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Thinking About Computing
Bruce Eckel's Web Log

5-2-03 Strong Typing vs. Strong Testing

In recent years my primary interest has become programmer productivity. Programmer cycles are expensive, CPU cycles are cheap, and I believe that we should no longer pay for the latter with the former.

How can we get maximal leverage on the problems we try to solve? Whenever a new tool (especially a programming language) appears, that tool provides some kind of abstraction that may or may not hide needless detail from the programmer. I have come, however, to always be on watch for a Faustian bargain, especially one that tries to convince me to ignore all the hoops I must jump through in order to achieve this abstraction. Perl is an excellent example of this — the immediacy of the language hides the meaningless details of building a program, but the unreadable syntax (based, I know, on backwards-compatibility with Unix tools like awk, sed and grep) is a counterproductive price to pay.

The last several years have clarified this Faustian bargain in terms of more traditional programming languages and their orientation towards strong type checking. This began with a 2-month love affair with Perl, which gave me productivity through rapid turnaround. (The affair was terminated because of Perl's reprehensible treatment of references and classes; only later did I see the real problems with the syntax.) Issues of strong-vs-weak typing were not visible with Perl, since you can't build projects large enough to see these issues and the syntax obscures everything in smaller programs.

After I had worked with Python (free at for awhile — a language which can build large, complex systems — I began noticing that despite an apparent carelessness about type checking, Python programs seemed to work quite well without much effort, and without the kinds of problems you would expect from a language that doesn't have the strong, static type checking that we've all come to "know" is the only correct way of solving the programming problem.

This became a puzzle to me: if strong static type checking is so important, why are people able to build big, complex Python programs (with much shorter time and effort than the strong static counterparts) without the disaster that I was so sure would ensue?

This shook my unquestioning acceptance of strong type checking (acquired when moving from C to C++, where the improvement was dramatic) enough that the next time I examined the issue of checked exceptions in Java, I asked "why"? which produced a big discussion wherin I was told that if I kept advocating unchecked exceptions, cities would fall and civilization as we know it would cease to exist. In Thinking in Java, 3rd edition, I went ahead and advocated the use of RuntimeException as a wrapper class to "turn off" checked exceptions. Every time I do it now, it seems right (I note that Martin Fowler came up with the same idea at roughly the same time), but I still get the occasional email that warns me that I am violating all that is right and true and probably the USA Patriot act, as well (hi, all you guys from the FBI! Welcome to my weblog!).

But deciding that checked exceptions seem like more trouble than they're worth (the checking, not the exception. I believe that a single, consistent error reporting mechanism is essential) did not answer the question "why does Python work so well, when conventional wisdom says it should produce massive failures?" Python and similar "weak" or "latently" typed languages are very lazy about type checking. Instead of putting the strongest possible constraints upon the type of objects, as early as possible (as C++ and Java do), languages like Ruby, Smalltalk and Python put the loosest possible constraints on types, and evaluate types only if they have to. That is, you can send any message to any object, and the language only cares that the object can accept the message — it doesn't require that the object be a particular type, as Java and C++ do. For example, if you have pets that can speak in Java, the code looks like this:

// Speaking pets in Java:
interface Pet {
  void speak();

class Cat implements Pet {
  public void speak() { System.out.println("meow!"); }

class Dog implements Pet {
  public void speak() { System.out.println("woof!"); }

public class PetSpeak {
  static void command(Pet p) { p.speak(); }
  public static void main(String[] args) {
    Pet[] pets = { new Cat(), new Dog() };
    for(int i = 0; i < pets.length; i++)

Note that command() must know exactly the type of argument it's going to accept — a Pet — and it will accept nothing else. Thus, I must create a hierarchy of Pet, and inherit Dog and Cat so that I can upcast them to the generic command() method.

For the longest time, I assumed that upcasting was an inherent part of object-oriented programming, and found the questions about same from ignorant Smalltalkers and the like to be annoying. But when I started working with Python I found the following curiosity. The above code can be translated directly into Python:

# Speaking pets in Python:
class Pet:
    def speak(self): pass

class Cat(Pet):
    def speak(self):
        print "meow!"

class Dog(Pet):
    def speak(self):
        print "woof!"

def command(pet):

pets = [ Cat(), Dog() ]

for pet in pets:

If you've never seen Python before, you'll notice that it redefines the meaning of a terse language, but in a very good way. You think C/C++ is terse? Let's throw away those curly braces — indentation already has meaning to the human mind, so we'll use that to indicate scope, instead. Argument types and return types? Let the language sort it out! During class creation, base classes are indicated in parentheses. 'def' means we are creating a function or method definition. On the other hand, Python is explicit about the 'this' argument (called 'self' by convention) for method definitions.

Note that command(pet) just says that it takes some object called 'pet', but it doesn't give any information about what the type of that object must be. That's because it doesn't care, as long as you can call speak(), or whatever else your function or method wants to do. We'll look at this more closely in a minute.

Also, command(pet) is just an ordinary function, which is OK in Python. That is, Python doesn't insist that you make everything an object, since sometimes a function is what you want.

In Python, lists and maps/dictionaries/associative arrays are both so important that they are built into the core of the language, so I don't need to import any special library to use them. Here:

pets = [ Cat(), Dog() ]

A list is created containing two new objects of type Cat and Dog. The constructors are called, but no "new" is necessary (and now you'll go back to Java and realize that no "new" is necessary there, either. It's just a redundancy inherited from C++).

Iterating through a sequence is also important enough that it's a native operation in Python:

for pet in pets:

selects each item in the list into the variable pet. Much clearer and more straightforward than the Java approach, I think.

