inherit and overwrite a property (better its method call)

[chris]

hi,
i am tinkering with properties of new style classes:

class Base(object):

def m(self):
return 'p of Base'
p = property(m)

class Sub(Base):
def m(self):
return 'p of Sub'

b = Base()
print b.p # prints 'p of Base'

s = Sub()
print s.p # prints 'p of Base'!?

i was thinking s.p would use the method m of class Sub and not Base. but
this does not work, both properties "p" of Base and Sub use method m of
baseclass Base.

so it seems i cannot overwrite the method p calls to get its value
without actually repeating the property definition in every subclass, or
is there a way? the following does work but i want to get rid of the
second p = property(m)...

class Sub(Base):
def m(self):
return 'p of Sub'
p = property(m)

print b.p # prints 'p of Base'
s = Sub()
print s.p # prints 'p of Sub'

am i missing something?
thanks for any advice
chris

[Peter Otten]

chris wrote:

hi,
i am tinkering with properties of new style classes:

class Base(object):

def m(self):
return 'p of Base'
p = property(m)

class Sub(Base):
def m(self):
return 'p of Sub'

b = Base()
print b.p # prints 'p of Base'

s = Sub()
print s.p # prints 'p of Base'!?

i was thinking s.p would use the method m of class Sub and not Base. but
this does not work, both properties "p" of Base and Sub use method m of
baseclass Base.

so it seems i cannot overwrite the method p calls to get its value
without actually repeating the property definition in every subclass, or
is there a way? the following does work but i want to get rid of the
second p = property(m)...

class Sub(Base):
def m(self):
return 'p of Sub'
p = property(m)

print b.p # prints 'p of Base'
s = Sub()
print s.p # prints 'p of Sub'

am i missing something?

I don't think so. Anyway, I have been tinkering too, and here's what I've
come up with so far:

class inheritableprop erty(property):
""" property with overridable accessor functions """

def funcByName(cls, fn):
# helper func
if fn is None: return None
result = getattr(cls, fn.__name__, None)
assert result is None or callable(result )
return result

class typewithinherit ableproperties( type):
def __init__(cls, name, bases, dict):
super(typewithi nheritableprope rties, cls).__init__(n ame, bases,
dict)
# replace inheritable properties with new instances where
# the accessors are determined by a name lookup in the actual class
for name in dir(cls):
a = getattr(cls, name)
if isinstance(a, inheritableprop erty):
setattr(cls, name, inheritableprop erty(
funcByName(cls, a.fget),
funcByName(cls, a.fset),
funcByName(cls, a.fdel)
))

class A(object):
__metaclass__ = typewithinherit ableproperties
def geta(self): return "A.a"
a = inheritableprop erty(geta)

class B(A):
def geta(self): return "B.a"

class C(A): pass
class D(B): pass

print A().a
print B().a
print C().a
print D().a

http://www.python.org/2.2/descrintro.html might also be of interest for you.
It has an example of properties based on naming conventions (class
autoprop).

Peter

[Nicodemus]

Peter Otten wrote:

chris wrote:
<snip>
http://www.python.org/2.2/descrintro.html might also be of interest for you.
It has an example of properties based on naming conventions (class
autoprop).

I ran into the same problem, and coded a property of my own. The problem
is that property() holds the actual functions, so when you overwrite
them in the subclass, the property doesn't know about it. The approach
below only saves the name of the function, and delays the lookup of the
actual function when needed.
class subclass_proper ty(object):
'''Creates an property just like a built-in property, except that the
functions that are part of the property can be changed in a subclass.
'''

def __init__(self, fget=None, fset=None, fdel=None, doc=None):
self.fget_name = fget and fget.__name__
self.fset_name = fset and fset.__name__
if isinstance(fdel , str):
doc = fdel
fdel = None
self.fdel_name = fdel and fdel.__name__
self.doc = doc or ''
def __get__(self, obj, objtype=None):
if obj is None:
return self
if self.fget_name is None:
raise AttributeError, "unreadable attribute"
fget = getattr(obj, self.fget_name)
return fget()
def __set__(self, obj, value):
if self.fset_name is None:
raise AttributeError, "can't set attribute"
fset = getattr(obj, self.fset_name)
fset(value)
def __delete__(self , obj):
if self.fdel_name is None:
raise AttributeError, "can't delete attribute"
fdel = getattr(obj, self.fdel_name)
fdel()
def __repr__(self):
p = []
if self.fget_name:
p.append(self.f get_name)
if self.fset_name:
p.append(self.f set_name)
if self.fdel_name:
p.append(self.f del_name)
if self.doc:
p.append(repr(s elf.doc))
return 'action.propert y(%s)' % ', '.join(p)

HTH,
Nicodemus.