I've read that it is possible to add a method to an existing object (i.e., not in the class definition) in Python.
I understand that it's not always good to do so. But how might one do this?
This question is related to
python
oop
methods
monkeypatching
What you're looking for is setattr I believe.
Use this to set an attribute on an object.
>>> def printme(s): print repr(s)
>>> class A: pass
>>> setattr(A,'printme',printme)
>>> a = A()
>>> a.printme() # s becomes the implicit 'self' variable
< __ main __ . A instance at 0xABCDEFG>
In Python monkeypatching generally works by overwriting a class or function's signature with your own. Below is an example from the Zope Wiki:
from SomeOtherProduct.SomeModule import SomeClass
def speak(self):
return "ook ook eee eee eee!"
SomeClass.speak = speak
This code will overwrite/create a method called peak in the class. In Jeff Atwood's recent post on monkey patching, he showed an example in C# 3.0 which is the current language I use for work.
Module new is deprecated since python 2.6 and removed in 3.0, use types
see http://docs.python.org/library/new.html
In the example below I've deliberately removed return value from patch_me() function.
I think that giving return value may make one believe that patch returns a new object, which is not true - it modifies the incoming one. Probably this can facilitate a more disciplined use of monkeypatching.
import types
class A(object):#but seems to work for old style objects too
pass
def patch_me(target):
def method(target,x):
print "x=",x
print "called from", target
target.method = types.MethodType(method,target)
#add more if needed
a = A()
print a
#out: <__main__.A object at 0x2b73ac88bfd0>
patch_me(a) #patch instance
a.method(5)
#out: x= 5
#out: called from <__main__.A object at 0x2b73ac88bfd0>
patch_me(A)
A.method(6) #can patch class too
#out: x= 6
#out: called from <class '__main__.A'>
This question was opened years ago, but hey, there's an easy way to simulate the binding of a function to a class instance using decorators:
def binder (function, instance):
copy_of_function = type (function) (function.func_code, {})
copy_of_function.__bind_to__ = instance
def bound_function (*args, **kwargs):
return copy_of_function (copy_of_function.__bind_to__, *args, **kwargs)
return bound_function
class SupaClass (object):
def __init__ (self):
self.supaAttribute = 42
def new_method (self):
print self.supaAttribute
supaInstance = SupaClass ()
supaInstance.supMethod = binder (new_method, supaInstance)
otherInstance = SupaClass ()
otherInstance.supaAttribute = 72
otherInstance.supMethod = binder (new_method, otherInstance)
otherInstance.supMethod ()
supaInstance.supMethod ()
There, when you pass the function and the instance to the binder decorator, it will create a new function, with the same code object as the first one. Then, the given instance of the class is stored in an attribute of the newly created function. The decorator return a (third) function calling automatically the copied function, giving the instance as the first parameter.
In conclusion you get a function simulating it's binding to the class instance. Letting the original function unchanged.
If it can be of any help, I recently released a Python library named Gorilla to make the process of monkey patching more convenient.
Using a function needle() to patch a module named guineapig goes as follows:
import gorilla
import guineapig
@gorilla.patch(guineapig)
def needle():
print("awesome")
But it also takes care of more interesting use cases as shown in the FAQ from the documentation.
The code is available on GitHub.
Since this question asked for non-Python versions, here's JavaScript:
a.methodname = function () { console.log("Yay, a new method!") }
Although Jasons answer works, it does only work if one wants to add a function to a class. It did not work for me when I tried to reload an already existing method from the .py source code file.
It took me for ages to find a workaround, but the trick seems simple... 1.st import the code from the source code file 2.nd force a reload 3.rd use types.FunctionType(...) to convert the imported and bound method to a function you can also pass on the current global variables, as the reloaded method would be in a different namespace 4.th now you can continue as suggested by "Jason Pratt" using the types.MethodType(...)
