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How to use the docstring in this property example

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  How to use the docstring in this property example Robert <rxjwg98@gmail.com> - 2016-01-20 20:00 -0800
    Re: How to use the docstring in this property example Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-01-21 15:50 +1100
    Re: How to use the docstring in this property example Peter Otten <__peter__@web.de> - 2016-01-21 18:15 +0100

#101939 — How to use the docstring in this property example

Hi,

I read below code snippet on link:
https://docs.python.org/2/library/functions.html#property

--------------
class C(object):
    def __init__(self):
        self._x = None

    def getx(self):
        return self._x

    def setx(self, value):
        self._x = value

    def delx(self):
        del self._x

    x = property(getx, setx, delx, "I'm the 'x' property.")
If c is an instance of C, c.x will invoke the getter, c.x = value will invoke the setter and del c.x the deleter.

If given, doc will be the docstring of the property attribute.
////////////////

I can use these:
c.x
c.x=42
del c.x

but I don't know how to get the doctring from the property attribute.
Could you show me how to do that?

Thanks,

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#101941

On Thursday 21 January 2016 15:00, Robert wrote:

> Hi,
> 
> I read below code snippet on link:
> https://docs.python.org/2/library/functions.html#property

Property docstrings are hard to get to. But with the class C you gave, this 
works in the interactive interpreter:

help(C.__dict__['x'])

displays:

    I'm the 'x' property.


C.__dict__['x'] will return the property object itself, without running the 
getter or setter, so you can access the dostring:


py> C.__dict__['x']
<property object at 0xb7171d9c>
py> C.__dict__['x'].__doc__
"I'm the 'x' property."



But what happens if you inherit the property from some other class?

py> class D(C): pass
... 
py> class E(D): pass
... 
py> E.__dict__['x']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'x'



In Python 3, you can use the inspect module:

py> obj = inspect.getattr_static(E, 'x')
py> inspect.getdoc(obj)
"I'm the 'x' property."



but in Python 2, you may need to walk the class's MRO by hand. Something 
like this:


def getattr_static(obj, name):
    if isinstance(obj, type):
        for cls in obj.__mro__:
            if name in cls.__dict__:
                return cls.__dict__[name]


-- 
Steve

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#101984

Robert wrote:

> Hi,
> 
> I read below code snippet on link:
> https://docs.python.org/2/library/functions.html#property
> 
> --------------
> class C(object):
>     def __init__(self):
>         self._x = None
> 
>     def getx(self):
>         return self._x
> 
>     def setx(self, value):
>         self._x = value
> 
>     def delx(self):
>         del self._x
> 
>     x = property(getx, setx, delx, "I'm the 'x' property.")
> If c is an instance of C, c.x will invoke the getter, c.x = value will
> invoke the setter and del c.x the deleter.
> 
> If given, doc will be the docstring of the property attribute.
> ////////////////
> 
> I can use these:
> c.x
> c.x=42
> del c.x
> 
> but I don't know how to get the doctring from the property attribute.
> Could you show me how to do that?

>>> c = C()
>>> type(c).x.__doc__
"I'm the 'x' property."

But usually you'll see it as part of the help on c:

>>> help(c)
Help on C in module __main__ object:

class C(__builtin__.object)
[...]
 |  Data descriptors defined here:
[...]
 |  x
 |      I'm the 'x' property.

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