Is there a way to create a class which instances respond to arbitrary method calls?
I know there is a the special method __getattr__(self, attr)
which would be called when someone is trying to access an attribute of an instance. I am searching for something similar that enables me to intercept method calls, too. The desired behavior would look something like this:
class A(object):
def __methodintercept__(self, method, *args, **kwargs): # is there a special method like this??
print(str(method))
>>> a = A()
>>> a.foomatic()
foomatic
EDIT
The other suggested questions do not address my case: I do not want to wrap another class or change the metaclass of a second class or similar. I just want to have a class that responds to arbitrary method calls.
Thanks to jonrshape I now know that __getattr__(self, attr)
will also be called when a method is called in the same way as it would be when an attribute is accessed. But how do i distinguish in __getattr__
if attr
comes from a method call or an attribute access and how to get the parameters of a potential method call?
This is the solution I was looking for when coming across this question:
class Wrapper:
def __init__(self):
self._inner = [] # or whatever type you want to wrap
def foo(self, x):
print(x)
def __getattr__(self, attr):
if attr in self.__class__.__dict__:
return getattr(self, attr)
else:
return getattr(self._inner, attr)
t = Test()
t.foo('abc') # prints 'abc'
t.append('x') # appends 'x' to t._inner
Criticisms very welcome. I wanted to add methods to the Browser class in the Splinter package, but it only exposes a function to return an instance, not the class itself. This approach permitted pseudo-inheritance, which meant I could declaratively decouple DOM code from website-specific code. (A better approach in hindsight might have been to use Selenium directly.)
unittest.mock.Mock
does this by default.
from unittest.mock import Mock
a = Mock()
a.arbitrary_method() # No error
a.arbitrary_method.called # True
a.new_method
a.new_method.called # False
a.new_method("some", "args")
a.new_method.called # True
a.new_method.assert_called_with("some", "args") # No error
a.new_method_assert_called_with("other", "args") # AssertionError
This is something I came up with, which will behave exactly as if the method exists.
First let's establish one thing: You cannot distinguish in __getattr__
if attr
comes from a function call or an "attribute access", because a class method is an attribute of your class. So someone can access that method even if they don't intend to call it, as in:
class Test:
def method(self):
print "Hi, I am method"
>> t = Test()
>> t.method # just access the method "as an attribute"
<bound method Test.method of <__main__.Test instance at 0x10a970c68>>
>> t.method() # actually call the method
Hi, I am method
Therefore, the closest thing I could think of is this behavior:
Create a class A, such that:
I will first write the class definition and then show how accessing a method that doesn't exist behaves exactly like accessing one that exists, whether you are just accessing it, or actually calling it.
Class definition:
class A(object):
def __init__(self):
self.x = 1 # set some attribute
def __getattr__(self,attr):
try:
return super(A, self).__getattr__(attr)
except AttributeError:
return self.__get_global_handler(attr)
def __get_global_handler(self, name):
# Do anything that you need to do before simulating the method call
handler = self.__global_handler
handler.im_func.func_name = name # Change the method's name
return handler
def __global_handler(self, *args, **kwargs):
# Do something with these arguments
print "I am an imaginary method with name %s" % self.__global_handler.im_func.func_name
print "My arguments are: " + str(args)
print "My keyword arguments are: " + str(kwargs)
def real_method(self, *args, **kwargs):
print "I am a method that you actually defined"
print "My name is %s" % self.real_method.im_func.func_name
print "My arguments are: " + str(args)
print "My keyword arguments are: " + str(kwargs)
I added the method real_method
just so I have something that actually exists in the class to compare its behavior with that of an 'imaginary method'
Here's the result:
>> a = A()
>> # First let's try simple access (no method call)
>> a.real_method # The method that is actually defined in the class
<bound method A.real_method of <test.A object at 0x10a9784d0>>
>> a.imaginary_method # Some method that is not defined
<bound method A.imaginary_method of <test.A object at 0x10a9784d0>>
>> # Now let's try to call each of these methods
>> a.real_method(1, 2, x=3, y=4)
I am a method that you actually defined
My name is real_method
My arguments are: (1, 2)
My keyword arguments are: {'y': 4, 'x': 3}
>> a.imaginary_method(1, 2, x=3, y=4)
I am an imaginary method with name imaginary_method
My arguments are: (1, 2)
My keyword arguments are: {'y': 4, 'x': 3}
>> # Now let's try to access the x attribute, just to make sure that 'regular' attribute access works fine as well
>> a.x
1
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