Skip to content
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
244 lines (177 sloc) 7.73 KB

Contributions are much appreciated. Please open a pull request or add an issue to discuss what you intend to work on. If the pull requests passes the CI and conforms to the existing style of specs, it will be merged.

File organization

Spec are grouped in 5 separate top-level groups:

  • command_line: for the ruby executable command-line flags (-v, -e, etc)
  • language: for the language keywords and syntax constructs (if, def, A::B, etc)
  • core: for the core methods (Fixnum#+, String#upcase, no need to require anything)
  • library: for the standard libraries methods (, YAML.parse, need to require the stdlib)
  • optional/capi: for functions available to the Ruby C-extension API

The exact file for methods is decided by the #owner of a method, for instance for #group_by:

> [].method(:group_by)
=> #<Method: Array(Enumerable)#group_by>
> [].method(:group_by).owner
=> Enumerable

Which should therefore be specified in core/enumerable/group_by_spec.rb.

MkSpec - a tool to generate the spec structure

If you want to create new specs, you should use mkspec, part of MSpec.

$ ../mspec/bin/mkspec -h

Creating files for unspecified modules or classes

For instance, to create specs for forwardable:

$ ../mspec/bin/mkspec -b library -rforwardable -c Forwardable

Specify core or library as the base.

Finding unspecified core methods

This is very easy, just run the command below in your spec directory. ruby must be a recent version of MRI.

$ ruby --disable-gem ../mspec/bin/mkspec

You might also want to search for:

it "needs to be reviewed for spec completeness"

which indicates the file was generated but the method unspecified.

Matchers and expectations

Here is a list of frequently-used matchers, which should be enough for most specs. There are a few extra specific matchers used in the couple specs that need it.

(1 + 2).should == 3 # Calls #==
(1 + 2).should_not == 5

File.should equal(File) # Calls #equal? (tests identity)
(1 + 2).should eql(3) # Calls #eql? (Hash equality)

1.should < 2
2.should <= 2
3.should >= 3
4.should > 3

"Hello".should =~ /l{2}/ # Calls #=~ (Regexp match)

[].should be_empty # Calls #empty?
[1,2,3].should include(2) # Calls #include?

(0.1 + 0.2).should be_close(0.3, TOLERANCE) # (0.2-0.1).abs < TOLERANCE
(0.0/0.0).should be_nan # Calls Float#nan?
(1.0/0.0).should be_positive_infinity
(-1.0/0.0).should be_negative_infinity

3.14.should be_an_instance_of(Float) # Calls #instance_of?
3.14.should be_kind_of(Numeric) # Calls #is_a?
Numeric.should be_ancestor_of(Float) # Float.ancestors.include?(Numeric)

3.14.should respond_to(:to_i) # Calls #respond_to?
Fixnum.should have_instance_method(:+)
Array.should have_method(:new)
# Also have_constant, have_private_instance_method, have_singleton_method, etc

-> {
  raise "oops"
}.should raise_error(RuntimeError, /oops/)

# To avoid! Instead, use an expectation testing what the code in the lambda does.
# If an exception is raised, it will fail the example anyway.
-> { ... }.should_not raise_error

-> {
}.should complain(/constant ::Fixnum is deprecated/) # Expect a warning


Different guards are available as defined by mspec. Here is a list of the most commonly-used guards:

ruby_version_is ""..."2.4" do
  # Specs for RUBY_VERSION < 2.4

ruby_version_is "2.4" do
  # Specs for RUBY_VERSION >= 2.4

platform_is :windows do
  # Specs only valid on Windows

platform_is_not :windows do
  # Specs valid on platforms other than Windows

platform_is :linux, :darwin do # OR

platform_is_not :linux, :darwin do # Not Linux and not Darwin

platform_is wordsize: 64 do
  # 64-bit platform

big_endian do
  # Big-endian platform

# In case there is a bug in MRI but the expected behavior is obvious
# First file a bug at
# It is better to use a ruby_version_is guard if there was a release with the fix
ruby_bug '#13669', ''...'2.5' do
  it "works like this" do
    # Specify the expected behavior here, not the bug

# Combining guards
guard -> { platform_is :windows and ruby_version_is ""..."2.5" } do
  # Windows and RUBY_VERSION < 2.5

guard_not -> { platform_is :windows and ruby_version_is ""..."2.5" } do
  # The opposite

# Custom guard
max_uint = (1 << 32) - 1
guard -> { max_uint <= fixnum_max } do

Custom guards are better than a simple if as they allow mspec commands to work properly.

In general, the usage of guards should be minimized as possible.

There are no guards to define implementation-specific behavior because the Ruby Spec Suite defines common behavior and not implementation details. Use the implementation test suite for these.

If an implementation does not support some feature, simply tag the related specs as failing instead.

Shared Specs

Often throughout Ruby, identical functionality is used by different methods and modules. In order to avoid duplication of specs, we have shared specs that are re-used in other specs. The use is a bit tricky however, so let's go over it.

Commonly, if a shared spec is only reused within its own module, the shared spec will live within a shared directory inside that module's directory. For example, the core/hash/shared/key.rb spec is only used by Hash specs, and so it lives inside core/hash/shared/.

When a shared spec is used across multiple modules or classes, it lives within the shared/ directory. An example of this is the shared/file/socket.rb which is used by core/file/socket_spec.rb, core/filetest/socket_spec.rb, and core/file/state/socket_spec.rb and so it lives in the root shared/.

Defining a shared spec involves adding a shared: true option to the top-level describe block. This will signal not to run the specs directly by the runner. Shared specs have access to two instance variables from the implementor spec: @method and @object, which the implementor spec will pass in.

Here's an example of a snippet of a shared spec and two specs which integrates it:

# core/hash/shared/key.rb
describe :hash_key_p, shared: true do
  it "returns true if the key's matching value was false" do
    { xyz: false }.send(@method, :xyz).should == true

# core/hash/key_spec.rb
describe "Hash#key?" do
  it_behaves_like :hash_key_p, :key?

# core/hash/include_spec.rb
describe "Hash#include?" do
  it_behaves_like :hash_key_p, :include?

In the example, the first describe defines the shared spec :hash_key_p, which defines a spec that calls the @method method with an expectation. In the implementor spec, we use it_behaves_like to integrate the shared spec. it_behaves_like takes 3 parameters: the key of the shared spec, a method, and an object. These last two parameters are accessible via @method and @object in the shared spec.

Sometimes, shared specs require more context from the implementor class than a simple object. We can address this by passing a lambda as the method, which will have the scope of the implementor. Here's an example of how this is used currently:

describe :kernel_sprintf, shared: true do
  it "raises TypeError exception if cannot convert to Integer" do
    -> {"%b", }.should raise_error(TypeError)

describe "Kernel#sprintf" do
  it_behaves_like :kernel_sprintf, -> (format, *args) {
    sprintf(format, *args)

describe "Kernel.sprintf" do
  it_behaves_like :kernel_sprintf, -> (format, *args) {
    Kernel.sprintf(format, *args)

In the above example, the method being passed is a lambda that triggers the specific conditions of the shared spec.


Do not leave any trailing space and follow the existing style.

You can’t perform that action at this time.