What's the difference between RSpec's subject and let? When should they be used or not?

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

How it works

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.

When to use it

An implicit subject (inferred from the example group) is hard to understand because

It's instantiated behind the scenes.

Whether it's used implicitly (by calling 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.

The one-liner example syntax doesn't have an example description (the string argument to 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

it can still put the instantiation of the subject far from where it's used (e.g. at the top of an example group with many examples that use it), which is still hard to follow, and

it has the other problems that the implicit subject does.

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 variables

How they work

let variables are just like named subjects except for two differences:

they're defined with let/let! instead of subject/subject!

they do not set the anonymous subject or allow expectations to be called on it implicitly.

When to use them

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,

that example will be hard to understand, because the reader will see the let! variable and wonder whether and how it affects the example

the example will be slower than it needs to be, because of the time taken to create the let! variablle

So 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.

The single-expectation-per-example fetish

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:

The anonymous subject isn't the subject of the examples — the method is the subject. Writing the test this way screws up the language, making it harder to think about.

As always with one-line examples, there isn't any room to explain the meaning of the expectations.

The subject has to be constructed for each example, which is slow.

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

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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

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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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