Python: changing methods and attributes at runtime
Solution 1:
This example shows the differences between adding a method to a class and to an instance.
>>> class Dog():
... def __init__(self, name):
... self.name = name
...
>>> skip = Dog('Skip')
>>> spot = Dog('Spot')
>>> def talk(self):
... print 'Hi, my name is ' + self.name
...
>>> Dog.talk = talk # add method to class
>>> skip.talk()
Hi, my name is Skip
>>> spot.talk()
Hi, my name is Spot
>>> del Dog.talk # remove method from class
>>> skip.talk() # won't work anymore
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: Dog instance has no attribute 'talk'
>>> import types
>>> f = types.MethodType(talk, skip, Dog)
>>> skip.talk = f # add method to specific instance
>>> skip.talk()
Hi, my name is Skip
>>> spot.talk() # won't work, since we only modified skip
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: Dog instance has no attribute 'talk'
Solution 2:
I wish to create a class in Python that I can add and remove attributes and methods.
import types
class SpecialClass(object):
@classmethod
def removeVariable(cls, name):
return delattr(cls, name)
@classmethod
def addMethod(cls, func):
return setattr(cls, func.__name__, types.MethodType(func, cls))
def hello(self, n):
print n
instance = SpecialClass()
SpecialClass.addMethod(hello)
>>> SpecialClass.hello(5)
5
>>> instance.hello(6)
6
>>> SpecialClass.removeVariable("hello")
>>> instance.hello(7)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'SpecialClass' object has no attribute 'hello'
>>> SpecialClass.hello(8)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: type object 'SpecialClass' has no attribute 'hello'
Solution 3:
A possibly interesting alternative to using types.MethodType
in:
>>> f = types.MethodType(talk, puppy, Dog)
>>> puppy.talk = f # add method to specific instance
would be to exploit the fact that functions are descriptors:
>>> puppy.talk = talk.__get__(puppy, Dog)
Solution 4:
I wish to create a class in Python that I can add and remove attributes and methods. How can I acomplish that?
You can add and remove attributes and methods to any class, and they'll be available to all instances of the class:
>>> def method1(self):
pass
>>> def method1(self):
print "method1"
>>> def method2(self):
print "method2"
>>> class C():
pass
>>> c = C()
>>> c.method()
Traceback (most recent call last):
File "<pyshell#62>", line 1, in <module>
c.method()
AttributeError: C instance has no attribute 'method'
>>> C.method = method1
>>> c.method()
method1
>>> C.method = method2
>>> c.method()
method2
>>> del C.method
>>> c.method()
Traceback (most recent call last):
File "<pyshell#68>", line 1, in <module>
c.method()
AttributeError: C instance has no attribute 'method'
>>> C.attribute = "foo"
>>> c.attribute
'foo'
>>> c.attribute = "bar"
>>> c.attribute
'bar'