使用元類將Python2轉換為Python3會導致錯誤的流程

Question

我有一個非常大的Python 2.7.6項目，我需要將其轉換為Python 3.4。 我使用了2to3腳本，但'metaclass'處理似乎被打破了。

我過濾了代碼以縮短並查明問題。 以下片段適用於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)

代碼按預期打印：

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

但是當使用Python 3.4運行代碼時，我只有最后一個打印：

Heir::define. Called.

無論是2to3失算或者還需要一些手工的工作。 不幸的是，我對元類很少有經驗。

Answer 1

您的原始代碼使用以下事實：類主體中的名稱 __metaclass__用作元類，但2to3修復程序僅查找直接賦值 ：

__metaclass__ = MetaClassName

而不是class __metaclass__語句或其他方式定義名稱（ from somemodule import MetaClassName as __metaclass__將在Python 2類體中工作， 2to3也會錯過）。

您可以通過將元類移動到單獨的class定義來解決此問題：

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)

無論如何 ，你必須這樣做才能在Python 3中定義元類，因為定義元類的機制被改為在運行類體之前而不是在期間確定元類（因此元類也可以影響該元素）。

現在2to3將正確檢測到類上有__metaclass__屬性並重寫它們以使用新的Python 3語法：

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

並且重構的代碼按預期工作：

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.