Inheriting property functions

[Dustan]

Looking at this interactive session:

>>class A(object):

def __init__(self, a):
self.a = a
def get_a(self): return self.__a
def set_a(self, new_a): self.__a = new_a
a = property(get_a, set_a)

>>class B(A):

b = property(get_a, set_a)


Traceback (most recent call last):
File "<pyshell#1 1>", line 1, in <module>
class B(A):
File "<pyshell#1 1>", line 2, in B
b = property(get_a, set_a)
NameError: name 'get_a' is not defined

>>class B(A):

b = a


Traceback (most recent call last):
File "<pyshell#1 3>", line 1, in <module>
class B(A):
File "<pyshell#1 3>", line 2, in B
b = a
NameError: name 'a' is not defined

>>>

B isn't recognizing its inheritence of A's methods get_a and set_a
during creation.

Why am I doing this? For an object of type B, it makes more sense to
reference the attribute 'b' than it does to reference the attribute
'a', even though they are the same, in terms of readability.

Is there any way to make this work as intended?

[Dustan]

Re: Inheriting property functions


Dustan wrote:

Looking at this interactive session:
>

>class A(object):

def __init__(self, a):
self.a = a
def get_a(self): return self.__a
def set_a(self, new_a): self.__a = new_a
a = property(get_a, set_a)
>
>

>class B(A):

b = property(get_a, set_a)
>
>
Traceback (most recent call last):
File "<pyshell#1 1>", line 1, in <module>
class B(A):
File "<pyshell#1 1>", line 2, in B
b = property(get_a, set_a)
NameError: name 'get_a' is not defined

>class B(A):

b = a
>
>
Traceback (most recent call last):
File "<pyshell#1 3>", line 1, in <module>
class B(A):
File "<pyshell#1 3>", line 2, in B
b = a
NameError: name 'a' is not defined

>>

>
B isn't recognizing its inheritence of A's methods get_a and set_a
during creation.
>
Why am I doing this? For an object of type B, it makes more sense to
reference the attribute 'b' than it does to reference the attribute
'a', even though they are the same, in terms of readability.

Clarification: The code given is obviously not the actual code. I doubt
it would really make a difference in types A and B given here, but for
what I'm actually doing, it makes a big difference.

Is there any way to make this work as intended?

[mdsteele@gmail.com]

Re: Inheriting property functions

Dustan wrote:

B isn't recognizing its inheritence of A's methods get_a and set_a
during creation.
>
Why am I doing this? For an object of type B, it makes more sense to
reference the attribute 'b' than it does to reference the attribute
'a', even though they are the same, in terms of readability.
>
Is there any way to make this work as intended?

Try this:

>>class A(object):

.... def __init__(self,a ):
.... self.__a = a
.... def get_a(self): return self.__a
.... def set_a(self,new_ a): self.__a = new_a
.... a = property(get_a, set_a)
....

>>class B(A):

.... b = property(A.get_ a,A.set_a)
....

>>bar = B(5)
>>bar.a

5

>>bar.b

5

The trouble is that get_a and set_a are attributes of the _class
object_ A. Instances of A (and hence, instances of B) will see them,
but the class B will not, so you have to point to them explicitly with
A.get_a and A.set_a.

[Robert Kern]

Re: Inheriting property functions

Dustan wrote:

Looking at this interactive session:
>

>>>class A(object):

def __init__(self, a):
self.a = a
def get_a(self): return self.__a
def set_a(self, new_a): self.__a = new_a
a = property(get_a, set_a)
>
>

>>>class B(A):

b = property(get_a, set_a)
>
>
Traceback (most recent call last):
File "<pyshell#1 1>", line 1, in <module>
class B(A):
File "<pyshell#1 1>", line 2, in B
b = property(get_a, set_a)
NameError: name 'get_a' is not defined

>>>class B(A):

b = a
>
>
Traceback (most recent call last):
File "<pyshell#1 3>", line 1, in <module>
class B(A):
File "<pyshell#1 3>", line 2, in B
b = a
NameError: name 'a' is not defined
>
B isn't recognizing its inheritence of A's methods get_a and set_a
during creation.

Inheritance really doesn't work that way. The code in the class suite gets
executed in its own namespace that doesn't know anything about inheritance. The
inheritance rules operate in attribute access on the class object later.

Try this:

class B(A):
b = property(A.get_ a, A.set_a)

or this:

class B(A):
b = A.a

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

[mdsteele@gmail.com]

Re: Inheriting property functions

Robert Kern wrote:

Inheritance really doesn't work that way. The code in the class suite gets
executed in its own namespace that doesn't know anything about inheritance. The
inheritance rules operate in attribute access on the class object later.

