使用元类将 Python2 转换为 Python3 会导致错误的流程

Question

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

我过滤了代码以缩短并查明问题所在。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.

但是当运行代码与 Python 3.4 我只有最后一个打印：

Heir::define. Called.

要么 2to3 计算错误，要么需要一些人工操作。不幸的是，我对元类没有什么经验。

Answer 1

您的原始代码使用 name class 正文中的 __metaclass__ 作为元 class，但是 2to3 修复程序只查找 直接赋值 :

__metaclass__ = MetaClassName

而不是 class __metaclass__ 语句或其他定义名称的方式（from somemodule import MetaClassName as __metaclass__ 将在 Python 2 class 正文中工作，而 2to3 将也想念那个）。

您可以通过将元 classes 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)

您将必须执行此操作以在 Python 3 中定义元classes，由于定义 metaclasses 的机制已更改为确定 metaclass before class body 是运行而不是在运行期间（这样 metaclass 也可以影响该步骤）。

现在 2to3 将正确检测到您的 class 上有一个 __metaclass__ 属性并重写它们以使用新的 Python 3 语法：

Whosebug-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

并且重构后的代码按预期工作：

Whosebug-2.7 $ bin/python -m lib2to3 -o ../Whosebug-3.4 -nw --no-diffs fixed.py lib2to3.main: Output in '../Whosebug-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 ../Whosebug-3.4/fixed.py. RefactoringTool: Files that were modified: RefactoringTool: fixed.py Whosebug-2.7 $ cd ../Whosebug-3.4 Whosebug-3.4 $ bin/python -V Python 3.4.2 Whosebug-3.4 $ bin/python fixed.py BaseMeta::new. Called. BaseMeta::new. Called. HeirMeta::new. Called. BaseMeta::new. Called. HeirMeta::new. Called. Heir::define. Called.

使用元类将 Python2 转换为 Python3 会导致错误的流程

Converting Python2 to Python3 with metaclasses resulted in a wrong flow

python

metaclass

python-2to3