Coding backwards

Summary Playing pretend on top of an API you haven't written yet can be a quick, effective way to spur ideas about design.

For a while I’ve wanted to write some personal finance software. Call me crazy, but I can’t find anything currently in existence that allows me to project my earnings and expenses1, then summarizes the net effect over a period of time. Basically, I want to be able to propose a budget to myself and simulate how it will play out.

So I sat down (as so many programmers have before me), ready to (almost certainly) reinvent the wheel, and I began to code. I wasn’t really feeling inspired, though. A good design wasn’t immediately clear to me, so I got slightly frustrated and stared aimlessly at the screen awhile.

Until I got an idea.

Quick flashback

Last winter, I interviewed at a hedge fund called Two Sigma in New York City. Regrettably, I bombed the interview, but it was a great experience nonetheless. My first technical interviewer was a guy who looked like he needed no less than three pots of coffee to show any sign of human emotion. He asked me the boilerplate questions about design patterns and data structures, but afterwards he asked me to tackle a considerably difficult data-handling problem in Java. I cracked open a terminal-based session of vi as he shook his head and muttered something about why wasn’t I using Eclipse.

Then, to my surprise, he told me to write out a bunch of class skeletons — class names, method signatures, and attributes. He wanted me to enumerate the cast of characters that I was going to be interacting with before I actually decided how the interactions would go down.

I walked out of the interview knowing I didn’t get the gig, but I was intrigued by this guy’s approach.

The backwards art of software design

I reminisced about the TwoSigma episode yesterday, as I was struggling to get down a design for my little personal-finance Python API. I decided that I’d do the stoic TwoSigma interviewer one better and write out a script using the yet unwritten API. I’d reverse-engineer a good design by pretending I’d already written one!

Ten minutes and a few backspaces later, I came up with this:

from miser import *

m = Miser("jobeirne")

g = Goal(amount = 16e3, # $16,000
         by = Date(2012, 8, 1)) # by Aug. 1, 2012


bills = [Expense(name = "MATH315 tuition",
                 amount = 1.3e3,
                 on = Date(2011, 8, 29)),

         Expense(name = "netflix",
                 amount = 14.,
                 on = MonthlyRecurring(15))] # 15th day of the month

income = [Income(name = "phase2",
                 amount = 1.5e3,
                 on = MonthlyRecurring(7, 22))]


print(m.summary(fromdt = Date(2011, 8, 20), 
                todt = Date(2012, 9, 1))) 

Skipping steps

In a few ways, the design I came up with coding backwards was better than what I originally proposed. Initially, I was going to have classes like MonthlyIncome and DailyExpense that mandated an overly complicated inheritance structure to avoid repeating periodicity code.

When I wrote a script in my fictional API, I decoupled the transactions and their frequency of occurrence into two disparate objects without really thinking about it; instead of my initial design,

MonthlyExpense(name = "netflix", amount = 14.);

I now had

Expense(name = "netflix", amount = 14., on = MonthlyRecurring(15))

which makes for a waaay more reasonable class tree (goodbye, multiple inheritance).

I was surprised at how satisfying the experiment was. A better design quickly appeared when I forced myself to preemptively eat my own dogfood. Instead of shoehorning use-cases into a class structure I’d already designed, I coded backwards and the opposite happened: a design evolved from daydreaming about an API that I’d like using.

Paint by numbers

Now that I had written out an interface I liked, development was simple: I started out with skeleton classes and then filled them out so as to match their behavior in the example usage I had written.

The process was straightforward, though I had to put some thought into how the classes would be structured internally. As I fleshed out the classes, I noticed a few ways I could improve upon the API I had come up with, so I made a few minor changes to the example usage. This iterative back-and-forth continued throughout development.


My experiment wasn’t without a catch; since this method of coding backwards is about as top-down as you can get, I fell into the pitfall of doing a ton of development without actually running the code to test correctness.

This, of course, led to a long bout of debugging at the end, which was a slight pain, especially since I’m so used to rapid prototyping. Maybe if I had test-driven the filling out of the classes, things would’ve gone down smoother.

The project was so small that it wasn’t a big deal, but I can see how my approach wouldn’t have scaled to something much larger. I’m usually a big fan of bottom-up development, but coding backwards put me in a mindset that made that a little less straightforward.

Conclusion & buzzword bingo

Clearly, this technique is inspired by test-driven development, and for all I know it may be old news to experienced programmers. Nonetheless, I found using it to be a fun experiment and a nice addition to my utility belt.

There is no silver bullet for development methodology. Coding backwards, or interface-driven development2, certainly won’t solve all your problems. It is, though, a quick and gratifying way to help writer’s block and gain some clarity when you sit down to write an API.

You can follow the development of Miser, my dead-simple personal-finance script, here on Github.

happened in the past, but I couldn’t find anything that would let me ask “what if I adhere to this budget?”

  1. Mint and other services will allow you to analyze expenses that have 

  2. use that one in a meeting.