关于元编程：在Python描述符中创建动态文档字符串

Creating dynamic docstrings in Python descriptor

我试图动态生成一些类定义(用于包装C++扩展)。以下描述符可以正常工作，除非我尝试使用help()访问字段的docstring，否则它会为描述符提供默认文档，而不是它自己的字段。但是，当我执行帮助(classname)时，它检索传递给描述符的docstring：

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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：

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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)

号

你知道怎样才能得到help(class.field)的descriptor.__doc__吗？是否有一种方法可以绕过这个问题，并为Doc提供类似getter函数的功能，而不必将Doc字符串存储在描述符中？

比如：

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更新：实际上，我从__get__的定义开始：

1 2	def __get__(self, obj, dtype=None): return obj.get_field(self.name)

。

问题是当我说：

1	help(DerivedClass.a)

python抛出了一个异常，表明我正试图调用None.get_field。因此，help()使用obj=None和dtype=DerivedClass调用__get__方法。这就是我决定在obj=none和dtype时返回fielddescriptor实例的原因！=无。我的印象是，help(xyz)试图展示xyz.__doc__。根据这个逻辑，如果__get__返回descriptor_instance，那么descriptor_instance.__doc__应该由help()打印，这是整个类〔help(DerivedClass)的情况，而不是单个字段〔help(DerivedClass.a)的情况。