On 10/01/13 02:26, Ed Owens wrote:
First, please start a new thread for a new topic, don't reply to an
existing one. Threaded mail/news readers are not fooled by a simple
change of subject so your post gets buried deep in an old thread.
Just create a brand new mail to the list address.
Anyway...
I'm working my way through Chun's book "Core Python Applications
Programming" and can't get one of the examples to actually work. In
trying to analyze the problem (good learning approach) I had troubles
understanding the interactions between the two classes of objects. As
an old FORTRAN programmer, I picked up my pencil to flowchart the code,
then realized I didn't know how to flowchart an OOP.
Us a message sequence chart(MSC). A vertical line represents an object.
Draw arrows between the lines to represent the messages passing between
them. Messages correspond to function calls in procedural programming.
MSCs are a very old design tool from the communications world but
adopted by OOP and included in UML (see below)
Google led me to UML (Unified Modeling Language) and OMG (apparently Oh
My God!!!). Looks more complicated than the code I'm trying to understand.
OMG (Object Management Group) is a body overseeeing many things OO
based. Design tools, integration technologies, languages and
specifications etc.
UML is a design language that, if used to its limit, can be used to
generate working code. As a result it has a lot of stuff in there to
represent fine grained detail. So yes, it is as complex as code because
in a sense that's what it is. Graphical code.
In addition UML is designed to completely represent a large software
system so it includes notations for designing networks and data centres
as well as application code.
The good news is that most people don't use UML like that and only use
about 20% of UML or less. The main diagrams you are likely to use as a
Python programmer are class diagram, MSC, activity diagram(flow charts!)
and state charts. Focus on those areas and don't obsess on the details
and UML becomes useful to ordinary mortals. I use UML in the v3 version
of my tutor in the OOP topic, the examples there hopefully show how UML
can be used even in simple designs.
It there a technique that people use to figure out how do design OOP
models, objects, and the information flow?
There are lots of them, that's why UML was invented to bring the best
bits together into a single Unified Modelling Language.
But OOP is more about a different way of thinking about the problem.
You have to start thinking about objects and their responsibilities.
Objects own data and provide services. Other objects use those services
to provide their services and so on. So we tend to get a bottom up style
of design. Build (or select from the library) the lowest level
objects then build the next layer up that consume those. Don;t be scared
to split objects into two(or more) if they start to get unweildy or to
combine two (or more) if they don;t seem to have a purpose or are
duplicating services.
CRC cards - not part of UML - are a good place to start when building
OOP systems. A very easy technique that does not require any fancy tools.
HTH,
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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