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脚本但是＆＃39;元类＆＃39;处理似乎被打破了。

我过滤了代码以缩短并查明问题。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.

2to3计算错误或需要手动工作。不幸的是，我对元类很少有经验。

Answer 1

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

__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__可以在Python 2类体中使用，而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)

你有来执行此操作来定义Python 3 中的元类，因为定义元类的机制已更改为确定元类之前 em>类主体是运行而不是在运行期间（因此元类也可以影响该步骤）。

现在2to3将正确检测到您的类上有__metaclass__属性并重写它们以使用新的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

并且重构的代码按预期工作：

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.

使用元类将Python2转换为Python3会导致错误的流程

1 个答案: