[Tutor] The Template Pattern
steve at pearwood.info
Sun Dec 19 04:33:03 CET 2010
>> 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.
def __init__(self, *args):
if type(self) is AbstractClass:
# 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:
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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