http://betterspecs.org/#subject has some info about subject
and let
. However, I am still unclear on the difference between them. Furthermore, the SO post What is the argument against using before, let and subject in RSpec tests? said it is better to not use either subject
or let
. Where shall I go? I am so confused.
let generates a method whose return value is memoized after the first call. This is known as lazy loading because the value is not loaded into memory until the method is called. Here is an example of how let is used within an RSpec test. let will generate a method called thing which returns a new instance of Thing .
Don't use should Do not use should when describing your tests. Use the third person in the present tense. Even better start using the new expectation syntax. See the should_not gem for a way to enforce this in RSpec and the should_clean gem for a way to clean up existing RSpec examples that begin with 'should.
RSpec lets you declare an “implicit subject” using `subject { … }` which allows for tests like `it { should be_valid }`. If you need to reference your test subject you should explicitly name it using `subject(:your_subject_name) { … }`.
Summary: RSpec's subject is a special variable that refers to the object being tested. Expectations can be set on it implicitly, which supports one-line examples. It is clear to the reader in some idiomatic cases, but is otherwise hard to understand and should be avoided. RSpec's let
variables are just lazily instantiated (memoized) variables. They aren't as hard to follow as the subject, but can still lead to tangled tests so should be used with discretion.
The subject is the object being tested. RSpec has an explicit idea of the subject. It may or may not be defined. If it is, RSpec can call methods on it without referring to it explicitly.
By default, if the first argument to an outermost example group (describe
or context
block) is a class, RSpec creates an instance of that class and assigns it to the subject. For example, the following passes:
class A
end
describe A do
it "is instantiated by RSpec" do
expect(subject).to be_an(A)
end
end
You can define the subject yourself with subject
:
describe "anonymous subject" do
subject { A.new }
it "has been instantiated" do
expect(subject).to be_an(A)
end
end
You can give the subject a name when you define it:
describe "named subject" do
subject(:a) { A.new }
it "has been instantiated" do
expect(a).to be_an(A)
end
end
Even if you name the subject, you can still refer to it anonymously:
describe "named subject" do
subject(:a) { A.new }
it "has been instantiated" do
expect(subject).to be_an(A)
end
end
You can define more than one named subject. The most recently defined named subject is the anonymous subject
.
However the subject is defined,
It's instantiated lazily. That is, the implicit instantiation of the described class or the execution of the block passed to subject
doesn't happen until subject
or the named subject is referred to in an example. If you want your explict subject to be instantiated eagerly (before an example in its group runs), say subject!
instead of subject
.
Expectations can be set on it implicitly (without writing subject
or the name of a named subject):
describe A do
it { is_expected.to be_an(A) }
end
The subject exists to support this one-line syntax.
An implicit subject
(inferred from the example group) is hard to understand because
is_expected
without an explicit receiver) or explicitly (as subject
), it gives the reader no information about the role or nature of the object on which the expectation is being called.it
in the normal example syntax), so the only information the reader has about the purpose of the example is the expectation itself.Therefore, it's only helpful to use an implicit subject when the context is likely to be well understood by all readers and there is really no need for an example description. The canonical case is testing ActiveRecord validations with shoulda matchers:
describe Article do
it { is_expected.to validate_presence_of(:title) }
end
An explict anonymous subject
(defined with subject
without a name) is a little better, because the reader can see how it's instantiated, but
A named subject provides an intention-revealing name, but the only reason to use a named subject instead of a let
variable is if you want to use the anonymous subject some of the time, and we just explained why the anonymous subject is hard to understand.
So, legitimate uses of an explicit anonymous subject
or a named subject are very rare.
let
variableslet
variables are just like named subjects except for two differences:
let
/let!
instead of subject
/subject!
subject
or allow expectations to be called on it implicitly.It's completely legitimate to use let
to reduce duplication among examples. However, do so only when it doesn't sacrifice test clarity. The safest time to use let
is when the let
variable's purpose is completely clear from its name (so that the reader doesn't have to find the definition, which could be many lines away, to understand each example) and it is used in the same way in every example. If either of those things isn't true, consider defining the object in a plain old local variable or calling a factory method right in the example.
let!
is risky, because it's not lazy. If someone adds an example to the example group that contains the let!
, but the example doesn't need the let!
variable,
let!
variable and wonder whether and how it affects the examplelet!
variablleSo use let!
, if at all, only in small, simple example groups where it's less likely that future example writers will fall into that trap.
There is a common overuse of subjects or let
variables that's worth discussing separately. Some people like to use them like this:
describe 'Calculator' do
describe '#calculate' do
subject { Calculator.calculate }
it { is_expected.to be >= 0 }
it { is_expected.to be <= 9 }
end
end
(This is a simple example of a method that returns a number for which we need two expectations, but this style can have many more examples/expectations if the method returns a more complicated value that needs many expectations and/or has many side effects that all need expectations.)
People do this because they've heard that one should have only one expectation per example (which is mixed up with the valid rule that one should only test one method call per example) or because they're in love with RSpec trickiness. Don't do it, whether with an anonymous or named subject or a let
variable! This style has several problems:
Instead, write a single example:
describe 'Calculator' do
describe '#calculate' do
it "returns a single-digit number" do
result = Calculator.calculate
expect(result).to be >= 0
expect(result).to be <= 9
end
end
end
Subject
and let
are just tools to help you tidy up and speed up your tests. People in the rspec community do use them so i wouldn't worry about whether it's ok to use them or not. They can be used similarly but serve slightly different purposes
Subject
allows you to declare a test subject, and then reuse it for any number of following test cases afterward. This reduces code repetition (DRYing up your code)
Let
is an alternative to before: each
blocks, which assign test data to instance variables. Let
gives you a couple of advantages. First, it caches the value without assigning it to an instance variable. Second, it is lazily evaluated, which means that it doesn't get evaluated until a spec calls for it. Thus let
helps you speed up your tests. I also think let
is easier to read
subject
is what is under test, usually an instance or a class. let
is for assigning variables in your tests, which are evaluated lazily vs. using instance variables. There are some nice examples in this thread.
https://github.com/reachlocal/rspec-style-guide/issues/6
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