When designing in Object Oriented Programming (OOP) the question I always ask myself when trying to figure out where some piece of code should go is, “Who does this behavior belong to?”.
If our class names are nouns, such as Orders, Books, ProductColors, ERPOrderSender, etc., then it is usually easy to figure out what behavior (or class method) belongs to which class.
Recently I had to add a condition to our order fulfillment workflow to look for suspected fraud orders, and stop them from being processed.
Originally, I had written a simple method to do this inside our ERPOrderSender Class, which is the class responsible for sending EDI order data to our ERP system.
After a while, these new suspected fraud orders start showing up in our “Stuck Orders” Report, and I needed a way to tell that report the reason these orders were stuck. Clearly the ERPOrderSender Class was no longer the right place to house the “Are you a suspected fraud order” code.
In this cause I created a new class called OrderReview, and a new method called getReviewStatus(). The new framework now works equally well when called the context of Order Fulfillment, and in Reporting.
OOP is great. It allows us to think in turns of system behavior. This makes our system easier to talk about in natural language. Which is just more fun – and makes our lives easier.
I’m looking to replace our Magento Ecommerce Server with something “Nice” – Something that is not complicated, not over abstracted, something that is well … fun to work with.
Saleor is an elegant solution. It is an Ecommerce app written in Python and Django. I was able to download it and get it up and running in about 2 hours. That says a lot.
Now, I need to write a product upload script. I could not find a diagram of the Product Schema, so I drew one. I did this by looking at products/models/base.py and images.py and the postgresql database tables. It is open for comments and corrections. Here it is:
Complexity is only Simplicity x 1000 — A mathematician friend once told me that. The idea struck a chord with me and has stuck with me ever since.
Everything in this world is complex. When we design computer systems our goals should be to encapsulate that complexity into simple business level concepts. They in turn can be broken down into smaller and smaller simple systems – and then no part of the system will be complex on its own.
Here is an example. Say you need to download and update pricing information on a regular bases. The new process should simple be called ‘Update Prices’.
I – The process could have the following simple breakdown:
- Update Prices
- Download Pricing
- Load New Pricing Data
- Update Pricing Data
II – The first one could further be broken down into
- Download Pricing:
- Get External Resource URI from config
- Get External Source Dir from config
- Get Local Destination Dir, from config
- SFTP Download files
III – The first one here could further be broken down into
- Get External Resource URI from config
- Create a Config Module that reads YAML config file
- Create environment variable to set config file name
- Provide getter methods for each config value
- Create YAML config files, (one for dev, staging, and prod)
IV – The first one here again, can be further be boken down into
- Create a Config Module that reads YAML config file
- Load YAML parser
- Set config filename = config.basedir + / + ENV(CONFIG)
- yaml.parse(read filename)
And nothing in it is complicated. So break it down. Make it simple! Everyone will be happy.
The Database, Config and Logging are at the core of all application development. Everything starts there.
A Database System stores all of the applications data. The storage itself can be anything from a loose collection of files like jpegs, a nosql db like mongodb, or a full fledge relational database like mysql or postgresql. This subsystem sits right below any Model Objects.
A Configuration System allows you to configure your application without changing code. It primarily determines where things are, such as: subdirectories, external web services, database credentials, log files etc. You will have a different configuration file for each type of environment: dev, stage, and prod
A Logging System should be built in from the word go. All backend tasks should be logged. One philosophy is that all events write a single line entry of Success or Failure. Each event can optionally also write multiple Warning or Debug logging messages. When done this way your logs are themselves databases of all system events. You can can easily answer questions like how many transactions processed today, and how many of them failed.
These three subsystems are needed for all application development. Here is a link to a library I have written that provides these functions. Which I reuse it all the time.
Python Application Development – Core Library Classes – https://github.com/dlink/vlib
You know you’re doing a great job when, adding new functionality to a code base, and you wind up removing more code then you’ve added. Less is More.
I once heard boast a programmer that he’d written million lines of C code. That seemed undesirable to me. Better you should boast that you’ve written an entire system in under 1,000 lines of code.
Doing more with less, requires good design and a lot of reusable code. Doing more with less means you have less to maintain, and less that can go wrong.
So pride yourself on how small your code base is, not how large it is.
I believe programs should be like gardens. They should be lovely to see. Gardens are things we walk around in and return to often. Gardens are things we wish to spend time in with others – to relax in and enjoy its beauty.
If we strive to make your programming systems like gardens, then our days as programmers will be filled with pleasure.