Right. That was what I should have said, but it came out wrong when I
tried to say it. (-:

-Matt

[Gabriel Genellina]

Re: Inheriting property functions

At Friday 20/10/2006 19:49, Dustan wrote:

>>class A(object):

def get_a(self): return self.__a
def set_a(self, new_a): self.__a = new_a
a = property(get_a, set_a)

>>class B(A):

b = property(get_a, set_a)

Use instead:

b = property(A.get_ a, A.set_a)


--
Gabriel Genellina
Softlab SRL

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[Dustan]

Re: Inheriting property functions


Robert Kern wrote:

Dustan wrote:

Looking at this interactive session:

>>class A(object):

def __init__(self, a):
self.a = a
def get_a(self): return self.__a
def set_a(self, new_a): self.__a = new_a
a = property(get_a, set_a)

>>class B(A):

b = property(get_a, set_a)


Traceback (most recent call last):
File "<pyshell#1 1>", line 1, in <module>
class B(A):
File "<pyshell#1 1>", line 2, in B
b = property(get_a, set_a)
NameError: name 'get_a' is not defined

>>class B(A):

b = a


Traceback (most recent call last):
File "<pyshell#1 3>", line 1, in <module>
class B(A):
File "<pyshell#1 3>", line 2, in B
b = a
NameError: name 'a' is not defined

B isn't recognizing its inheritence of A's methods get_a and set_a
during creation.

>
Inheritance really doesn't work that way. The code in the class suite gets
executed in its own namespace that doesn't know anything about inheritance. The
inheritance rules operate in attribute access on the class object later.
>
Try this:
>
class B(A):
b = property(A.get_ a, A.set_a)
>
or this:
>
class B(A):
b = A.a
>
--
Robert Kern
>
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

Thanks for your help, and mdsteele's.

[Aahz]

Re: Inheriting property functions

In article <1161383286.543 397.46700@m73g2 000cwd.googlegr oups.com>,
Dustan <DustanGroups@g mail.comwrote:

>

>>>class A(object):

> def __init__(self, a):
> self.a = a
> def get_a(self): return self.__a
> def set_a(self, new_a): self.__a = new_a
> a = property(get_a, set_a)
>
>

>>>class B(A):

> b = property(get_a, set_a)

BTW, since you're almost certainly going to run into this quickly given
the direction your code is taking (and also to fix some bugs):

class A(object):
def __init__(self, a):
self._a = a

def get_a(self):
return self._a

def _get_a(self):
return self.get_a()

def set_a(self, new_a):
self._a = new_a

def _set_a(self, new_a):
self.set_a(new_ a)

a = property(_get_a , _set_a)

class B(A):
def get_a(self):
return str(self._a)

Thank Alex Martelli for this demonstration that programming is all built
on one basic trick: add another layer of indirection. However, I leave
you to figure out on your own why this is better.

Note carefully that I changed __a to _a. You almost never want to use
double-underscore private names because of the way they cause problems
with inheritance.

PS: Please do NOT post code with TABs
--
Aahz (aahz@pythoncra ft.com) <* http://www.pythoncraft.com/

"If you don't know what your program is supposed to do, you'd better not
start writing it." --Dijkstra

[Diez B. Roggisch]

Re: Inheriting property functions

Dustan schrieb:

Looking at this interactive session:
>

>>>class A(object):

def __init__(self, a):
self.a = a
def get_a(self): return self.__a
def set_a(self, new_a): self.__a = new_a
a = property(get_a, set_a)
>
>

>>>class B(A):

b = property(get_a, set_a)
>
>
Traceback (most recent call last):
File "<pyshell#1 1>", line 1, in <module>
class B(A):
File "<pyshell#1 1>", line 2, in B
b = property(get_a, set_a)
NameError: name 'get_a' is not defined

>>>class B(A):

b = a
>
>
Traceback (most recent call last):
File "<pyshell#1 3>", line 1, in <module>
class B(A):
File "<pyshell#1 3>", line 2, in B
b = a
NameError: name 'a' is not defined
>
B isn't recognizing its inheritence of A's methods get_a and set_a
during creation.
>
Why am I doing this? For an object of type B, it makes more sense to
reference the attribute 'b' than it does to reference the attribute
'a', even though they are the same, in terms of readability.

I think you are having a code smell here. However, this is how you do it:

class B(A):
b = A.a


Diez

[Bruno Desthuilliers]

Re: Inheriting property functions

mdsteele@gmail. com a écrit :
(snip)

The trouble is that get_a and set_a are attributes of the _class
object_ A. Instances of A (and hence, instances of B) will see them,
but the class B will not,

Yes it does:

>>class A(object):

.... aa = "aa"
....

>>class B(A):pass

....

>>B.aa

'aa'

>>>

so you have to point to them explicitly with
A.get_a and A.set_a.

Actually, while the solution is good, the explanation is wrong.