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.