In the fine tradition of Hollywood (who these days is remaking everything from old movies, comic books, and video games in lieu of coming up with original ideas) I am doing a remake of the very first post on my blog (which itself was a repost from my Artima.com blog from Feb 2004). The original post created quite a stir at the time, and lead to many of the ideas that have evolved my understanding of TDD and now BDD.
Once upon a time (early 2004) 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 (in Java but you get the idea):
a = CreateObject("Address")
a.GetAddressParts("ADDR1$ADDR2$CITY IL 60563$COUNTRY")
AssertEquals("ADDR1", a.Addr1)
AssertEquals("ADDR2", a.Addr2)
AssertEquals("CITY IL 60563", a.CityStatePostalCd)
AssertEquals("Country", a.Country)
They didn’t see how to achieve this with one assertion per test, as there are obviously four things to test in this case. I decided that rather than simply reply, I would write some tests and code to illustrate my view on the matter, and offer a solid response.
For the original post, I chose Squeak Smalltalk and Java. In this remake I’m going to use just Ruby. In the original I said “For the sake of conciseness, I’ll omit any required accessors.” Well.. this time around I’m not going to use any accessors, since I am more & more of the opinion that they are quite simply an idea whose sole purpose is to undermine the very essence of Object-Oriented software. I’ll also be using rSpec rather than an xUnit framework.
So, where to start? Well, it often makes sense to start with something simple to quickly and easily get some code written and working. Then it can be extended and evolved. Here the simplest case is: ADDR1$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 examples.
If we want to avoid accessors (and for this exercise, I do) we need another way to see inside the object were working with. This is an address.. let’s say (for the sake of argument & this exercise) that one of the purposes of having an address is to generate address labels of some sort. We could do that with an address label maker object that would interrogate the address object for it’s contents and use them. A better way is to have an address label builder that the address object gives information to. It would have methods such as street_number=, csp=, etc. This is something that we can use Mocks Objects for in our examples.
So, let’s start with an example for ADDR1:
context "An address with street & csp" do
specify "should capture street information" do
builder = mock("builder", null_object => true)
builder.should_receive(:addr1=).once.with("ADDR1")
addr = Address.from_string("ADDR1$CITY IL 60563")
addr.use(builder)
end
end
To get this to pass we need an Address class, a from_string factory method, and a use method.
class Address
def self.from_string(address_string)
Address.new
end
def use(a_builder)
end
end
This lets our spec run and fail:
An address with street & csp
- should capture street information (FAILED - 1)
1)
Spec::Api::MockExpectationError in 'An address with street & csp should capture street information'
Mock 'builder' expected 'street=' once, but received it 0 times
./address_spec.rb:7:in `'
Finished in 0.003395 seconds
1 specification, 1 failure
Now let’s make that work:
class Address
def self.from_string(address_string)
addr1, rest = address_string.split('$')
Address.new(addr1)
end
def initialize(addr1)
@addr1 = addr1
end
def use(a_builder)
a_builder.addr1 = @addr1
end
end
This makes the example work:
An address with street & csp
- should capture street information
Finished in 0.003003 seconds
1 specification, 0 failures
That’s well & good. The next test is for CSP.
specify "should capture csp information" do
builder = mock("builder", :null_object => true)
builder.should_receive(:csp=).once.with("CITY IL 60563")
addr = Address.from_string("ADDR1$CITY IL 60563")
addr.use(builder)
end
Resulting in:
An address with street & csp
- should capture street information
- should capture street information (FAILED - 1)
1)
Spec::Api::MockExpectationError in 'An address with street & csp should capture street information'
Mock 'builder' expected 'csp=' once, but received it 0 times
./address_spec.rb:15:in `'
Finished in 0.005866 seconds
2 specifications, 1 failure
Address#from_string will need to be extended (and we need to add csp support):
class Address
def self.from_string(address_string)
addr1, csp, rest = address_string.split('$')
Address.new(addr1, csp)
end
def initialize(addr1, csp)
@addr1 = addr1
@csp = csp
end
def use(a_builder)
a_builder.addr1 = @addr1
a_builder.csp = @csp
end
end
So. We have two tests for this one situation. Notice the duplication in the tests… the creation of builder & address. This is the context. After refactoring, we have:
require 'address'
context "An address with street & csp" do
setup do
@builder = mock("builder", :null_object => true)
@addr = Address.from_string("ADDR1$CITY IL 60563")
end
specify "should capture street information" do
@builder.should_receive(:addr1=).once.with("ADDR1")
@addr.use(@builder)
end
specify "should capture csp information" do
@builder.should_receive(:csp=).once.with("CITY IL 60563")
@addr.use(@builder)
end
end
So, a context that creates the Address instance from the string as well as the mock, and very simple examples that focus on each aspect of that context.
