在Python中动态地将基类与实例混合

Question

是否可以将基类添加到对象实例（不是类！）在运行时？类似于Object#extend在Ruby中的工作原理：

class Gentleman(object):
  def introduce_self(self):
    return "Hello, my name is %s" % self.name

class Person(object):
  def __init__(self, name):
    self.name = name

p = Person("John")
# how to implement this method?
extend(p, Gentleman)
p.introduce_self() # => "Hello, my name is John"

Answer 1

稍微干净的版本：

def extend_instance(obj, cls):
    """Apply mixins to a class instance after creation"""
    base_cls = obj.__class__
    base_cls_name = obj.__class__.__name__
    obj.__class__ = type(base_cls_name, (base_cls, cls),{})

Answer 2

虽然已经有答案了，但这里有一个函数：

def extend(instance, new_class):
    instance.__class__ = type(
          '%s_extended_with_%s' % (instance.__class__.__name__, new_class.__name__), 
          (instance.__class__, new_class), 
          {},
          )

Answer 3

这将动态定义一个新类GentlePerson，并将p的类重新分配给它：

class Gentleman(object):
  def introduce_self(self):
    return "Hello, my name is %s" % self.name

class Person(object):
  def __init__(self, name):
    self.name = name

p = Person("John")
p.__class__ = type('GentlePerson',(Person,Gentleman),{})
print(p.introduce_self())
# "Hello, my name is John"

根据您的请求，这将修改p的基，但不会更改p的原始类Person。因此，Person的其他实例不受影响（如果调用introduce_self，则会引发AttributeError）。

---

虽然在这个问题中没有直接问到这个问题，但是我要补充的是，对于Google和好奇者来说，也可以动态地改变一个类的基，但是（AFAIK）只有当这个类没有直接从object继承时：

class Gentleman(object):
  def introduce_self(self):
    return "Hello, my name is %s" % self.name

class Base(object):pass
class Person(Base):
  def __init__(self, name):
    self.name = name

p = Person("John")
Person.__bases__=(Gentleman,object,)
print(p.introduce_self())
# "Hello, my name is John"

q = Person("Pete")
print(q.introduce_self())
# Hello, my name is Pete