When you’re programming, and the program you’re working on behaves in some way you don’t expect, then by definition there’s some assumption you’ve made about the system that’s incorrect.
Today I was working on adding some fields to a Django admin interface, and I got an unexpected error message when I tried to create a new object in the admin interface and save it. The code was very similar to other code I had previously written that already worked, so I tried to figure out what was wrong by identifying the difference.
The important difference I couldn’t see was that the name of a variable was the source of the problem. In this particular case, the field name (image) was colliding with the field name in a grandparent class, which caused the error to occur. I hadn’t considered that the name of the variable was a meaningful difference that could cause a problem, until I did a search on Stack Overflow and found someone who encountered a similar problem.
This is one of the hardest parts of programming: being able to systematically test your assumptions until you discover which of your current beliefs are false.
Surfing Complexity 