Monthly Archives: February 2011

The gauntlet software pattern

I find code like this really confusing:


if a and b and c:

    if d or e:
        return True

    elif f and g:
        return True

else:
    return False

Furthermore, updating those rules is nasty. I’m very likely to get something wrong, especially when “if a” is really some elaborate method call with several parameters.

It get a little more obvious when I flatten it out like this:


if a and b and c and (d or e or (f and g)):
    return True

else:
    return False

That looks OK for this contrived example, but imagine I need to stick in another clause that says also return True when a and g are True. It ain’t so easy to arrange the code any more.

Over the years, I’ve run into this situation a lot. Usually, I’m doing something like registering a new user, and I want to rule out that this new user is already in the database. So I have to look up the user on a variety of fields, like maybe email address, their user name, their mobile number, etc.

When I can boil the action down into something like “do this, unless any of these scenarios apply” I write the code in this format:


# Beginning of the gauntlet
if not a:
    return False

if not b:
    return False

if not c:
    return False

if not d or e:
    return False
# End of the gauntlet

return True

I put the fastest reasons to disqualify the action at the beginning. The really elaborate stuff belongs at the bottom.

Now,