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POCreating dynamic docstrings in Python descriptor
 

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    <p>I am trying to generate some class definitions dynamically (for wrapping a C++ extension). The following descriptor works fine except when I try to access the docstring for a field using help(), it gives default documentation for the descriptor rather than the field it self. However when I do help(classname), it retrieves the docstring passed to the descriptor:</p> <pre><code>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) </code></pre> <p>"python test.py" prints:</p> <pre> 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) </pre> <p>Any idea how one can get the <code>descriptor.__doc__</code> for <code>help(class.field)</code> ? And is there a way to bypass this and have something like a getter function for doc in stead of having to store the doc string in the descriptor?</p> <p>like:</p> <pre><code>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) </code></pre> <p>UPDATE: Actually I started with this definition of <code>__get__</code>:</p> <pre><code>def __get__(self, obj, dtype=None): return obj.get_field(self.name) </code></pre> <p>The problem with this was that when I said:</p> <pre><code>help(DerivedClass.a) </code></pre> <p>Python threw an Exception indicating that I was trying to call <code>None.get_field</code>. Thus <code>help()</code> is calling the <code>__get__</code> method with <code>obj=None</code> and <code>dtype=DerivedClass</code>. That is why I decided to return the FieldDescriptor instance when obj=None and dtype!=None. My impression was <code>help(xyz)</code> tries to display <code>xyz.__doc__</code>. By that logic, if <code>__get__</code> returns <code>descriptor_instance</code>, then <code>descriptor_instance.__doc__</code> should be printed by help(), which is the case for the whole class [<code>help(DerivedClass)</code>], but not for the single field [<code>help(DerivedClass.a)</code>].</p>
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    1. COI'm sure it's all there, but could you clarify which calls are giving you the wrong help output? It's too much effort to guess what you expected by reading the code.
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    2. COAs jsbueno pointed out, it is help(DerivedClass.a) that displays the documentation for the descriptor instead of documentation for the field (saved in descriptor.__doc__).
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