在不调用__init__的情况下动态创建类型（self）的实例？

Question

这很难解释。我有一个应该支持方法copy_stateonly()的类。它应该返回一个残缺的对象版本，它只包含我想要的（复制的）数据成员。我希望这个例子更好地解释它：

# everything inherits from this
class SuperBase:
    def __init__(self):
        self.state_var = 3 # this should be copied into future objects
        self.non_state_var = 0 # we don't want to copy this

    def copy_stateonly(self):
        newobj = # ??????????? create instance without calling __init__
        newobj.state_var = self.state_var
        return newobj

# some clases inherit from this
class Base(SuperBase):
    def __init__(self):
        SuperBase.__init__(self)
        self.isflying = True # we want to copy this, this is state
        self.sprite = "sprites/plane_generic.png" # we must drop this

    def copy_stateonly(self):
        newobj = SuperBase.copy_stateonly(self)
        newobj.isflying = self.isflying
        return newobj

class A144fighter(Base):
    def __init__(self, teamname): # note required __init__ argument
        Base.__init__(self)
        self.colors = ["black", "grey"] # we want to copy this, this is state
        self.name = teamname # we must drop this

    def copy_stateonly(self):
        newobj = Base.copy_stateonly(self)
        newobj.colors = self.colors[:]
        return newobj

plane = A144fighter("team_blue")
plane_state = plane.copy_stateonly() # this should return an A144fighter object with only state_var, flying and colors set.

Python 2.7

Answer 1

我不知道如何在不调用__init__()的情况下创建经典类的新实例（这是您在示例中使用的）。可以使用

创建新样式类的新实例（object的后代）

object.__new__(cls)

其中cls是您要创建的对象类型。

另一种方法是使用copy.copy()进行复制，可能会覆盖__getstate__()和__setstate__()来定义应该复制的内容。

修改：要创建经典类cls的新实例而不调用__init__()，您可以使用以下黑客：

class EmptyClass:
    pass

new_instance = EmptyClass()
new_instance.__class__ = cls
new_instance.__dict__.update(whatever)

Answer 2

# everything inherits from this
class SuperBase:
    def __init__(self):
        self.state_var = 3 # this should be copied into future objects
        self.non_state_var = 0 # we don't want to copy this

    def __getstate__(self):
        return { 'state_var' : self.state_var }

    def __str__(self):
        return self.__class__.__name__ + '(' + str(vars(self)) + ')'

# some clases inherit from this
class Base(SuperBase):
    def __init__(self):
        SuperBase.__init__(self)
        self.isflying = True # we want to copy this, this is state
        self.sprite = "sprites/plane_generic.png" # we must drop this

    def __getstate__(self):
        state = SuperBase.__getstate__(self)
        state['isflying'] = self.isflying
        return state

class A144fighter(Base):
    def __init__(self, teamname): # note required __init__ argument
        Base.__init__(self)
        self.colors = ["black", "grey"] # we want to copy this, this is state
        self.name = teamname # we must drop this

    def __getstate__(self):
        state = Base.__getstate__(self)
        state['colors'] = self.colors[:]
        return state

plane = A144fighter("team_blue")
print plane

import copy
print copy.copy(plane)

# or manually:
import types
print types.InstanceType(plane.__class__, plane.__getstate__())

Answer 3

请记住，每个对象都有一个名为__class__的属性。如果你<object>.__class__它，它将返回该对象的类对象（如果这是有意义的）。类对象是可调用的，因此您可以在末尾添加括号以创建该类的新实例。

newobj = self.__class__()