want to modify)

>

Now I'd like to change one method for one object only (after it has
been created) without adding any overhead
to the call of the other object's methods.

Thanks for all of your answers:

Here an example with three of the suggested solutions:
(I didn't succeed in implementing Jason's solution with my
example)

############### ############### ############### ###########
import threading
# some objects
a = threading.Event ()
b = threading.Event ()
c = threading.Event ()
d = threading.Event ()

def run_dly(o): # a test function
print o,"start",
o.wait(1)
print "stop"

# unmodified test
run_dly(a)
run_dly(b)
run_dly(c)
run_dly(d)

# The new Method
def verbose_wait(se lf,dly):
print "VERBOSE",
threading._Even t.wait(self,dly )

### Implemented with partial
from functools import partial
b.wait = partial(verbose _wait,b)

### with __get__ for new classes
c.wait = verbose_wait.__ get__(c,type(c) )

## with new for old classes
import new
d.wait = new.instancemet hod(verbose_wai t,d,type(d))

run_dly(a)
run_dly(b)
run_dly(c)
run_dly(d)
############# end
thanks again

Hofer

**Bruno Desthuilliers** · Oct 15 '08, 11:15 AM

Re: replace mothod for only one object but not for a class

hofer a écrit :

Hi
>

>>hofer a écrit :
>>>I have multiple objects all belonging to the same class
>>> (which I didn't implement and whose code I don't want to modify)
>>>Now I'd like to change one method for one object only (after it has
>>>been created) without adding any overhead
>>>to the call of the other object's methods.

>
Thanks for all of your answers:
>
Here an example with three of the suggested solutions:
(I didn't succeed in implementing Jason's solution with my
example)
>
############### ############### ############### ###########
import threading
# some objects
a = threading.Event ()
b = threading.Event ()
c = threading.Event ()
d = threading.Event ()
>
def run_dly(o): # a test function
print o,"start",
o.wait(1)
print "stop"
>
# unmodified test
run_dly(a)
run_dly(b)
run_dly(c)
run_dly(d)
>
# The new Method
def verbose_wait(se lf,dly):
print "VERBOSE",
threading._Even t.wait(self,dly )

Note that given your use case, you could have used a decorator here
instead...

def verbose(method) :
def _verbose(*args, **kw):
print "%s called on %s" % (method.__name_ _, method.im_self)
print "args : ", args, " - kwargs : ", kw
return method(*args, **kw)

_verbose.__name __ = "verbose wrapper for %s" % method.__name__
return _verbose

b.wait = verbose(b.wait)

Or if you want a more extensible - and "reversible " - solution:

class VerboseMethod(o bject):
def __init__(self, method, before=None, after=None):
# we only want bound methods here
obj = getattr(method, "im_self", None)
if obj is None:
err = "%s expected a bound method, got %s" % (
type(self), method
)
raise ValueError(err)

self._method = method
self._before = before
self._after = after

def _verbose_before (self, *args, **kw):
"""
You subclass VerboseMethod and taylor this to your own needs,
or alternatively pass a 'before' callback to VerboseMethod
that will get called with method, *args, **kw
"""
if callable(self._ before):
self._before(se lf._method, *args, **kw)
return
# default
m = self._method
print "%s about to be called on %s" % (m.__name__, m.im_self)
print "args : ", args, " - kwargs : ", kw

def _verbose_after( self, result, *args, **kw):
"""
You subclass VerboseMethod and taylor this to your own needs,
or alternatively pass an 'after' callback to VerboseMethod
that will get called with method, result, *args, **kw
"""
if callable(self._ after):
self._after(sel f._method, result, *args, **kw)
return
# default
m = self._method
print "%s called on %s" % (m.__name__, m.im_self)
print "args : ", args, " - kwargs : ", kw
print "result : ", result

def __call__(self, *args, **kw):
self._verbose_b efore(*args, **kw)
result = self._method(*a rgs, **kw)
self._verbose_a fter(result, *args, **kw)
return result

def drop(self):
"""restore the original method..."""
obj = self._method.im _self
delattr(obj, self._method.__ name__)

class B(object):
def __init__(self, name):
self.name = name

def wait(self, dly=42):
return "%s.wait(%s )" % (self.name, dly)

b1 = B('b1')
b2 = B('b2')

print b1.wait()
print b2.wait()

b1.wait = VerboseMethod(b 1.wait)
print b1.wait()
print b2.wait()

b1.wait.drop()
print b1.wait()
print b2.wait()

def before(m, *args, **kw):
print "test before"
print m, args, kw

def after(m, r, *args, **kw):
print "test after"
print m, r, args, kw

b1.wait = VerboseMethod(b 1.wait, before=before, after=after)
print b1.wait()
print b2.wait()

b1.wait.drop()
print b1.wait()
print b2.wait()

HTH

replace mothod for only one object but not for a class

replace mothod for only one object but not for a class

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