Example:
# this class resides inside ReloadCodeDemo.py
class A:
def bar( self ):
print "bar1"
def reloadCode(self, methodName):
''' use this function to reload any function of class A'''
import types
import ReloadCodeDemo as ReloadMod # import the code as module
reload (ReloadMod) # force a reload of the module
myM = getattr(ReloadMod.A,methodName) #get reloaded Method
myTempFunc = types.FunctionType(# convert the method to a simple function
myM.im_func.func_code, #the methods code
globals(), # globals to use
argdefs=myM.im_func.func_defaults # default values for variables if any
)
myNewM = types.MethodType(myTempFunc,self,self.__class__) #convert the function to a method
setattr(self,methodName,myNewM) # add the method to the function
if __name__ == '__main__':
a = A()
a.bar()
# now change your code and save the file
a.reloadCode('bar') # reloads the file
a.bar() # now executes the reloaded code
Apart from what others said, I found that __repr__ and __str__ methods can't be monkeypatched on object level, because repr() and str() use class-methods, not locally-bounded object methods:
# Instance monkeypatch
[ins] In [55]: x.__str__ = show.__get__(x)
[ins] In [56]: x
Out[56]: <__main__.X at 0x7fc207180c10>
[ins] In [57]: str(x)
Out[57]: '<__main__.X object at 0x7fc207180c10>'
[ins] In [58]: x.__str__()
Nice object!
# Class monkeypatch
[ins] In [62]: X.__str__ = lambda _: "From class"
[ins] In [63]: str(x)
Out[63]: 'From class'
I think that the above answers missed the key point.
Let's have a class with a method:
class A(object):
def m(self):
pass
Now, let's play with it in ipython:
In [2]: A.m
Out[2]: <unbound method A.m>
Ok, so m() somehow becomes an unbound method of A. But is it really like that?
In [5]: A.__dict__['m']
Out[5]: <function m at 0xa66b8b4>
It turns out that m() is just a function, reference to which is added to A class dictionary - there's no magic. Then why A.m gives us an unbound method? It's because the dot is not translated to a simple dictionary lookup. It's de facto a call of A.__class__.__getattribute__(A, 'm'):
In [11]: class MetaA(type):
....: def __getattribute__(self, attr_name):
....: print str(self), '-', attr_name
In [12]: class A(object):
....: __metaclass__ = MetaA
In [23]: A.m
<class '__main__.A'> - m
<class '__main__.A'> - m
Now, I'm not sure out of the top of my head why the last line is printed twice, but still it's clear what's going on there.
Now, what the default __getattribute__ does is that it checks if the attribute is a so-called descriptor or not, i.e. if it implements a special __get__ method. If it implements that method, then what is returned is the result of calling that __get__ method. Going back to the first version of our A class, this is what we have:
In [28]: A.__dict__['m'].__get__(None, A)
Out[28]: <unbound method A.m>
And because Python functions implement the descriptor protocol, if they are called on behalf of an object, they bind themselves to that object in their __get__ method.
Ok, so how to add a method to an existing object? Assuming you don't mind patching class, it's as simple as:
B.m = m
Then B.m "becomes" an unbound method, thanks to the descriptor magic.
And if you want to add a method just to a single object, then you have to emulate the machinery yourself, by using types.MethodType:
b.m = types.MethodType(m, b)
By the way:
In [2]: A.m
Out[2]: <unbound method A.m>
In [59]: type(A.m)
Out[59]: <type 'instancemethod'>
In [60]: type(b.m)
Out[60]: <type 'instancemethod'>
In [61]: types.MethodType
Out[61]: <type 'instancemethod'>
Consolidating Jason Pratt's and the community wiki answers, with a look at the results of different methods of binding:
Especially note how adding the binding function as a class method works, but the referencing scope is incorrect.
#!/usr/bin/python -u
import types
import inspect
## dynamically adding methods to a unique instance of a class
# get a list of a class's method type attributes
def listattr(c):
for m in [(n, v) for n, v in inspect.getmembers(c, inspect.ismethod) if isinstance(v,types.MethodType)]:
print m[0], m[1]
# externally bind a function as a method of an instance of a class
def ADDMETHOD(c, method, name):
c.__dict__[name] = types.MethodType(method, c)
class C():
r = 10 # class attribute variable to test bound scope
def __init__(self):
pass
#internally bind a function as a method of self's class -- note that this one has issues!