Note that we don’t have any direct expectations in these examples… What’s up? Well, the mock is autoverifying itself at the end of each example, and that is where our expectations are.. on @address’s interaction with the mock.
The next simplest case is the obvious choice for the next context:
context "An address with street, csp, and country" do
setup do
@builder = mock("builder", :null_object => true)
@addr = Address.from_string("ADDR1$CITY IL 60563$COUNTRY")
end
end
This set of tests will include ones for addr1 and csp as before (since that behaviour is required in this new context) as well as a new test for country:
specify "should capture country information" do
@builder.should_receive(:country=).once.with("COUNTRY")
@addr.use(@builder)
end
As before, an instance variable and such need to be added to the Address class.
This drives Address to evolve:
class Address
def self.from_string(address_string)
addr1, csp, country, rest = address_string.split('$')
Address.new(addr1, csp, country)
end
def initialize(addr1, csp, country)
@addr1 = addr1
@csp = csp
@country = country
end
def use(a_builder)
a_builder.addr1 = @addr1
a_builder.csp = @csp
a_builder.country = @country
end
end
Now that we are supporting country, we can go back to the first context and add an example specifying that no country information should be provided to the builder:
specify "should not capture country information" do
@builder.should_not_receive(:country=)
@addr.use(@builder)
end
A slight tweak to Address#use will make this work:
def use(a_builder)
a_builder.addr1 = @addr1
a_builder.csp = @csp
a_builder.country = @country unless @country.nil?
end
From here on, the evolution gets a bit more complex, as we add the ADDR2 option to the mix.
The final code and output is:
address_spec.rb
require 'address'
context "An address with street & csp" do
setup do
@builder = mock("builder", :null_object => true)
@addr = Address.from_string("ADDR1$CITY IL 60563")
end
specify "should capture street information" do
@builder.should_receive(:addr1=).once.with("ADDR1")
@addr.use(@builder)
end
specify "should capture csp information" do
@builder.should_receive(:csp=).once.with("CITY IL 60563")
@addr.use(@builder)
end
specify "should not capture country information" do
@builder.should_not_receive(:country=)
@addr.use(@builder)
end
end
context "An address with street, csp, and country" do
setup do
@builder = mock("builder", :null_object => true)
@addr = Address.from_string("ADDR1$CITY IL 60563$COUNTRY")
end
specify "should capture street information" do
@builder.should_receive(:addr1=).once.with("ADDR1")
@addr.use(@builder)
end
specify "should capture csp information" do
@builder.should_receive(:csp=).once.with("CITY IL 60563")
@addr.use(@builder)
end
specify "should capture country information" do
@builder.should_receive(:country=).once.with("COUNTRY")
@addr.use(@builder)
end
end
address.rb
class Address
def self.from_string(address_string)
addr1, csp, country, rest = address_string.split('$')
Address.new(addr1, csp, country)
end
def initialize(addr1, csp, country)
@addr1 = addr1
@csp = csp
@country = country
end
def use(a_builder)
a_builder.addr1 = @addr1
a_builder.csp = @csp
a_builder.country = @country unless @country.nil?
end
end
spec output
An address with street & csp
- should capture street information
- should capture csp information
- should not capture country information
An address with street, csp, and country
- should capture street information
- should capture csp information
- should capture country information
Finished in 0.014 seconds
6 specifications, 0 failures
Conclusion
So we took a situation that was thought to require multiple assertions/expectations in a test/example and did it in such as way as to have only one per.
The key is that instead of using a single context with a complex (i.e. multiple expectation) example for each situation, we made each of those situations into a separate context. Now each example focuses on a very small, specific aspect of the behaviour dealt with by its context.
I’m convinced writing examples like this is a useful approach. One advantage is that the resulting examples are 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 context centered approach that is the recommended way to organize your examples. We set up the object being worked on in setup, and wrote examples of it’s behaviour in that particular context in individual specify clauses.
As I was writing this back in early 2004, something clicked. I saw these test methods (as they are in jUnit/test::unit) as specifications of tiny facets of the required behavior. Thus, it made 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. This epiphany was one of the main drivers in my deepening understanding of what has been called Test Driven Development (and which we are now considering an aspect of Behaviour Driven Development)
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 certainly not BDD), and doesn’t buy you all of the benefits that are possible. 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/examples as small, focused and simple as possible.
Aiming for one expectation each is one way to get there.
Tags: bdd code rspec ruby tdd
