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Project: RSpec Mocks 2.14

Mutating constants

Mutating Constants


Support is provided for stubbing constants. Like with method stubs, the
stubbed constants will be restored to their original state when an
example completes.

stub_const("Foo", fake_foo)
Foo # => fake_foo

Stubbed constant names must be fully qualified; the current module
nesting is not considered.

module MyGem
  class SomeClass; end

module MyGem
  describe "Something" do
    let(:fake_class) { Class.new }

    it "accidentally stubs the wrong constant" do
      # this stubs ::SomeClass (in the top-level namespace),
      # not MyGem::SomeClass like you probably mean.
      stub_const("SomeClass", fake_class)

    it "stubs the right constant" do
      stub_const("MyGem::SomeClass", fake_class)

When you stub a constant that is a module or a class, nested
constants on the original module or class are not transferred
by default, but you can use the :transfer_nested_constants
option to tell rspec-mocks to transfer them:

class CardDeck
  SUITS = [:Spades, :Diamonds, :Clubs, :Hearts]
  NUM_CARDS = 52

fake_class = Class.new
stub_const("CardDeck", fake_class)
CardDeck # => fake_class
CardDeck::SUITS # => raises uninitialized constant error
CardDeck::NUM_CARDS # => raises uninitialized constant error

stub_const("CardDeck", fake_class, :transfer_nested_constants => true)
CardDeck::SUITS # => [:Spades, :Diamonds, :Clubs, :Hearts]
CardDeck::NUM_CARDS # => 52

stub_const("CardDeck", fake_class, :transfer_nested_constants => [:SUITS])
CardDeck::SUITS # => [:Spades, :Diamonds, :Clubs, :Hearts]
CardDeck::NUM_CARDS # => raises uninitialized constant error


Support is also provided for hiding constants. Hiding a constant temporarily
removes it; it is restored to its original value after the test completes.

FOO = 42
FOO => NameError: uninitialized constant FOO

Like stubbed constants, names must be fully qualified.

Hiding constants that are already undefined has no effect.



  1. Stub Defined Constant
  2. Stub Undefined Constant
  3. Hide Defined Constant

Last published over 7 years ago by myronmarston.