4

我有一个非常大的 Python 2.7.6 项目，需要将其转换为 Python 3.4。我使用了 2to3 脚本，但“元类”处理似乎被破坏了。

我过滤了代码以缩短和查明问题。以下片段适用于 Python 2.7.6：

class Base(object):
    class __metaclass__(type):
        def __new__(cls, classname, bases, dict):
            new = type.__new__(cls, classname, bases, dict)
            new.classname = classname
            print ("Base::__metaclass__::new. Called.")
            return new                 

class Heir(Base):
    class __metaclass__(Base.__metaclass__):
        def __new__(self, *args):
            new = Base.__metaclass__.__new__(self, *args)
            print ("Heir::__metaclass__::new. Called.")
            return new

    @classmethod
    def define(cls, nexttype):
        print ("Heir::define. Called.")

class HeirOfHeir(Heir):
    pass

Heir.define(HeirOfHeir)

代码按预期打印：

Base::__metaclass__::new. Called.
Base::__metaclass__::new. Called.
Heir::__metaclass__::new. Called.
Base::__metaclass__::new. Called.
Heir::__metaclass__::new. Called.
Heir::define. Called.

但是当使用 Python 3.4 运行代码时，我只有最后一个打印：

Heir::define. Called.

要么2to3计算错误，要么需要一些手动工作。不幸的是，我对元类几乎没有经验。

4

1 回答 1

9

您的原始代码使用类主体中的名称 __metaclass__用作元类这一事实，但2to3修复程序仅查找直接分配：

__metaclass__ = MetaClassName

而不是class __metaclass__定义名称的语句或其他方式（from somemodule import MetaClassName as __metaclass__将在 Python 2 类主体中工作并且2to3也会错过）。

您可以通过将元类移动到单独的class定义来解决此问题：

class BaseMeta(type):
    def __new__(cls, classname, bases, dict):
        new = type.__new__(cls, classname, bases, dict)
        new.classname = classname
        print ("BaseMeta::new. Called.")
        return new                 

class Base(object):
    __metaclass__ = BaseMeta

class HeirMeta(BaseMeta):
    def __new__(self, *args):
        new = BaseMeta.__new__(self, *args)
        print ("HeirMeta::new. Called.")
        return new

class Heir(Base):    
    __metaclass__ = HeirMeta

    @classmethod
    def define(cls, nexttype):
        print ("Heir::define. Called.")

class HeirOfHeir(Heir):
    pass

Heir.define(HeirOfHeir)

无论如何，您必须这样做才能在 Python 3 中定义元类，因为定义元类的机制已更改为在类主体运行之前而不是在运行期间确定元类（因此元类也可以影响该步骤）。

现在2to3将正确检测到您的类中有一个__metaclass__属性并重写这些属性以使用新的 Python 3 语法：

stackoverflow-2.7 $ bin/python -m lib2to3 fixed.py 
RefactoringTool: Skipping implicit fixer: buffer
RefactoringTool: Skipping implicit fixer: idioms
RefactoringTool: Skipping implicit fixer: set_literal
RefactoringTool: Skipping implicit fixer: ws_comma
RefactoringTool: Refactored fixed.py
--- fixed.py    (original)
+++ fixed.py    (refactored)
@@ -5,8 +5,8 @@
         print ("BaseMeta::new. Called.")
         return new                 

-class Base(object):
-    __metaclass__ = BaseMeta
+class Base(object, metaclass=BaseMeta):
+    pass

 class HeirMeta(BaseMeta):
     def __new__(self, *args):
@@ -14,9 +14,7 @@
         print ("HeirMeta::new. Called.")
         return new

-class Heir(Base):    
-    __metaclass__ = HeirMeta
-
+class Heir(Base, metaclass=HeirMeta):    
     @classmethod
     def define(cls, nexttype):
         print ("Heir::define. Called.")
RefactoringTool: Files that need to be modified:
RefactoringTool: fixed.py

重构后的代码按预期工作：

stackoverflow-2.7 $ bin/python -m lib2to3 -o ../stackoverflow-3.4 -nw --no-diffs fixed.py 
lib2to3.main: Output in '../stackoverflow-3.4' will mirror the input directory '' layout.
RefactoringTool: Skipping implicit fixer: buffer
RefactoringTool: Skipping implicit fixer: idioms
RefactoringTool: Skipping implicit fixer: set_literal
RefactoringTool: Skipping implicit fixer: ws_comma
RefactoringTool: Refactored fixed.py
RefactoringTool: Writing converted fixed.py to ../stackoverflow-3.4/fixed.py.
RefactoringTool: Files that were modified:
RefactoringTool: fixed.py
stackoverflow-2.7 $ cd ../stackoverflow-3.4
stackoverflow-3.4 $ bin/python -V
Python 3.4.2
stackoverflow-3.4 $ bin/python fixed.py 
BaseMeta::new. Called.
BaseMeta::new. Called.
HeirMeta::new. Called.
BaseMeta::new. Called.
HeirMeta::new. Called.
Heir::define. Called.
于 2015-02-24T14:03:38.340 回答