[Tutor] Null Object Design Pattern Question

[Francis Moore]

> From: Tim Johnson [mailto:tim at johnsons-web.com] 

> which has an example of using a Null() object.
> Unfortunately, I'm having a problem wrapping my brain 
> around the usefulness of this object compared to just using None.

Tim,

I've never used a Null Object pattern (NOP) in Python but I've used it 
quite a few times in C++. The NOP is primarily handy for removing lots
of 
conditional code. For instance, imagine a program where we have an
option 
to log user output. We should then allow the user to switch logging on
or 
off (to speed up the program). So, in our program we would normally need
to 
use lots of conditional code to see whether logging is turned off or 
not i.e.

if logging_on:
    Logger.Log(message)
elif:
    pass

etc...

An easier way is to code a base class called Logger and two concrete
classes 
derived from Logger called RealLogger and NullLogger (an instance of the
NOP). 
At the beginning of the program we create an instance of the correct
class 
(RealLogger or NullLogger) depending on whether the user has logging
switched 
on or not and bind it to an instance of the Logger class. We can then 
use the objects polymorphically, by just calling Logger.Log(message) and
the 
correct class will be called. 
There is now no need to write conditional code and check against None. 
The correct object does the right thing. The RealLogger logs all
messages, the 
NullLogger lets the message pass through. Using the same line of code.

If logging is widespread throughout your application this can provide a
huge 
reduction in the number of lines and make the application logic clearer.

Hope this helps,
Francis. 
  
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