Functions to use, and generate monkeypatches for classes.

monkeypatch.py

def monkeypatch(target, name = None, func = None, backup_prefix = "default_"):
    '''
    Decorator.
    Binds target.name = func
    Creates target.{backup_prefix + name} = target.name if property exists.
    This allows the patched properties to depend on the existence of the original
    property on the target.
    '''
    if not func: return lambda func: monkeypatch(target, name, func)
    if not name and func.__name__[0] is not '<': name = func.__name__.strip('_')
    if hasattr(target, name) and not hasattr(target, backup_prefix + name):
        setattr(target, backup_prefix + name, getattr(target, name))
    setattr(target, name, func)
    return func

def targeted_patch(target):
    '''
    Decorator generator.
    Returns a decorator shorthand equal to monkeypatch(target, func = decorated_function)
    Example:
    from django.contrib.auth.models import User
    user_patch = targeted_patch(User)

    @user_patch
    def _get_username_questioningly(self):
        return self.username + "?"

    u = User.objects.get(username = 'bob')
    u.get_username_questioningly() #=> 'bob?
    '''
    return lambda func: monkeypatch(target, func = func)