Defensive programming aims to anticipate failure points and improve software quality by making code more reliable, maintainable, and robust. It involves programming with the assumption that things can go wrong in the real world where software interacts with users and external systems. Defensive practices include validating external input, writing code with teams in mind through standards and modularity, adding error handling and logging, and assuming other developers' code could contain bugs. The goal is to create programs that work as intended and are protected from invalid or malicious data.

1. Code - Fu:
Defensive Programming
With great power comes great responsibility ~ Peter Paker’s Dad’s Brother Ben

2. 1980
A radiation therapy machine
was directly responsible for
at least 5 patient deaths
1996
A US$1 billion exploded
40 seconds after takeoff
2012
Apple Maps gives us
directions to nowhere
2016
ABSA bank double
charges clients
Why learn another design pattern?
Because: BUGS!

3. Intro to Code-Fu
Defensive programming, simply
put, is programming with the intent
to anticipate likely failure points.
Defensive programming practices
are often used where high
reliability, safety or security is
needed.

4. Intro to Code-Fu cont...
Defensive programming aims to improve software and source code, in terms
of:
1. General quality (reduce bugs)
2. Comprehensive code
a. Clean
b. Readable
c. Understandable
d. Reliable
e. Maintainable
f. Testable
g. Robustness
3. Predictable behaviour

5. Mastering the
ways of Code-Fu
Defensive programming is really
programming for the real world
where if anything can go wrong it
will.

6. Real world programs
1. Are not in isolation, you can expect user input or external input
1. Are built & maintained by teams and the individuals making the team can
change over time (imagine open source projects)
1. Need to work

7. External/User input
● Never trust external/user input
● Remember there are also users with malicious intent
What helps
● Protect your code from invalid data (data validation)
● User whitelists not blacklists
● Use database abstraction (e.g. PDO for PHP)

8. We work in Teams
● Trust your fellow developers but DON’T trust their code
● Defensive Programming is much like Defensive Driving assume that
everyone around us can potentially and possibly make mistakes.
● Teams can consists of individuals working from different planets
What helps
● Enforcing coding standards (naming conversions included)
● Write SOLID code (Encapsulation)
● Write code for the maintainer (The maintainer can be future you)
● Parameter checks at the beginning of a function/method
● Unit tests, Regression testing, Integration tests

9. Bad programming
// ES5

10. Check params + Encapsulation
// ES5

11. Programs need to work
● Imagine paying R200 per month for Software that works 30% of the time
What helps
● Add diagnostic code, logging and tracing (Bugsnag/Sentry)
● Security tests

12. As a Defensive programmer
you need to have a healthy
dosage of paranoia

13. Too much Paranoia

14. Good/Bad?

15. QUESTIONS?