I'd like to wrap every method of a particular class in python, and I'd like to do so by editing the code of the class minimally. How should I go about this?
An elegant way to do it is described in Michael Foord's Voidspace blog in an entry about what metaclasses are and how to use them in the section titled A Method Decorating Metaclass. Simplifying it slightly and applying it to your situation resulted in this:
from types import FunctionType
from functools import wraps
def wrapper(method):
@wraps(method)
def wrapped(*args, **kwrds):
# ... <do something to/with "method" or the result of calling it>
return wrapped
class MetaClass(type):
def __new__(meta, classname, bases, classDict):
newClassDict = {}
for attributeName, attribute in classDict.items():
if isinstance(attribute, FunctionType):
# replace it with a wrapped version
attribute = wrapper(attribute)
newClassDict[attributeName] = attribute
return type.__new__(meta, classname, bases, newClassDict)
class MyClass(object):
__metaclass__ = MetaClass # wrap all the methods
def method1(self, ...):
# ...etc ...
In Python, function/method decorators are just function wrappers plus some syntactic sugar to make using them easy (and prettier).
Python 3 Compatibility Update
The previous code uses Python 2.x metaclass syntax which would need to be translated in order to be used in Python 3.x, however it would then no longer work in the previous version. This means it would need to use:
class MyBase(metaclass=MetaClass)
...
instead of:
class MyBase(object):
__metaclass__ = MetaClass"
...
If desired, it's possible to write code which is compatible both both Python 2.x and 3.x, but doing so requires using a slightly more complicated technique which dynamically creates a new base class that inherits the desired metaclass, thereby avoiding errors due to the syntax differences between the two versions of Python. This is basically what Benjamin Peterson's six module's with_metaclass() function does.
from types import FunctionType
from functools import wraps
def wrapper(method):
@wraps(method)
def wrapped(*args, **kwrds):
print('{!r} executing'.format(method.__name__))
return method(*args, **kwrds)
return wrapped
class MetaClass(type):
def __new__(meta, classname, bases, classDict):
newClassDict = {}
for attributeName, attribute in classDict.items():
if isinstance(attribute, FunctionType):
# replace it with a wrapped version
attribute = wrapper(attribute)
newClassDict[attributeName] = attribute
return type.__new__(meta, classname, bases, newClassDict)
def with_metaclass(meta):
""" Create an empty class with the supplied bases and metaclass. """
return type.__new__(meta, "TempBaseClass", (object,), {})
if __name__ == '__main__':
# Inherit metaclass from a dynamically-created base class.
class MyClass(with_metaclass(MetaClass)):
@staticmethod
def a_static_method():
pass
@classmethod
def a_class_method(cls):
pass
def a_method(self):
pass
instance = MyClass()
instance.a_static_method() # Not decorated.
instance.a_class_method() # Not decorated.
instance.a_method() # -> 'a_method' executing
You mean programatically set a wrapper to methods of a class?? Well, this is probably a really bad practice, but here's how you may do it:
def wrap_methods( cls, wrapper ):
for key, value in cls.__dict__.items( ):
if hasattr( value, '__call__' ):
setattr( cls, key, wrapper( value ) )
If you have class, for example
class Test( ):
def fire( self ):
return True
def fire2( self ):
return True
and a wrapper
def wrapper( fn ):
def result( *args, **kwargs ):
print 'TEST'
return fn( *args, **kwargs )
return result
then calling
wrap_methods( Test, wrapper )
will apply wrapper to all methods defined in class Test. Use with caution! Actually, don't use it at all!
If extensively modifying default class behavior is the requirement, MetaClasses are the way to go. Here's an alternative approach.
If your use case is limited to just wrapping instance methods of a class, you could try overriding the __getattribute__ magic method.
from functools import wraps
def wrapper(func):
@wraps(func)
def wrapped(*args, **kwargs):
print "Inside Wrapper. calling method %s now..."%(func.__name__)
return func(*args, **kwargs)
return wrapped
Make sure to use functools.wraps while creating wrappers, even more so if the wrapper is meant for debugging since it provides sensible TraceBacks.
import types
class MyClass(object): # works only for new-style classes
def method1(self):
return "Inside method1"
def __getattribute__(self, name):
attr = super(MyClass, self).__getattribute__(name)
if type(attr) == types.MethodType:
attr = wrapper(attr)
return attr
Using python decorators is the cleanest method of doing this, as it looks like you want to debug or at least trace the code it appears.