def addmethod(self, method, name):
self.__dict__[name] = types.MethodType( method, self.__class__ )
# predfined function to compare with
def f0(self, x):
print 'f0\tx = %d\tr = %d' % ( x, self.r)
a = C() # created before modified instnace
b = C() # modified instnace
def f1(self, x): # bind internally
print 'f1\tx = %d\tr = %d' % ( x, self.r )
def f2( self, x): # add to class instance's .__dict__ as method type
print 'f2\tx = %d\tr = %d' % ( x, self.r )
def f3( self, x): # assign to class as method type
print 'f3\tx = %d\tr = %d' % ( x, self.r )
def f4( self, x): # add to class instance's .__dict__ using a general function
print 'f4\tx = %d\tr = %d' % ( x, self.r )
b.addmethod(f1, 'f1')
b.__dict__['f2'] = types.MethodType( f2, b)
b.f3 = types.MethodType( f3, b)
ADDMETHOD(b, f4, 'f4')
b.f0(0) # OUT: f0 x = 0 r = 10
b.f1(1) # OUT: f1 x = 1 r = 10
b.f2(2) # OUT: f2 x = 2 r = 10
b.f3(3) # OUT: f3 x = 3 r = 10
b.f4(4) # OUT: f4 x = 4 r = 10
k = 2
print 'changing b.r from {0} to {1}'.format(b.r, k)
b.r = k
print 'new b.r = {0}'.format(b.r)
b.f0(0) # OUT: f0 x = 0 r = 2
b.f1(1) # OUT: f1 x = 1 r = 10 !!!!!!!!!
b.f2(2) # OUT: f2 x = 2 r = 2
b.f3(3) # OUT: f3 x = 3 r = 2
b.f4(4) # OUT: f4 x = 4 r = 2
c = C() # created after modifying instance
# let's have a look at each instance's method type attributes
print '\nattributes of a:'
listattr(a)
# OUT:
# attributes of a:
# __init__ <bound method C.__init__ of <__main__.C instance at 0x000000000230FD88>>
# addmethod <bound method C.addmethod of <__main__.C instance at 0x000000000230FD88>>
# f0 <bound method C.f0 of <__main__.C instance at 0x000000000230FD88>>
print '\nattributes of b:'
listattr(b)
# OUT:
# attributes of b:
# __init__ <bound method C.__init__ of <__main__.C instance at 0x000000000230FE08>>
# addmethod <bound method C.addmethod of <__main__.C instance at 0x000000000230FE08>>
# f0 <bound method C.f0 of <__main__.C instance at 0x000000000230FE08>>
# f1 <bound method ?.f1 of <class __main__.C at 0x000000000237AB28>>
# f2 <bound method ?.f2 of <__main__.C instance at 0x000000000230FE08>>
# f3 <bound method ?.f3 of <__main__.C instance at 0x000000000230FE08>>
# f4 <bound method ?.f4 of <__main__.C instance at 0x000000000230FE08>>
print '\nattributes of c:'
listattr(c)
# OUT:
# attributes of c:
# __init__ <bound method C.__init__ of <__main__.C instance at 0x0000000002313108>>
# addmethod <bound method C.addmethod of <__main__.C instance at 0x0000000002313108>>
# f0 <bound method C.f0 of <__main__.C instance at 0x0000000002313108>>
Personally, I prefer the external ADDMETHOD function route, as it allows me to dynamically assign new method names within an iterator as well.
def y(self, x):
pass
d = C()
for i in range(1,5):
ADDMETHOD(d, y, 'f%d' % i)
print '\nattributes of d:'
listattr(d)
# OUT:
# attributes of d:
# __init__ <bound method C.__init__ of <__main__.C instance at 0x0000000002303508>>
# addmethod <bound method C.addmethod of <__main__.C instance at 0x0000000002303508>>
# f0 <bound method C.f0 of <__main__.C instance at 0x0000000002303508>>
# f1 <bound method ?.y of <__main__.C instance at 0x0000000002303508>>
# f2 <bound method ?.y of <__main__.C instance at 0x0000000002303508>>
# f3 <bound method ?.y of <__main__.C instance at 0x0000000002303508>>
# f4 <bound method ?.y of <__main__.C instance at 0x0000000002303508>>
You can use lambda to bind a method to an instance:
def run(self):
print self._instanceString
class A(object):
def __init__(self):
self._instanceString = "This is instance string"
a = A()
a.run = lambda: run(a)
a.run()
Output:
This is instance string
Preface - a note on compatibility: other answers may only work in Python 2 - this answer should work perfectly well in Python 2 and 3. If writing Python 3 only, you might leave out explicitly inheriting from object, but otherwise the code should remain the same.
Adding a Method to an Existing Object Instance
I've read that it is possible to add a method to an existing object (e.g. not in the class definition) in Python.
I understand that it's not always a good decision to do so. But, how might one do this?
I don't recommend this. This is a bad idea. Don't do it.
