AttributeError: type object x has no attribute y and some other inconsistencies with Python 3.4

Question

我正在学习 Python 类和 meta类。

下面是从文章 "Metaclasses Demystified" at the now-defunct website cleverdevil.org.

修改而来的示例

# metaclass methods

class Meta(type):
    def show(cls):
        return 'I am a Meta class method'

class Mistake(object):
    __metaclass__ = Meta

但是我遇到这个打印语句的错误：

>>> print(Mistake.show())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'Mistake' has no attribute 'show'

下面是同类的另一个例子。

# data hiding
class Fruit:
    __price = 0

    def show(self):
        self.__price += 1
        print (self.__price)

objFruit = Fruit()
objFruit.show()
objFruit.show()
objFruit.show()
print (objFruit._Fruit.__price) # error

此外，我收到 print 'hello' 的错误，但 print('hello') 有效。

我不明白所有这些事情背后的背景。

Answer 1

您遇到三个不同的问题：

元类语法是 different in Python 2.x and 3.x:
PEP 3115: New Metaclass Syntax. Instead of:
```
class C:
    __metaclass__ = M
    ...
```
you must now use:
```
class C(metaclass=M):
    ...
```
The module-global __metaclass__ variable is no longer supported. (It was a crutch to make it easier to default to new-style classes without deriving every class from object.)
您的代码导致的错误如下所示：
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Fruit' object has no attribute '_Fruit'
```
...不难理解：你的Fruit对象objFruit没有属性_Fruit。双下划线名称修饰 doesn't work the way you appear to think。这有效：
```
print(objFruit._Fruit__price)  # no period
```
在Python3、print is a function.

注意：碰巧你在这个问题中提出的三个独立的问题解决起来相当微不足道，但一般来说，如果你有不止一个问题，你应该ask about each in a separate question.

AttributeError: type object x has no attribute y and some other inconsistencies with Python 3.4

AttributeError: type object x has no attribute y and some other inconsistencies with Python 3.4

python

oop

metaclass

class

python-3.x