The output is the same as the Java version, and one observation we can make here is that Python makes an excellent pseudocode, with the wonderful attribute that it can actually be executed. This means you can quickly try out ideas in Python, and when you get one that works, you can rewrite it in Java/C++/C# or your language of choice. Or maybe you will realize that the problem is solved in Python, so why bother rewriting it? (That's usually as far as I get). I've taken to giving exercise hints in Python during seminars, because I'm then not giving away the whole picture, but people can see the form that I'm looking for in a solution so they can move ahead. And I'm able to verify that the form is correct. This is why Python is often called "executable pseudocode."

But the interesting part is this: because the command(pet) method doesn't care about the type it's getting, I don't have to upcast. So I can rewrite the Python program without using base classes:

# Speaking pets in Python, but without base classes:
class Cat:
    def speak(self):
        print "meow!"

class Dog:
    def speak(self):
        print "woof!"

class Bob:
    def bow(self):
        print "thank you, thank you!"
    def speak(self):
        print "hello, welcome to the neighborhood!"
    def drive(self):
        print "beep, beep!"

def command(pet):

pets = [ Cat(), Dog(), Bob() ]

for pet in pets:

Since command(pet) only cares that it can send the speak() message to its argument, I've removed the base class Pet, and even added a totally non-pet class called Bob which happens to have a speak() method, so it also works in the command(pet) function.

At this point, a strong, statically-typed language would be sputtering with rage, insisting that this kind of sloppiness will cause disaster and mayhem. Clearly, at some point the "wrong" type will be used with command() or will otherwise slip through the system. The benefit of simpler, clearer expression of concepts is simply not worth the danger. Even if that benefit is a productivity increase of 5 to 10 times over that of Java or C++.

What happens when such a problem occurs in a Python program — an object somehow gets where it shouldn't be? Python reports all errors as exceptions, like Java and C# does and like C++ ought to do. So you do find out that there's a problem, but it's virtually always at run time. "Aha!" you say, "There's your problem: you can't guarantee the correctness of your program because you don't have the necessary compile-time type checking."

When I wrote Thinking in C++, 1st edition, I incorporated a very crude form of testing: I wrote a program that would automatically extract all the code from the book (using comment markers placed in the code to find the beginning and ending of each listing), and then build makefiles that would compile all the code. This way I could guarantee that all the code in my books compiled and so, I reasoned, I could say "if it's in the book, it's correct." I ignored the nagging voice that said "compiling doesn't mean it executes properly," because it was a big step to automate the code verification in the first place (as anyone who looks at programming books knows, most authors still don't put much effort into verifying code correctness). But naturally, some of the examples didn't run right, and when enough of these were reported over the years I began to realize I could no longer ignore the issue of testing. I came to feel so strongly about this that in the third edition of Thinking in Java, I wrote:

If it's not tested, it's broken.

That is to say, if a program compiles in a strong, statically typed language, it just means that it has passed some tests. It means that the syntax is guaranteed to be correct (Python checks syntax at compile time, as well. It just doesn't have as many syntax contraints). But there's no guarantee of correctness just because the compiler passes your code. If your code seems to run, that's also no guarantee of correctness.

The only guarantee of correctness, regardless of whether your language is strongly or weakly typed, is whether it passes all the tests that define the correctness of your program. And you have to write some of those tests yourself. These, of course, are unit tests. In Thinking in Java, 3rd Edition, we filled the book with unit tests, and they paid off over and over again. Once you become "test infected," you can't go back.

It's very much like going from old C to C++. Suddenly, the compiler was performing many more tests for you and your code was getting right, faster. But those syntax tests can only go so far. The compiler cannot know how you expect the program to behave, so you must "extend" the compiler by adding unit tests (regardless of the language you're using). If you do this, you can make sweeping changes (refactoring code or modifying design) in a rapid manner because you know that your suite of tests will back you up, and immediately fail if there's a problem — just like a compilation fails when there's a syntax problem.

But without a full set of unit tests (at the very least), you can't guarantee the correctness of a program. To claim that the strong, static type checking constraints in C++, Java, or C# will prevent you from writing broken programs is clearly an illusion (you know this from personal experience). In fact, what we need is

Strong testing, not strong typing.

So this, I assert, is an aspect of why Python works. C++ tests happen at compile time (with a few minor special cases). Some Java tests happen at compile time (syntax checking), and some happen at run time (array-bounds checking, for example). Most Python tests happen at runtime rather than at compile time, but they do happen, and that's the important thing (not when). And because I can get a Python program up and running in far less time than it takes you to write the equivalent C++/Java/C# program, I can start running the real tests sooner: unit tests, tests of my hypothesis, tests of alternate approaches, etc. And if a Python program has adequate unit tests, it can be as robust as a C++, Java or C# program with adequate unit tests (although the tests in Python will be faster to write).

Robert Martin is one of the long-time inhabitants of the C++ community. He's written books and articles, consulted, taught, etc. A pretty hard-core, strong- static type checking guy. Or so I would have thought, until I read this weblog entry. Robert came to more or less the same conclusion I have, but he did so by becoming "test infected" first, then realizing that the compiler was just one (incomplete) form of testing, then understanding that a weakly-typed language could be much more productive but create programs that are just as robust as those written in strongly-typed languages, by providing adequate testing.

Of course, Martin also recieved the usual "how can you possibly think this?" comments. Which is the very question that lead me to begin struggling with the strong/weak typing concepts in the first place. And certainly both of us began as strong static type checking advocates. It's interesting that it takes an earth-shaking experience — like becoming test-infected or learning a different kind of language — to cause a re-evaluation of beliefs.

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