There are three distinct things here.
- API definition - something we do all the time.
- "Defensive Programming" - something that may or may not actually exist.
- Paranoid Schizophrenic programming - a symptom of larger problems; this exists far too often.
It's not that complicated, there's a simple 3-element checklist for API design. Unless "someone" is out to break your API. Whatever that means.
A related topic is this kind of thing on Stack Overflow: How Do I Protect Python Code? and Secure Plugin System For Python Application.
Following the Rules
When we define an API for a module, we define some rules. Failure to follow the rules is -- simply -- bad behavior. And, just as simply, when someone breaks the API rules, the module can't work. Calling the API improperly is the as same as trying to install and execute a binary on the wrong platform.
It's the obligation of the designer to specify what will happen when the rules are followed. While it might be nice to specify what will happen if the rules are not followed, it is not an obligation.
Here's my canonical example.
def sqrt( n ):
"""sqrt(n) -> x such that x**2 == n, where n >= 0."""
The definition of what will happen is stated. The definition of what happens when you attempt sqrt(-1) is not defined. It would be nice if sqrt(-1) raises an exception, and it would be nice to include that in the documentation, but it isn't an obligation of the designer. It's entirely possible that sqrt(-1) could return 0. Or (0+1j). Or nan.
Item one on the checklist: define what the function will do.
And note that there's a world of difference between failing, and being used improperly. We're talking about improper use here; failure is unrelated.
When I remind people that they are only obligated to specify the correct behavior, some folks say "That's just wrong! An API document should specify every behavior! You can't omit the most important behavior -- the edge cases!"
Ummm... That position makes no sense.
There are lots and lots of situations unspecified in the API documentation. What about sqrt(2) when the underlying math libraries are mis-installed? What about sqrt(2) when the OS has been corrupted by a virus in the math libraries? What about sqrt(2) when the floating-point processor has been partially fried? What about sqrt(2) when the floating-point processor has been replaced by a nearly-equivalent experimental chipset that doesn't raise exceptions properly?
Indeed, there are an infinite number of situations not specified in the API documentation. For the most part, there is only one situation defined in the API documentation: the proper use. All other situations may as well be left unspecified. Sometimes, a few additional behaviors are specified, but only when those behaviors provide value in diagnosing problems.
An API with thoughtful documentation will at least list the exceptions that are most likely to be raised. What's important is that it does not include an exhaustive list of exceptions. Again, that's an absurd position -- why list MemoryError on every single function definition?
What's important about things like exceptions and error conditions is the diagnostic value of this information. A good designer will provide some diagnostic hints instead of lots of words covering every "possible" case.
If there's no helpful diagnostic value, don't specify it. For example, there's little good to be done by adding a "Could raise MemoryError" on every method function description. It's true, but it isn't helpful. Except in a rare case of an API function that -- if used wrong -- will raise a MemoryError; in this rare case you're providing diagnostic information that can be helpful. You are overwriting the API, but you're being helpful.
Item two on the checklist: provide diagnostic hints where they're actually meaningful and helpful.
How much error checking should our sqrt() function do?
- None? Just fail to produce an answer, or perhaps throw an exception?
- Minimal. This is easy to define, but many folks are unhappy with minimal.
- More than minimal but not everything. This is troubling.
- Everything. This is equally troubling.
No error checking is easiest. And it fits with our philosophy. If our sqrt function is used improperly -- i.e., someone broke the rule and provided a negative number -- then any exception (or nan value) will propagate to the caller and we're in good shape. We didn't overspecify -- we provided a wrong answer when someone asked a wrong question.
Again, we're not talking about some failure to process the data. We're talking about being called in a senseless way by a client that's not following the rules.
There's a subtlety to this, however.
A Non-Math Examples
Yesterday, I tried to use a postal scale to measure the temperature in my oven. The scale read 2.5 oz.
What does that mean?
I asked an ill-formed question. I got something back. It isn't an answer -- the question was ill-formed -- but it looks like an answer. It's a number where I expected a number.
Here's another one. "Which is heavier, the number 7 or the color green?" Any answer ("7", "green" or "splice the main brace") is valid when confronted with a question like that.
Perhaps I should have run a calibration (or "unit") test first.
The Termination Question
In the case of a function like square root, there is an additional subtlety. If we're using logarithms to compute square root, our log function may raise an exception for sqrt(-1) or it may return nan; either of which work out well - an ill-formed question gets an improper answer.
However, we might be using a search algorithm that will fail to terminate (a bisection algorithm, or Newton's method, for example.) Failure to terminate is a much, much worse thing. In this case -- and this case only -- we have to actually do some validation on the range of inputs.
Termination is undecidable by automated means. It's a design feature that we -- as programmers -- must assert independently of any lint, compiler or testing discipline.
Note that this is not "defensive programming". This is ordinary algorithm design. Every loop structure must terminate. If we're trying a simple bisection algorithm and we have not bracketed a root properly (because, for example, it's a complex number), the bisection won't terminate. A root-finding bisection algorithm must actually do two two things to assure termination: check the range of the inputs and limit the number of iterations.
This isn't defensive programming because we're not checking that a mysterious "someone" is abusing the API. We're asserting that our loop terminates.
