[Tutor] The Template Pattern

[Karim]

Hello Steven,

Could take some time when possible to answer to my follow-up question?

Thank you!
Cheers
Karim

On 12/14/2010 12:19 AM, Karim wrote:
> On 12/13/2010 11:47 PM, Steven D'Aprano wrote:
>> Karim wrote:
>>>
>>> Hello all,
>>>
>>> I am seeking for information about the template pattern applied to 
>>> python.
>>> Could you explain some implementation or anything else? it would be 
>>> helpful.
>>
>> Design patterns are means to an end, not an end in themselves. You 
>> shouldn't say "I want to use the template pattern", you should say "I 
>> want to solve this problem, what solution is appropriate?" and then 
>> only use the template pattern if it is appropriate.
>>
>
> Steven, thank you for answering ;o)
>
> I don't want to use this pattern absolutely. But I discussed this 
> topic with a C++ programmer.
> He wanted to translate part of C++ code in python that implements this 
> pattern in C++.
> I already have implemented a Strategy pattern in python to read 
> different file format into
> my database and generate different output format file like the code 
> below:
>
> class Input(object):
>     """The basic Input class, leaving out the details of the 
> algorithm."""
>     def __init__( self, strategy ):
>         self.strategy = strategy
>         self.lastRead = None
>
>     def read(self, filePath):
>     """Load into database memory an input file.
>     @arg: filePath - string - The input file path.
>     @return: The created database object.
>     """
>     self.lastRead = self.strategy.read()
>     return self.lastRead
>
>
> 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?
>
> 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> ?!
>
>> In any case, the template design pattern is one of the simpler design 
>> patterns. Here's an example, adapted from Wikipedia's article on the 
>> Template Method Pattern:
>>
>>
>> class AbstractGame:
>>     """An abstract class that is common to several games in which
>>     players play against the others, but only one is playing at a
>>     given time.
>>     """
>>     def __init__(self, *args, **kwargs):
>>         if self.__class__ is AbstractGame:
>>             raise TypeError('abstract class cannot be instantiated')
>>
>>     def playOneGame(self, playersCount):
>>         self.playersCount = playersCount
>>         self.initializeGame()
>>         j = 0
>>         while not self.endOfGame():
>>             self.makePlay(j)
>>             j = (j + 1) % self.playersCount
>>         self.printWinner()
>>
>>     def initializeGame(self):
>>         raise TypeError('abstract method must be overridden')
>>
>>     def endOfGame(self):
>>         raise TypeError('abstract method must be overridden')
>>
>>     def makePlay(self, player_num):
>>         raise TypeError('abstract method must be overridden')
>>
>>     def printWinner(self):
>>         raise TypeError('abstract method must be overridden')
>>
>>
>> Now to create concrete (non-abstract) games, you subclass 
>> AbstractGame and override the abstract methods.
>>
>> class Chess(AbstractGame):
>>     def initializeGame(self):
>>         # Put the pieces on the board.
>>         ...
>>
>>     def makePlay(player):
>>         # Process a turn for the player
>>         ...
>>
>> etc.
>
>
> One more Thanks for you example Steven!
> Regards
> Karim
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