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Class Inheritance - What am I doing wrong?

My example:

class A(object):

def __init__(self, name):
self.__name = name

def getName(self):
return self.__name

class B(A):

def __init__(self,name=None):
super(A,self).__init__()

def setName(self, name):
self.__name = name

if __name__ == '__main__':

a = A('class a')
print a.getName()

b = B('class b')
print b.getName()

b.setName('class b, reset')
print b.getName()

I get the following error:

mtinky:~ brian$ python teste.py
class a
Traceback (most recent call last):
File "teste.py", line 23, in <module>
print b.getName()
File "teste.py", line 7, in getName
return self.__name
AttributeError: 'B' object has no attribute '_A__name'

Am I *not* using super() correctly? Also, did I define my the class B
constructor correctly?
Jun 27 '08 #1
8 973
On Apr 24, 10:22*pm, Brian Munroe <brian.e.mun...@gmail.comwrote:
My example:

class A(object):

* * * * def __init__(self, name):
* * * * * * * * self.__name = name

* * * * def getName(self):
* * * * * * * * return self.__name

class B(A):

* * * * def __init__(self,name=None):
* * * * * * * * super(A,self).__init__()

* * * * def setName(self, name):
* * * * * * * * self.__name = name

if __name__ == '__main__':

* * * * a = A('class a')
* * * * print a.getName()

* * * * b = B('class b')
* * * * print b.getName()

* * * * b.setName('class b, reset')
* * * * print b.getName()

I get the following error:

mtinky:~ brian$ python teste.py
class a
Traceback (most recent call last):
* File "teste.py", line 23, in <module>
* * print b.getName()
* File "teste.py", line 7, in getName
* * return self.__name
AttributeError: 'B' object has no attribute '_A__name'

Am I *not* using super() correctly? *Also, did I define my the class B
constructor correctly?
Exactly, you used it wrong. It's super(B, self).

But before you start using super() everywhere, read this:

http://fuhm.net/super-harmful/

I love Python, but super() is one of those tricky things...
Jun 27 '08 #2
Brian Munroe <br************@gmail.comwrites:
My example:

class A(object):

def __init__(self, name):
self.__name = name

def getName(self):
return self.__name

class B(A):

def __init__(self,name=None):
super(A,self).__init__()

def setName(self, name):
self.__name = name

if __name__ == '__main__':

a = A('class a')
print a.getName()

b = B('class b')
print b.getName()

b.setName('class b, reset')
print b.getName()

I get the following error:

mtinky:~ brian$ python teste.py
class a
Traceback (most recent call last):
File "teste.py", line 23, in <module>
print b.getName()
File "teste.py", line 7, in getName
return self.__name
AttributeError: 'B' object has no attribute '_A__name'

Am I *not* using super() correctly? Also, did I define my the class B
constructor correctly?
You have fallen victim to the Name Mangling Trap [1]. Replace the
leading double underscore in the __name attribute with a single one,
and Python shall calm down and let your code behave as you expect it
to.

That is, if you also pass the name parameter to super(A,self).__init__
in B's __init__ method

[1] http://docs.python.org/ref/atom-identifiers.html

--
Arnaud
Jun 27 '08 #3
Arnaud Delobelle <ar*****@googlemail.comwrites:
That is, if you also pass the name parameter to super(A,self).__init__
in B's __init__ method
Oops. should be super(B, self).__init__(name), of course.

--
Arnaud
Jun 27 '08 #4
Brian Munroe wrote:
My example:

class A(object):

def __init__(self, name):
self.__name = name

def getName(self):
return self.__name

class B(A):

def __init__(self,name=None):
super(A,self).__init__()

def setName(self, name):
self.__name = name

if __name__ == '__main__':

a = A('class a')
print a.getName()

b = B('class b')
print b.getName()

b.setName('class b, reset')
print b.getName()

I get the following error:

mtinky:~ brian$ python teste.py
class a
Traceback (most recent call last):
File "teste.py", line 23, in <module>
print b.getName()
File "teste.py", line 7, in getName
return self.__name
AttributeError: 'B' object has no attribute '_A__name'

Am I *not* using super() correctly? Also, did I define my the class B
constructor correctly?
--
http://mail.python.org/mailman/listinfo/python-list
Tell us what you are trying to do and what you expected to happen.

If you are trying to do simple inheritance, you don't need the supers,
and you should not invoke the name mangling implied by the double
underscore.

If you *are* trying to use the name mangling, then you still don't need
the super.
Gary Herron
Jun 27 '08 #5
Ok, so thanks everyone for the helpful hints. That *was* a typo on my
part (should've been super(B...) not super(A..), but I digress)

I'm building a public API. Along with the API I have a few custom
types that I'm expecting API users to extend, if they need too. If I
don't use name mangling, isn't that considered bad practice (read not
defensive programming) to not protect those 'private' fields?
Jun 27 '08 #6
Brian Munroe <br************@gmail.comwrites:
Ok, so thanks everyone for the helpful hints. That *was* a typo on my
part (should've been super(B...) not super(A..), but I digress)

I'm building a public API. Along with the API I have a few custom
types that I'm expecting API users to extend, if they need too. If I
don't use name mangling, isn't that considered bad practice (read not
defensive programming) to not protect those 'private' fields?
The problem is that you are using name mangling for an attribute which
is accessed by several generations of a class hierarchy. Name
mangling is only useful for attributes you *don't* want to share with
subclasses (or bases).

In python, use attributes starting with a single underscore (such as
_name). It tells users that they shouldn't mess with them. By
design, python doesn't include mechanisms equivalent to the Java / C++
'private'.

--
Arnaud
Jun 27 '08 #7
Brian Munroe a écrit :
Ok, so thanks everyone for the helpful hints. That *was* a typo on my
part (should've been super(B...) not super(A..), but I digress)

I'm building a public API. Along with the API I have a few custom
types that I'm expecting API users to extend, if they need too. If I
don't use name mangling, isn't that considered bad practice (read not
defensive programming) to not protect those 'private' fields?
There would be a lot to say about whether defensive programming is a
good practice or not. At least in the context of hi-level languages with
exception handling and automatic memory management.

Anyway:
- there's just no way to make anything truly "private" in Python
- the convention is that attributes (including methods - they are
attributes too) whose name starts with a single leading underscore are
implementation stuff and should not be messed with. IOW, the contract is
"mess with them and you're on your own".
- name mangling is only useful when you really need to make sure an
implementation attribute won't be *accidentally* shadowed. These
attributes should only be used by methods that are not themselves
supposed to be overridden or extended by user code. FWIW, I must have
used them less than half a dozen times in 7+ years (and I wrote more
than a couple mini-frameworks, with lot of black-juju metaclasses and
custom descriptors magic in them).

So just use single-leading-underscore for implementation attributes, and
document what the users are supposed to extend and what they're supposed
to leave alone.

My 2 cents.
Jun 27 '08 #8
On Apr 24, 10:11 pm, Arnaud Delobelle <arno...@googlemail.comwrote:
In python, use attributes starting with a single underscore (such as
_name). It tells users that they shouldn't mess with them. By
design, python doesn't include mechanisms equivalent to the Java / C++
'private'.
Arnaud, Gabriel:

Ok, Ok, I'll trust my users to not abuse my API :) I didn't realize
that 'private' meant 'private, even to sub-classes' - it is all
becoming clear to me now!

Thanks for the help, and I'm going to re-read 'Python is Not Java'
right about now (I've spent the past few months knee-deep in Java, I
guess I need to cleanse myself)
Jun 27 '08 #9

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