Summary
For some time I've been thinking about how TDD tests can be as simple, as expressive, and as elegant as possible. This article explores a bit about what it's like to make tests as simple and decomposed as possible: aiming for a single assertion in each test.
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A while ago there was a bit of fuss on the testdrivendevelopment
Yahoo group about the idea of limiting yourself to one assertion per
test method, which is a guideline that others and I offer for TDD
work.
An address parser was the example of a situation where it was
argued that multiple assertions per test made sense. Date was
formatted with one address per line, each in one of the following
formats:
ADDR1$ADDR2$CSP$COUNTRY
ADDR1$ADDR2$CSP
ADDR1$CSP$COUNTRY
ADDR1$CSP
The poster went on to say:
My first inclination is/was to write a test like this:
a = CreateObject("Address")
a.GetAddressParts("ADDR1$ADDR2$CITY IL 60563$COUNTRY")
AssertEquals("ADDR1", a.Addr1)
AssertEquals("ADDR2", a.Addr2)
AssertEquals("CITY IL 60563", a.CityStatePostalCd)
AssertEquals("Country", a.Country)
They didn't see how to achieve this with one assertion per test, as
there are obviously four things to test in this case. I decided that
rather than simply reply, I would write some tests and code to
illustrate my view on the matter, and offer a solid response.
For this problem, I chose Squeak Smalltalk (see www.squeak.org) and Java. For the sake of
conciseness, I'll omit any required accessors.
So, where to start? Well, when doing TDD it often makes sense to
start with something simple to quickly and easily get some code
written and working. Then it can be extended and evolved in response
to further test driving. Here the simplest case is: ADDR1$CSP. There
are two requirements in the parsing of this example: that the ADDR1
was recognized, and that the CSP was recognized. Viewed this way, we
need two tests. We start with one for ADDR1:
public void testAddr1() throws Exception {
Address anAddress = new Address("ADDR1$CITY IL 60563");
assertEquals("ADDR1", anAddress.getAddr1());
}
To get this to pass we need an Address class and a from: factory
method, which creates an instance and has it parse the address string.
For brevity, I'll skip the "return literal" step.
public class Address {
private String addr1;
public Address(String aString) {
parse(aString);
}
private void parse(String aString) {
StringTokenizer parts = new StringTokenizer(aString, "$");
addr1 = parts.nextToken();
}
}
That's well & good. The next test is for CSP.
Squeak:
testCsp
| anAddress |
anAddress := Address from: 'ADDR1$CITY IL 60563'.
self assert: anAddress csp equals: 'CITY IL 60563'
Java:
public void testCsp() throws Exception {
Address anAddress = new Address("ADDR1$CITY IL 60563");
assertEquals("CITY IL 60563", anAddress.getCsp());
}
Address>>parse: will need to be extended (and we need to add a csp
instance variable and accessors):
Squeak:
parse: aString
| parts |
parts := aString findTokens: '$'.
addr1 := (parts at: 1).
csp := (parts at: 2)
Java:
private void parse(String aString) {
StringTokenizer parts = new StringTokenizer(aString, "$");
addr1 = parts.nextToken();
csp = parts.nextToken();
}
So. We have two tests for this one situation. Notice the duplication
in the tests... the creation of the instance of Address that is being
probed. This is the fixture. After refactoring, we have:
public class Addr1CspTests extends TestCase {
private Address anAddress;
protected void setUp() throws Exception {
anAddress = new Address("ADDR1$CITY IL 60563");
}
public void testAddr1() throws Exception {
assertEquals("ADDR1", anAddress.getAddr1());
}
public void testCsp() throws Exception {
assertEquals("CITY IL 60563", anAddress.getCsp());
}
}
So, a fixture that creates the Address instance from the string, and
very simple tests that focus on each aspect of that fixture.
The next simplest case is the obvious choice for the next fixture:
Squeak:
setUp
anAddress := Address from: 'ADDR1$CITY IL 60563$COUNTRY'
Java:
protected void setUp() throws Exception {
anAddress = new Address("ADDR1$CITY IL 60563$COUNTRY");
}
This set of tests will include ones for addr1 and csp as before
(refactoring this to remove that duplication is left to the reader) as
well as a new test for country:
Squeak:
testCountry
self assert: anAddress country equals: 'COUNTRY'
Java:
public void testCountry() throws Exception {
assertEquals("COUNTRY", anAddress.getCountry());
}
As before, an instance variable and associated accessors need to be
added to the Address class.
This drives Address>>parse: to evolve:
Squeak:
parse: aString
| parts |
parts := aString findTokens: '$'.
addr1 := (parts at: 1).
csp := (parts at: 2).
country := (parts at: 3 ifAbsent: [''])
Java:
private void parse(String aString) {
StringTokenizer parts = new StringTokenizer(aString, "$");
addr1 = parts.nextToken();
csp = parts.nextToken();
country = parts.hasMoreTokens() ? parts.nextToken() : "";
}
From here on, the evolution gets a bit more complex, as we add the
ADDR2 option to the mix.
Conclusion
So we took a situation that was thought to require multiple assertions
in a test and did it in such as way as to have only one assertion per
test.
The key is that instead of using a single TestCase subclass with a
complex (i.e. multiple assertion) tests for each situation, we made
each of those situations into a separate fixture. Each fixture is
implemented by a separate subclass of TestCase. Now each test focuses
on a very small, specific aspect of that particular fixture.
I'm convinced writing tests like this is a useful approach. One
advantage is that the resulting tests simpler and easier to
understand. Just as important, and maybe more so, is that by adding
the specification of the behavior one tiny piece at a time, you drive
toward evolving the code in small, controllable, understandable steps.
It also fits better into the test fixture centered approach that is
the recommended way to organize your tests. We set up the object to
test in the setUp method, and tested each aspect of it in individual
tests methods.
As I've been writing this, something clicked. I see these test
methods as specifications of tiny facets of the required behavior.
Thus, it makes sense to me to be as gradual as possible about it,
driving the evolution of the code in the smallest steps possible.
Striving for one assertion per test is a way to do that.
If, however, you view test methods as strictly performing
verification, then I can see how it might be seen to make sense to
invoke some code and then test all the postconditions. But this view
is not TDD, and doesn't buy you all of the benefits of TDD. I contend
that central to TDD is this notion of working in the smallest steps
possible, both for the finest-grained long-term verification, and for
the most flexible design evolution. Furthermore, this is best done by
striving to keep tests as small, focused and simple as possible.
Aiming for one assertion per test is one way to get there.
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