A simple rule to help improve convention and maintainability: Camelcasing should be used when dealing with Objects, Everything else should use underscores.
- Class definitions should be CapCase
- Objects should be mixedCase
- Class Methods should be mixedCase
class BookAuthors(object): # <-- Class Definition
def getAuthors(self, book_id): # <-- Class Method
bookAuthors = BookAuthors() # <-- Object
When not dealing with objects we should use underscores and lowercase
- Simple instance variables
- Filenames / Modules
- SQL schema, table and column names
num_books = len(books)
fp_debug = open('/tmp/debug.log', 'a')
sql = 'select book_id, author_id from books.book_authors where book_id = ?'
This varies slightly from the PEP8 Naming conventions. The PEP8 suggest class methods and non-class methods (or functions) be treated the same way, with underscores. However I prefer naming class methods with mixedCase to remind us we’re in the domain of objects.
With regard to sql schemas, table and column names, because column names can become very long, underscores makes them more readable.
Defensive programming is like defensive driving: Anticipate everything that might go wrong. If a function is passed an Id to a database table, do not assume that it is a valid Id, or an integer, or even that it has a value.
What is most important in defensive programming is to communicate clear and precise error messages when things are not as they aught be.
Here are some examples error messages:
Less then ideal error messages:
AttributeError: 'NoneType' object has no attribute 'last_name'
_mysql_exceptions.OperationalError: (1054, "Unknown column 'Jerry' in 'where clause'")
IndexError: list index out of range
BookError: Book not found: id = Jerry
AuthorError: Author not found: id = 506
getCustomers command: Expected 3 parameters, only 2 given.
FoomWebsiteError: Unable to read from http://foom.com: HTTP 500
These better error messages are not hard to do if we think about it ahead of time. Here are some examples:
class BookError(Exception): pass
def get(self, id):
results = self.db.query(select * from books where id = ?, id)
if not results:
raise Bookerror('Book not found: id = %s' % id
URL = 'foom.com'
class FoomWebsiteError(Exception): pass
def scrapePage(path, params):
website = Website(URL)
page = website.go(path, params)
raise WebsiteError('Unable to read from %s: %s'
% (URL, website.error)
lines = page.split('<p>')
name = lines
It is okay to have bugs if they are easy to find and easy to fix. Applying a little defensive programming to everything we write make debugging a breeze, and helps everyone using the system.
Whether building a new system, or trying to untangle some untenable rats nest of code — One should have an image of the ideal state in their mind.
If we draw up the best plan we can, thinking in terms of the perfect – regardless of its immediate feasibility – we can then put that on the wall and work towards it.
All changes to code come in two forms: Bug Fixes and Feature Enhancements. There are many decisions to make in each case. By holding up an Ideal, it helps us with those decisions. We choose the path that helps the code converge on some well conceived plan rather than letting it oscillating around, as is often the case.
The ideal plan is rarely realized but that’s not the point. By aiming at one target we help the general direction of all the arrows.
So we must spend a lot of time designing the ideal. To do that we need to create documents such as these:
– Problem Statements
– Use Case Diagrams
– Data Model Entity Relationship Diagrams (ERD),
– Class Diagrams
– Sequence Diagrams
– Wireframes, and
– Mock Reports
It is not enough to simple understand a single aspect of the system and go to work implementing it. We must take the extra time to see how that component fits into the larger whole. The benefits of which creates flexible, easy to use and fun to maintain code.
It was Abe Lincoln who said: Give me six hours to chop down a tree and I will spend the first four sharpening the axe.
What is the benefit of using a framework like Ruby-on-Rails, Pylons or Drupal? Simply put, it helps us start and develop code from nothing quickly. But it does not have long lasting staying power. If you plan to be using the system for years to come – the advantage of the quick start up is out weighed by the restrictions placed upon you by the framework.
So often in my experience working on legacy systems in various industries the Framework itself becomes the enemy of quick bug fixes and feature enhancements.
I prefer to use libraries rather than using frameworks. We all need libraries because we don’t want to reinvent everything like database connectivity, yaml parsing, json parsing, and interesting things like zipcode distance calculations, etc.
But frameworks are something we can write ourselves very easily. The advantage of doing so is the ability to understand and have complete control over everything in the system, and only have those things we really need, and nothing else.
Lightweight “frameworks” like Cherrypy and Flask for python and Sinatra for ruby, which can be considered libraries for http-routing, rather than frameworks, are more elegant solutions than full fledged MVC frameworks.