在Python描述符中创建动态文档字符串

Question

我试图动态生成一些类定义(用于包装C++扩展).以下描述符工作正常,除非我尝试使用help()访问字段的docstring时,它为描述符而不是自己的字段提供默认文档.但是当我做帮助(classname)时,它会检索传递给描述符的docstring:

class FieldDescriptor(object):
    def __init__(self, name, doc='No documentation available.'):
        self.name = name
        self.__doc__ = doc

    def __get__(self, obj, dtype=None):
        if obj is None and dtype is not None:
            print 'Doc is:', self.__doc__
            return self
        return obj.get_field(self.name)

    def __set__(self, obj, value):
        obj.set_field(self.name, value)

class TestClass(object):
    def __init__(self):
        self.fdict = {'a': None, 'b': None}

    def get_field(self, name):
        return self.fdict[name]

    def set_field(self, name, value):
        self.fdict[name] = value

fields = ['a', 'b']
def define_class(class_name, baseclass):
    class_obj = type(class_name, (baseclass,), {})
    for field in fields:
        setattr(class_obj, field, FieldDescriptor(field, doc='field %s in class %s' % (field, class_name)))
    globals()[class_name] = class_obj


if __name__ == '__main__':
    define_class('DerivedClass', TestClass)
    help(DerivedClass.a)
    help(DerivedClass)
    v = DerivedClass()
    help(v.a)

"python test.py"打印:

Doc is: field a in class DerivedClass
Help on FieldDescriptor in module __main__ object:

class FieldDescriptor(__builtin__.object)
 |  Methods defined here:
 |  
 |  __get__(self, obj, dtype=None)
 |  
 |  __init__(self, name, doc='No documentation available.')
 |  
 |  __set__(self, obj, value)
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  __dict__
 |      dictionary for instance variables (if defined)
 |  
 |  __weakref__
 |      list of weak references to the object (if defined)

Doc is: field a in class DerivedClass
Doc is: field b in class DerivedClass
Help on class DerivedClass in module __main__:

class DerivedClass(TestClass)
 |  Method resolution order:
 |      DerivedClass
 |      TestClass
 |      __builtin__.object
 |  
 |  Data descriptors defined here:
 |  
 |  a
 |      field a in class DerivedClass
 |  
 |  b
 |      field b in class DerivedClass
 |  
 |  ----------------------------------------------------------------------
 |  Methods inherited from TestClass:
 |  
 |  __init__(self)
 |  
 |  get_field(self, name)
 |  
 |  set_field(self, name, value)
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors inherited from TestClass:
 |  
 |  __dict__
 |      dictionary for instance variables (if defined)
 |  
 |  __weakref__
 |      list of weak references to the object (if defined)

Help on NoneType object:

class NoneType(object)
 |  Methods defined here:
 |  
 |  __hash__(...)
 |      x.__hash__()  hash(x)
 |  
 |  __repr__(...)
 |      x.__repr__()  repr(x)

任何想法,如何能够拿到descriptor.__doc__的help(class.field)？是否有一种方法可以绕过这个并为doc提供类似getter函数的东西,而不必将doc字符串存储在描述符中？

喜欢:

class FieldDescriptor(object):
    def __init__(self, name, doc='No documentation available.'):
        self.name = name
        self.__doc__ = doc

    def __get__(self, obj, dtype=None):
        if obj is None and dtype is not None:
            print 'Doc is:', self.__doc__
            return self
        return obj.get_field(self.name)

    def __set__(self, obj, value):
        obj.set_field(self.name, value)

    # This is what I'd like to have
    def __doc__(self, obj, dtype):
       return dtype.generate_docstring(self.name)

更新:其实我从这个定义开始__get__:

def __get__(self, obj, dtype=None):
    return obj.get_field(self.name)

问题是当我说:

help(DerivedClass.a)

Python抛出一个Exception表示我试图调用None.get_field.因此用和help()调用__get__方法.这就是为什么我决定在obj = None和dtype!= None时返回FieldDescriptor实例.我的印象是试图展示.通过该逻辑,如果返回,那么应该由help()打印,这是整个类[ ]的情况,但不是单个字段[ ].obj=Nonedtype=DerivedClasshelp(xyz)xyz.__doc____get__descriptor_instancedescriptor_instance.__doc__help(DerivedClass)help(DerivedClass.a)

Answer 1

发生的事情是，当您请求时help(DerivedClass.a)- python 计算括号内的表达式 - 这是描述符__get__方法返回的对象 - 并且它们搜索该对象的帮助（包括文档字符串）。

实现此功能（包括动态文档字符串生成）的一种方法是让您的__get__方法重新生成具有所需文档字符串的动态生成的对象。但是这个对象本身需要成为原始对象的适当代理对象，并且会给您的代码带来一些开销 - 以及许多特殊情况。

不管怎样，让它像你想要的那样工作的唯一方法是修改它本身返回的对象__get__，以便它们的行为像你希望的那样。

我建议，如果您在帮助中想要的只是像您正在做的那样的一些信息，也许您希望从您返回的对象__get__属于定义方法__repr__（而不仅仅是__doc__字符串）的类。