Here's a couple of reasons:
Thus, I suggest that you not do this unless you have a really good reason. It is far better to define the correct method in the class definition or less preferably to monkey-patch the class directly, like this:
Foo.sample_method = sample_method
Since it's instructive, however, I'm going to show you some ways of doing this.
Here's some setup code. We need a class definition. It could be imported, but it really doesn't matter.
class Foo(object):
'''An empty class to demonstrate adding a method to an instance'''
Create an instance:
foo = Foo()
Create a method to add to it:
def sample_method(self, bar, baz):
print(bar + baz)
__get__Dotted lookups on functions call the __get__ method of the function with the instance, binding the object to the method and thus creating a "bound method."
foo.sample_method = sample_method.__get__(foo)
and now:
>>> foo.sample_method(1,2)
3
First, import types, from which we'll get the method constructor:
import types
Now we add the method to the instance. To do this, we require the MethodType constructor from the types module (which we imported above).
The argument signature for types.MethodType is (function, instance, class):
foo.sample_method = types.MethodType(sample_method, foo, Foo)
and usage:
>>> foo.sample_method(1,2)
3
First, we create a wrapper function that binds the method to the instance:
def bind(instance, method):
def binding_scope_fn(*args, **kwargs):
return method(instance, *args, **kwargs)
return binding_scope_fn
usage:
>>> foo.sample_method = bind(foo, sample_method)
>>> foo.sample_method(1,2)
3
A partial function applies the first argument(s) to a function (and optionally keyword arguments), and can later be called with the remaining arguments (and overriding keyword arguments). Thus:
>>> from functools import partial
>>> foo.sample_method = partial(sample_method, foo)
>>> foo.sample_method(1,2)
3
This makes sense when you consider that bound methods are partial functions of the instance.
If we try to add the sample_method in the same way as we might add it to the class, it is unbound from the instance, and doesn't take the implicit self as the first argument.
>>> foo.sample_method = sample_method
>>> foo.sample_method(1,2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sample_method() takes exactly 3 arguments (2 given)
We can make the unbound function work by explicitly passing the instance (or anything, since this method doesn't actually use the self argument variable), but it would not be consistent with the expected signature of other instances (if we're monkey-patching this instance):
>>> foo.sample_method(foo, 1, 2)
3
You now know several ways you could do this, but in all seriousness - don't do this.
I find it strange that nobody mentioned that all of the methods listed above creates a cycle reference between the added method and the instance, causing the object to be persistent till garbage collection. There was an old trick adding a descriptor by extending the class of the object:
def addmethod(obj, name, func):
klass = obj.__class__
subclass = type(klass.__name__, (klass,), {})
setattr(subclass, name, func)
obj.__class__ = subclass
There are at least two ways for attach a method to an instance without types.MethodType:
>>> class A:
... def m(self):
... print 'im m, invoked with: ', self
>>> a = A()
>>> a.m()
im m, invoked with: <__main__.A instance at 0x973ec6c>
>>> a.m
<bound method A.m of <__main__.A instance at 0x973ec6c>>
>>>
>>> def foo(firstargument):
... print 'im foo, invoked with: ', firstargument
>>> foo
<function foo at 0x978548c>
1:
>>> a.foo = foo.__get__(a, A) # or foo.__get__(a, type(a))
>>> a.foo()
im foo, invoked with: <__main__.A instance at 0x973ec6c>
>>> a.foo
<bound method A.foo of <__main__.A instance at 0x973ec6c>>
2:
>>> instancemethod = type(A.m)
>>> instancemethod
<type 'instancemethod'>
>>> a.foo2 = instancemethod(foo, a, type(a))
>>> a.foo2()
im foo, invoked with: <__main__.A instance at 0x973ec6c>
>>> a.foo2
<bound method instance.foo of <__main__.A instance at 0x973ec6c>>
Useful links:
Data model - invoking descriptors
Descriptor HowTo Guide - invoking descriptors
What Jason Pratt posted is correct.
>>> class Test(object):
... def a(self):
... pass
...
>>> def b(self):
... pass
...
>>> Test.b = b
>>> type(b)
<type 'function'>
>>> type(Test.a)
<type 'instancemethod'>
>>> type(Test.b)
<type 'instancemethod'>
As you can see, Python doesn't consider b() any different than a(). In Python all methods are just variables that happen to be functions.
from types import MethodType
def method(self):
print 'hi!'
setattr( targetObj, method.__name__, MethodType(method, targetObj, type(method)) )
With this, you can use the self pointer
Source: Stackoverflow.com