Scope constants to an rspec context

Question

I often want to do

context "empty stack" do
  SOME_CONSTANT = "value"
  it "should be empty" do
    # use SOME_CONSTANT
  end
end

context "populated stack" do
  SOME_CONSTANT = "a different value"
  it "should have some items" do
    # use SOME_CONSTANT
  end
end

ruby doesn't scope constants to closures so they leak out. Does anyone have a trick for declaring constants that are scoped to a context?

Bodkin

Answer 1

Change the declaration of the constant:
from SOME_CONSTANT = "value"
to self::SOME_CONSTANT = "value"

RSpec creates an anonymous class for each set of specs (context in your example) that it comes across. Declaring a constant without self:: in an anonymous class makes it available in global scope, and is visible to all the specs. Changing the constant declaration to self:: ensures that it is visible only within the anonymous class.

Swoosh

To use those constants inside an it block do: self.class::SOME_CONSTANT – Grapery

Answer 2

Having used rspec for longer now I think the more idiomatic approach is to use let.

context "empty stack" do
  let(:some_constant){ "value" }

  it "should be empty" do
    puts some_constant
  end
end

context "populated stack" do
  let(:some_constant){ "a different value" }

  it "should have some items" do
    puts some_constant
  end
end

Bodkin

I totally recommend using the let constructs. Read more about let here: relishapp.com/rspec/rspec-core/docs/helper-methods/let-and-let – Gurl

I want to use let but it's not available in the before blocks. – Messere

That's not true. Definitions with let are available in before, before(:each) and before(:suite). They only don't work with before(:all). – Bernie

before(:all) exact the case why I am here :) – Whitmer

Answer 3

The correct way to do it is to use stub_const

RSpec.describe "stubbing FOO" do
  it "can stub undefined constant FOO" do
    stub_const("FOO", 5)
    expect(FOO).to eq(5)
  end

  it "undefines the constant when the example completes" do
    expect { FOO }.to raise_error(NameError)
  end
end

https://relishapp.com/rspec/rspec-mocks/v/3-9/docs/mutating-constants/stub-undefined-constant

Agueda