Thanks a lot to point this out!
Karim
On 12/19/2010 04:33 AM, Steven D'Aprano wrote:
Karim wrote:
class InputStrategy( object ):
"""This InputStrategy class is an abstract interface to various
read strategy objects.
"""
def read(self, filePath):
"""Abstract method to load into database memory an input file.
@arg: filePath - string - The input file path.
@return: The created database object.
"""
raise NotImplementedError
By the way, I use NotImplementedError in my abstract method of the
abstract class. You are using TypeError exception.
Is there a general rule for that? NotImplementedError is ok?
For abstract methods that should be overridden by a subclass, I would
use NotImplementedError, as you do. That is a clear convention for
saying "this method must be overridden".
But __init__ is a special case. The __init__ method isn't merely not
implemented. It *is* implemented, and it does two jobs: it performs
any common initialization for the class and all subclasses, and it
also prevents the AbstractCLass from being instantiated.
class AbstractClass:
def __init__(self, *args):
if type(self) is AbstractClass:
raise TypeError
# do common initialization here
Trying to instantiate an AbstractClass is always an error:
instance = AbstractClass(args)
and the error is a *type* error -- you are trying to instantiate a
type that can't be. Python already does that, with the singletons
None, NotImplemented (different from NotImplementedError!), and Ellipsis:
>>> type(None)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot create 'NoneType' instances
TypeError means you have an error in your code. NotImplementedError
means the code is incomplete.
I knew from Java experience that template method pattern is only a
kind of particular case of Stategy where you delegate
parts of algorithm by abstract methods overriding. Indeed the base
class implements some common features and let
derived classes implement particular parts by polymorphism.
Now with your example, I don't understand why he had problems to
implement from C++ ?
Or perhaps is he mixing it with C++ feature template <class T> ?!
You'd have to ask him. Maybe he's just not a very good programmer, or
was having a bad day.
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