Question

我有一个永远不会被实例化的基类。这个基类有不同的子类。每个子类定义某些类变量，其中所有子类的名称相同，但值将不同。例如

class Base:
    def display(self): 
        print self.logfile, self.loglevel
class d1(Base):
    logfile = "d1.log"
    loglevel = "debug"
    def temp(self):
        Base.display(self)
class d2(Base):
    logfile = "d2.log"
    loglevel = "info"
    def temp(self):
        Base.display(self)

设计这个的正确方法是什么，以便我可以强制执行，如果明天任何新的子类被定义，实现子类的人应该为这些类变量提供一些值而不是错过定义它们？

Answer 1

一个不需要实例化类以进行检查的替代方法是创建一个元类：

class BaseAttrEnforcer(type):
    def __init__(cls, name, bases, d):
        if 'loglevel' not in d:
            raise ValueError("Class %s doesn't define loglevel attribute" % name)
        type.__init__(cls, name, bases, d)

class Base(object):
    __metaclass__ = BaseAttrEnforcer
    loglevel = None

class d1(Base):
    logfile = "d1.log"
    loglevel = "debug"

class d2(Base):
    logfile = "d2.log"
    loglevel = "info"

class d3(Base):
    logfile = "d3.log"
    # I should fail

Answer 2

这应该有效

>>> class Base(object):
...  def __init__(self):
...   if not hasattr(self, "logfile"):
...    raise Exception("not implemented")
... 
>>> class d1(Base):
...  logfile='logfile1.log'
... 
>>> class d2(Base):
...  pass
... 
>>> d1()
<__main__.d1 object at 0x7d0d0>
>>> d2()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __init__
not implemented

Answer 3

您可以通过ciphor建议的构造函数中的简单检查来执行此操作，但您也可以在基类中使用 abc.abstractproperty 装饰器来确保属性类似于您想要的属性被定义为。

然后解释器将检查实例化实例时是否创建了日志文件：

import abc
#It is almost always a good idea to have your base class inherit from object
class Base(object):  
    __metaclass__ = abc.ABCMeta
    @abc.abstractproperty
    def logfile(self):
        raise RuntimeError("This should never happen")

class Nice(Base):
    @property
    def logfile(self):
        return "actual_file.log"

class Naughty(Base):
    pass

d=Nice()  #This is fine
print d.logfile  #Prints actual_file.log
d=Naughty()  #This raises an error: 
#TypeError: Can't instantiate abstract class Base with abstract methods logfile

请参阅 http://docs.python.org/library/abc.html 并且可能更有用： http://www.doughellmann.com/PyMOTW/abc/ 了解更多详情。

还有一个注意事项 - 当您的子类在原始示例中调用Base.display（self）时，让它们调用self.display（）会更有意义。该方法继承自base，这样就避免了对基类进行硬编码。如果你有更多的子类，那么它也会使继承链变得更清晰。

Answer 4

也许您可以在Base类的 init 函数中添加检查代码，如下所示：

class Base:
    logfile = ""
    loglevel = ""
    def __init__(self):
        if len(self.logfile) == 0 or len(self.loglevel) == 0:
            print 'WARNING: logfile & loglevel must be set!'
    def display(self): 
        print self.logfile, self.loglevel
class d1(Base):
    logfile = "d1.log"
    loglevel = "debug"
    def temp(self):
        Base.display(self)
class d2(Base):
    logfile = "d2.log"
    loglevel = "info"
    def temp(self):
        Base.display(self)

python中每个子类中类变量的不同值

4 个答案: