Minitest::Substitute

Simple Minitest helper to replace values such as an instance variable of an object or an environment variable for the duration of a block or a group of tests.

This comes in very handy when you have to deviate from default configuration in order to test some aspects of your code.

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• Author: Sven Schwyn - Bitcetera

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This gem is cryptographically signed in order to assure it hasn't been tampered with. Unless already done, please add the author's public key as a trusted certificate now:

gem cert --add <(curl -Ls https://bitcetera.com/downloads/gem-public_cert.pem)

Add the following to the Gemfile or gems.rb of your Bundler powered Ruby project:

gem 'minitest-substitute'

And then install the bundle:

bundle install --trust-policy MediumSecurity

Finally, require this gem in your test_helper.rb or spec_helper.rb:

require 'minitest/substitute'

Rails 7 has polluted Object for everybody by introducing Object#with. To prevent collisions, Minitest::Substitute has switched from with to substitute as of version 1.0.0.

After having updated this gem, you'll have to adapt all your tests accordingly:

# Version 0.x.x
with '@version', 2, on: config do
  config.instance_variable_get('@version')   # => 2
end

# Version 1.x.x
substitute '@version', 2, on: config do
  config.instance_variable_get('@version')   # => 2
end

To substitute the value of an instance variable for the duration of a block:

class Config
  def initialize
    @version = 1
  end
end

config = Config.new

config.instance_variable_get('@version')     # => 1
substitute '@version', 2, on: config do
  config.instance_variable_get('@version')   # => 2
end
config.instance_variable_get('@version')     # => 1

⚠️ The target on is set explicitly in this case. If you omit this argument, self will be used as target by default.

Class variables can be substituted as well:

class Config
  @@counter = 0
end

Config.class_variable_get('@@counter')     # => 0
substitute '@@counter', 42, on: Config do
  Config.class_variable_get('@@counter')   # => 42
end
Config.class_variable_get('@@counter')     # => 0

Same goes for global variables:

$verbose = false   # => false
substitute '$verbose', true do
  $verbose         # => true
end
$verbose           # => false

And it works for globals like ENV as well which comes in handy when you have to temporarily override the value of an environment variable:

ENV['EDITOR']     # => 'vi'
substitute "ENV['EDITOR']", 'nano' do
  ENV['EDITOR']   # => 'nano'
end
ENV['EDITOR']     # => 'vi'

You can even substitute constants, however, you have to use their absolute name starting with :::

module Animals
  DOG_MAKES = 'woof'
  CAT_MAKES = 'meow'
end

Animals::DOG_MAKES     # => 'woof'
substitute '::Animals::DOG_MAKES', Animals::CAT_MAKES do
  Animals::DOG_MAKES   # => 'meow'
end
Animals::DOG_MAKES     # => 'woof'

Remember that class declarations are assigned to constants as well:

class Dog
  self.makes
    'woof'
  end
end

class Cat
  self.makes
    'meow'
  end
end

Dog.makes     # => 'woof'
substitute '::Dog', Cat do
  Dog.makes   # => 'meow'
end
Dog.makes     # => 'woof'

It's safe to nest multiple substitute statements.

When using spec notation, you can change a value for all tests within a describe group:

class Config
  def initialize
    @version = 1
  end
end

describe Config do
  subject do
    Config.new
  end

  describe 'original version' do
    it "returns the original version" do
      _(subject.instance_variable_get('@version')).must_equal 1
    end
  end

  describe 'sustituted version' do
    substitute '@version', 2, on: Config

    it "returns the substituted version" do
      _(subject.instance_variable_get('@version')).must_equal 2
    end
  end
end

⚠️ The target on is set explicitly in this case. If you omit this argument, :subject will be used as target by default which refers to the subject defined by the subject {} helper.

Alternatively, you can pass the substitution value as a block. This block will be evaluated once in the context of the test, in other words, you can use assignments done with let inside the block:

class Config
  def initialize
    @version = 1
  end
end

describe Config do
  subject do
    Config.new
  end

  let :version do
    2
  end

  describe 'original version' do
    it "returns the original version" do
      _(subject.instance_variable_get('@version')).must_equal 1
    end
  end

  describe 'sustituted version' do
    substitute '@version', on: Config do
      version   # set using "let" above
    end

    it "returns the substituted version" do
      _(subject.instance_variable_get('@version')).must_equal 2
    end
  end
end

If both a substitution value and a substitution block are present, the latter takes precedence.

It's safe to use multiple substitute statements within one describe block.

(The spec integration is borrowed from minitest-around for elegance and compatibility.)

To install the development dependencies and then run the test suite:

bundle install
bundle exec rake    # run tests once
bundle exec guard   # run tests whenever files are modified

You're welcome to submit issues and contribute code by forking the project and submitting pull requests.

The gem is available as open source under the terms of the MIT License.