Item 3 on the checklist: reject values that would lead loops to not terminate properly.
def sqrt( n ):
"""sqrt(n) -> x; such that x**2 == n; where n >= 0"""
assert n >= 0
Incorrect Error Checking
Once we start checking for loop termination, folks say that "we're on a slippery slope" and ask where's that "fine line" between the minimal level of error checking (loops will terminate) and the paranoid schizophrenic level of error checking.
It isn't a slope. It's a cliff. Beyond loop termination, there's (almost) nothing more that's relevant.
By "almost", I mean that languages like Python have a tiny realm where an additional assertion about the arguments is appropriate.
Because of duck typing, many algorithms in Python can be written very generically. Very generically. Sorting, for example, can be applied to lists of -- almost -- anything. Except, of course, it isn't meaningful for things with no useful __cmp__ function. And in the case of things like a dictionary, what's the basis for comparison?
In the case of dynamic languages and duck typing, it's possible that an algorithm will terminate, producing a wrong answer. (BTW, this one reason why Python has / and // as distinct division operators -- to assure that ints and floats can be used interchangeably and the algorithm still works.)
Item 4 on the checklist: When you have a known problem with a type, reject only those types that are a problem. This is very rare, BTW. Mostly it occurs with overlapping types (lists and tuples, floats and ints.) Most well-designed algorithms work with a wide variety of types. Except in the overlapping types situation, Python will raise exceptions for types that don't work; make use of this.
What About "Business Rules"?
By "business rules" most people mean value ranges or codes that are defined by some externality. As in "the claim value must be a number between the co-pay and the life-time limit".
This is not a "Defensive Programming" issue. This is just a policy statement written into the code. Your API won't break if the claim value is less than the co-pay. Your users will be pissed off, but that's a separate problem.
Also, you rarely raise an exception for business rules. Usually, you'll collect business rule violations into a formal error report or log. For example, Django's Forms will collection a dictionary of validation errors. Each element in the dictionary has a list of problems with a particular field on the form.
What About "Someone" Who Can't Use The API?
Here's where the conversation goes awry.
First, if this is a hypothetical "someone", you need to relax. Consider these use cases. Are you worried that "someone" will download your software, install it, configures it, start to use it, and refuse to follow the documented API? Are you worried that they will send you angry emails saying that they insist on doing the wrong thing and your software doesn't work? You don't need "defensive programming", you need to either add the features they want or steer them to a package that does what they're expecting.
Here's another version of a hypothetical someone: you're working as part of a larger team, and you provide a package with an API. Are you worried that a team member will refuse to follow the documented API? Are you worried that they will send you angry emails saying that they insist on doing the wrong thing and your software doesn't work? This isn't a call for "defensive programming," this is a call for a conversation. Perhaps you built the wrong thing. Perhaps you API documentation isn't as crystal-clear as you thought.
Someone Really Is Using It Wrong
A common situation is someone who's actually using the API wrong. The conversation didn't help, they refuse to change their software. Or you can't easily call them out on it because -- for example -- your boss wrote detailed specs for you, which you followed, but someone else isn't following. What can you do? The specification contradicts the actual code that uses the API.
Is this a place where we can apply "Defensive Programming"?
This is a call for some diagnostic support. You need error messages and logs that help you diagnose the problem and locate the root cause.
The issue with "Defensive Programming" is that it conflates two unrelated use cases.
- API Design.
- Unwilling (or unable) to Follow Instructions. (UFI™)
API design has four simple rules.
- Document what it does.
- For diagnostic aid, in common edge cases, document other things it might do. Specifically, describe conditions that are root causes of exceptions or weird answers. Sometimes a subclass of exception is handy for handling this.
- Be sure that it terminates. If necessary, validate arguments to determine if termination can't happen and raise exceptions.
- In rare cases, check the data types to be sure the algorithm will actually work. Most of the time, wrong data types will simply throw exceptions; leverage that built-in behavior.
If (1) someone refuses to follow the rules and (2) complains that it's your API and (3) you elect to make changes, then...
First, you can't prevent this. There's no "defensive programming" to head this off.
Second, know that what you're doing is wrong. Making changes when someone else refuses to follow the rules and blames you is enabling someone else's bad behavior. But, we'll assume you have to make changes for external political reasons.
Third -- and most important -- you're relaxing the API to tolerate ordinarily invalid data.
Expanding What's "Allowed"
When someone refuses to follow the API -- and demands you make a change -- you're having this conversion.
Them: "I need you to 'handle' sqrt(-1)."
You: "Square Root is undefined for negative numbers."
Them: "I know that, but you need to 'handle' it."
You: "There's no answer, you have to stop requesting sqrt(-1)."
Them: "Can't change it. I'm going to make sqrt(-1) requests for external political reasons. I can't stop it, prevent it or even detect it."
You: "What does 'handle' mean?"
At this point, they usually want you to do something that lets them limp along. Whatever they ask you to do is crazy. But you've elected to cover their erroneous code in your module. You're writing diagnostic code for their problem, and you're burying it inside your code.
If you're going to do this, you're not doing "defensive programming", you're writing some unnecessary code that diagnoses a problem elsewhere. Label it this way and make it stand out. It isn't "defensive" programming. It's "dysfunctional co-dependent relationship" programming.