descriptor & docstring

[cyril giraudon]

Hello,

I try to use python descriptors to define attributes with default
value (the code is reported below).
But apparently, it breaks the docstring mechanism.

help(Basis) shows the right help but help(Rectangle) shows only two
lines :
"
Help on class Rectangle in module basis2:

Rectangle = <class 'basis2.Rectang le'>
"
If the Rectangle.lengt h attribute is removed, the help is OK.

Secondly, the __doc__ attribute of a PhysicalValue instance doesn't
seem to be read.

I don't understand.

Any idea ?

Thanks a lot

Cyril.

# A descriptor class with default value handling
class PhysicalValue(o bject):
"""
A physical value descriptor
"""
def __init__(self, default_value):
self.default_va lue = default_value
self.__doc__ = "Hello from Physical Value"

def __get__(self, obj, type=None):
if obj.__dict__.ha s_key(self.key) :
return getattr(obj, self.key)
else:
return self.default_va lue

def __set__(self, obj, value):
if value is DefaultValue:
setattr(obj, self.key, self.default_va lue)
else:
setattr(obj, self.key, value)

# A meta class which adds instance attributes
# If hasattr(cls, "attr") then add "_attr" attribute.
class MyMetaClass(typ e):
def __init__(cls, name, bases, dct):
super(MyMetaCla ss, cls).__init__(n ame, bases, dct)
print "Add property to ", name
def init(self):
pvl = [item for item in cls.__dict__.it ems()
if isinstance(item[1], PhysicalValue)]
for pv in pvl:
print "Add _%s property to %s" % (pv[0], name)
cls.__dict__[pv[0]].key = "_" + pv[0]
setattr(self, "_" + pv[0], getattr(self, pv[0]))
cls.__init__ = init

# A basis class
class Basis(object):
"""
Tempest basis class
"""
__metaclass__ = MyMetaClass

# A concrete class
class Rectangle(Basis ):
"""
A beautiful Rectangle
"""
length = PhysicalValue(1 2.)

[cyril giraudon]

A precision, I use python 2.5.2 under linux mandiva 2007.0

Cyril.

[George Sakkis]

On Apr 28, 10:59*pm, "Gabriel Genellina"

def property_defaul t(prop_name, default_value=N one, doc=None):

* * *attr_name = '_'+prop_name

* * *def fget(self, attr_name=attr_ name,
* * * * * * * * * * default_value=d efault_value):
* * * * *return getattr(self, attr_name, default_value)

* * *def fset(self, value,
* * * * * * * * * * attr_name=attr_ name,
* * * * * * * * * * default_value=d efault_value):
* * * * *if value == default_value:
* * * * * * *delattr(self, attr_name)
* * * * *else:
* * * * * * *setattr(self, attr_name, value)

* * *return property(fget=f get, fset=fset, doc=doc)

When setting the same value as the default, the instance attribute is *
removed (so the default will be used when retrieving the value later). I *
think this is what you intended to do.

Note that this will fail if the value is already equal to the default
and you try to reset it to the default, so it needs an extra
hasattr(self, attr_name) before the delattr. Regardless, I would be
surprised with the following behaviour:

>>r = Rectangle()
r.length = 4
type(r.length )

<type 'int'>

>>r.length = 12
type(r.length )

<type 'float'>

Another simpler alternative would be to (ab)use a decorator:

def defaultproperty (func):
attr = '_' + func.__name__
default = func.func_defau lts[0]
return property(
fget = lambda self: getattr(self, attr, default),
fset = lambda self,value: setattr(self, attr, value),
doc = func.__doc__)
class Rectangle(objec t):
'''A beautiful Rectangle'''

@defaultpropert y
def length(default= 12.0):
'''This is the length property'''

George

[Gabriel Genellina]

En Tue, 29 Apr 2008 01:29:40 -0300, George Sakkis
<ge***********@ gmail.comescrib ió:

On Apr 28, 10:59*pm, "Gabriel Genellina"

>def property_defaul t(prop_name, default_value=N one, doc=None):

* * *attr_name = '_'+prop_name

* * *def fget(self, attr_name=attr_ name,
* * * * * * * * * * default_value=d efault_value):
* * * * *return getattr(self, attr_name, default_value)

* * *def fset(self, value,
* * * * * * * * * * attr_name=attr_ name,
* * * * * * * * * * default_value=d efault_value):
* * * * *if value == default_value:
* * * * * * *delattr(self, attr_name)
* * * * *else:
* * * * * * *setattr(self, attr_name, value)

* * *return property(fget=f get, fset=fset, doc=doc)

When setting the same value as the default, the instance attribute is *
removed (so the default will be used when retrieving the value later).
I think this is what you intended to do.

Note that this will fail if the value is already equal to the default
and you try to reset it to the default, so it needs an extra
hasattr(self, attr_name) before the delattr. Regardless, I would be
surprised with the following behaviour:

>>>r = Rectangle()
r.length = 4
type(r.lengt h)

<type 'int'>

>>>r.length = 12
type(r.lengt h)

<type 'float'>

Yep, probably the best thing to do is to always call setattr and avoid
special cases.

Another simpler alternative would be to (ab)use a decorator:

def defaultproperty (func):
attr = '_' + func.__name__
default = func.func_defau lts[0]
return property(
fget = lambda self: getattr(self, attr, default),
fset = lambda self,value: setattr(self, attr, value),
doc = func.__doc__)
class Rectangle(objec t):
'''A beautiful Rectangle'''

@defaultpropert y
def length(default= 12.0):
'''This is the length property'''

Nice, although that empty function looks somewhat strange...

--
Gabriel Genellina