Noel Rappin Writes Here

Re-refactoring

Posted on September 13, 2006


Here’s a little riff inspired by one of the examples in Martin Fowler’s book Refactoring, which is another great programming book that deserves an appreciation post one of these days. This was actually also spawned by code that I’ve read, and later realized that Fowler did a similar example. Thing is, I don’t think Fowler went far enough in this case.

Here’s the example. (page 243 for those of you playing the home game). But, since it’s Python Week here, I’ll translate to Python.

if isSpecialDeal():
 total = price \* 0.95
 send()
else:
 total = price \* 0.98
 send()

Fowler correctly notices the duplicate call to send(), and refactors to:

if isSpecialDeal():
 total = price \* 0.95
else:
 total = price \* 0.98
send()

This is fine as far as it goes, but as I see it, there’s a second duplication in this snippet – the formula for calculating the total. I’d rather see something like this (using the new Python 2.5 ternary syntax:

multiplier = (0.95 if isSpecialDeal() else 0.98)
total = price \* multipiler
send()

There are a couple of advantages to this last snippet. We’ve separated the calculation of the total from the act of gathering the data for that calculation. This makes the actual formula for the total clearer, and allows you to easily spawn the multiplier getter off to it’s own method if it gets more complicated. Plus we’ve removed more duplication, and I think made the code structure match the logical structure of the calculation a little bit better.

This is a simple example, and you could quibble with it. The general idea of separating conditional logic from calculations is a solid way to clean up code and make it easier to maintain in the long run.

Before I leave… I’m not sold on the syntax for the Python ternary yet. I’m told that the syntax was chosen over the perhaps more consistent if cond then x else y end because it was felt that in most use cases you’d have a clear preferred choice and a clear alternate choice, and putting the preferred choice before the conditional emphasized that. I don’t know if that matches how I’d use a ternary. Although I guess it’s reminiscent of listcomp syntax. I need to use it in real programs to know for sure.



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