Great job ignoring the substance. I thought it'd be better to come up with an unrealistic but simple example rather than to paste in the source for Pylons...
Just play along and imagine a more complex world. If it makes you happier, pretend the getter is intended to return the string version of the numbers or some other operation that would make just using instance.a wrong.
But using instance.a is not wrong because you can override __getattr__. And if overriding __getattr__ is not the right thing then what you are writing is not a getter. Under no circumstances is it justified IMHO to write a get_X method. (And if your language doesn't let you override __getattr__ then you need a better language.) IMHO of course.
Just play along and imagine a more complex world. If it makes you happier, pretend the getter is intended to return the string version of the numbers or some other operation that would make just using instance.a wrong.