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Project: RSpec Core 3.1

Publisher: RSpec

Randomization can be reproduced across test runs

In Ruby, randomness is seeded by calling srand and passing it the seed that
you want to use. By doing this, subsequent calls to rand, shuffle,
sample, etc. will all be randomized the same way given the same seed is
passed to srand.

RSpec takes care not to seed randomization directly when taking action that
involves randomness (such as random ordering of examples).

Since RSpec does not ever invoke srand, this means that you are free to
choose which, if any, mechanism is used to seed randomization.

There is an example below of how to use RSpec's seed for this purpose if you
wish to do so.

If you would like to manage seeding randomization without any help from RSpec,
please keep the following things in mind:

  • The seed should never be hard-coded.

    The first example below only does this to show that seeding randomization
    with a seed other than the one used by RSpec will correctly seed
    randomization.

  • Report the seed that was chosen.

    The randomization that was used for a given test run can not be reproduced
    if no one knows what seed was used to begin with.

  • Provide a mechanism to feed the seed into the tests.

    Without this, the call to srand will have to be hard-coded any time it
    is necessary to replicate a given test run's randomness.

Background
Given
a file named "spec/random_spec.rb" with: 
require 'spec_helper'

RSpec.describe 'randomized example' do
  it 'prints random numbers' do
    puts 5.times.map { rand(99) }.join("-")
  end
end
Scenarios
Specifying a seed using `srand` provides predictable randomization
Given
a file named "spec/spec_helper.rb" with: 
srand 123
When
I run rspec
Then
the output should contain "66-92-98-17-83"
Passing the RSpec seed to `srand` provides predictable randomization
Given
a file named "spec/spec_helper.rb" with: 
srand RSpec.configuration.seed
When
I run rspec --seed 123
Then
the output should contain "66-92-98-17-83"

Last published about 7 years ago by myronmarston.