Kent Johnson said unto the world upon 2005-10-08 07:08:
> Brian van den Broek wrote:
<snip problem spec of wanting a class and its subclass to each call a
sanity-check on the arguments to __init__ from within the __init__>
>>
>>Here's a sketch of where I'm at:
<code below trimmed for repost>
>>class _BaseClass(object):
>>
>> def __init__(self, arg1, arg2):
>> self.arg1 = arg1
>> self.arg2 = arg2
>> if type(self) == _BaseClass:
>> self._validate_args()
>>
>> def _validate_args(self):
>> if not type(self.arg1) in (int, long):
>> raise TypeError
>>
>>class SubClass(_BaseClass):
>>
>> def __init__(self, arg1, arg2, arg3, arg4):
>> super(SubClass, self).__init__(arg1, arg2)
>> self.arg3 = arg3
>> self.arg4 = arg4
>> if type(self) == SubClass:
>> self._validate_args()
>>
>> def _validate_args(self):
>> super(SubClass, self)._validate_args()
>> if not isinstance(self.arg3, basestring):
>> raise TypeError
>>
>>
>>This works as desired, but leaves two problems:
>>
>>1) I suspect there may be a better way, as a) this doesn't feel quite
>>right and b) in general with Python, it seems that if you are tempted
>>to type test, you should rethink, and
Thanks for the reply, Kent.
> I can think of two alternatives:
<snip alternative which is clear to me, leaving:>
> - Have two validate functions in each class - the shared
_validate_args() and a class-specific _validate_SubClassArgs(). Call
the class-specific version from __init__() and the shared one from
other clients. Then you would have
>
> class _BaseClass(object):
> def __init__(self):
> ...
> self._validate_BaseClass()
>
> def _validate_args(self):
> super(_BaseClass, self)._validate_args()
> self._validate_BaseClass()
>
> and similar code in SubClass.
I think I see the idea. But I take it you didn't mean to have:
> def _validate_args(self):
> super(_BaseClass, self)._validate_args()
within a method of _BaseClass (a subclass of object), as that will
produce:
AttributeError: 'super' object has no attribute '_validate_args'
>>2) I originally had __BaseClass rather than _BaseClass. But, with that
>>naming, cannot figure out how to write the line
>>
>> if type(self) == __BaseClass:
>>
>>so as to make it work. I know about the name mangling with __somename
>>names, but I don't know how to manage it in this case.
>>
>>The attempt of 4 lines up produces:
>>
>>NameError: global name '_BaseClass__BaseClass' is not defined
>>
>>This confuses me. I don't see why the error msg prepends '_BaseClass'
>>as that name appears nowhere. That confusion aside, I've no idea how
>>to effect what I want.
>
>
> The __ name only gets mangled inside the class definition. The
> class name itself is not getting mangled. From the language
> reference:
>
> Private name mangling: When an identifier that textually occurs in
> a class definition begins with two or more underscore characters
> and does not end in two or more underscores, it is considered a
> private name of that class. Private names are transformed to a
> longer form before code is generated for them.
OK, thanks. But I'm still not quite there.
Consider this example code:
class _OneUnderBase(object):
def __init__(self):
if type(self) == _OneUnderBase:
print "From _OneUnderBase"
else:
print "From subclass",
class __TwoUnderBase(object):
def __init__(self):
if type(self) == __TwoUnderBase: # What to write here
print "From __TwoUnderBase"
else:
print "From subclass",
class Sub1(_OneUnderBase):
def __init__(self):
super(Sub1, self).__init__()
print "Sub1"
class Sub2(__TwoUnderBase):
def __init__(self):
super(Sub2, self).__init__()
print "Sub2"
s1 = Sub1()
s2 = Sub2()
When run, this gives:
From subclass Sub1
Traceback (most recent call last):
File "C:/Python24/foofoofoo.py", line 26, in -toplevel-
s2 = Sub2()
File "C:/Python24/foofoofoo.py", line 22, in __init__
super(Sub2, self).__init__()
File "C:/Python24/foofoofoo.py", line 10, in __init__
if type(self) == __TwoUnderBase: # What to write here
NameError: global name '_TwoUnderBase__TwoUnderBase' is not defined
>>>
What should I write in the if test of __TwoUnderBase.__init__() to
make it work? (Nevermind the desired behaviour here could be obtained
without the type test. How to work that test is the point I'm
interested in.)
Thanks,
Brian vdB
_______________________________________________
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
