Lookout is a unit testing framework for Ruby that puts your results in focus. Tests (expectations) are written as follows

expect 2 do
  1 + 1

expect ArgumentError do
  Integer('1 + 1')

expect Array do
  [1, 2, 3].select{ |i| i % 2 == 0 }

expect [2, 4, 6] do
  [1, 2, 3].map{ |i| i * 2 }

Lookout is designed to encourage – force, even – unit testing best practices such as

This is done by

Other important points are

The way Lookout works has been heavily influenced by expectations, by Jay Fields. The code base was once also heavily based on expectations, based at Subversion revision 76. A lot has happened since then and all of the work past that revision are due to Nikolai Weibull.


Install Lookout with

% gem install lookout


Lookout allows you to set expectations on an object’s state or behavior. We’ll begin by looking at state expectations and then take a look at expectations on behavior.

Expectations on State: Literals

An expectation can be made on the result of a computation:

expect 2 do
  1 + 1

Most objects, in fact, have their state expectations checked by invoking #== on the expected value with the result as its argument.

Checking that a result is within a given range is also simple:

expect 0.099..0.101 do
  0.4 - 0.3

Here, the more general #=== is being used on the Range.


Strings of course match against Strings:

expect 'ab' do

but we can also match a String against a Regexp:

expect %r{a substring} do
  'a string with a substring'

(Note the use of %r{…} to avoid warnings that will be generated when Ruby parses expect /…/.)


Checking that the result includes a certain module is done by expecting the Module.

expect Enumerable do

This, due to the nature of Ruby, of course also works for classes (as they are also modules):

expect String do
  'a string'

This doesn’t hinder us from expecting the actual Module itself:

expect Enumerable do

or the Class:

expect String do

for obvious reasons.

As you may have figured out yourself, this is accomplished by first trying #== and, if it returns false, then trying #=== on the expected Module. This is also true of Ranges and Regexps.


Truthfulness is expected with true and false:

expect true do

expect false do

Results equaling true or false are slightly different:

expect TrueClass do

expect FalseClass do

The rationale for this is that you should only care if the result of a computation evaluates to a value that Ruby considers to be either true or false, not the exact literals true or false.


Expecting output on an IO object is also common:

expect output("abc\ndef\n") do |io|
  io.puts 'abc', 'def'

This can be used to capture the output of a formatter that takes an output object as a parameter.


Expecting warnings from code isn’t very common, but should be done:

expect warning('this is your final one!') do
  warn 'this is your final one!'

expect warning('this is your final one!') do
  warn '%s:%d: warning: this is your final one!' % [__FILE__, __LINE__]

$VERBOSE is set to true during the execution of the block, so you don’t need to do so yourself. If you have other code that depends on the value of $VERBOSE, that can be done with #with_verbose

expect nil do
  with_verbose nil do


You should always be expecting errors from – and in, but that’s a different story – your code:

expect ArgumentError do
  Integer('1 + 1')

Often, not only the type of the error, but its description, is important to check:

expect StandardError.new('message') do
  raise StandardError.new('message')

As with Strings, Regexps can be used to check the error description:

expect StandardError.new(/mess/) do
  raise StandardError.new('message')

Queries Through Symbols

Symbols are generally matched against symbols, but as a special case, symbols ending with ? are seen as expectations on the result of query methods on the result of the block, given that the method is of zero arity and that the result isn’t a Symbol itself. Simply expect a symbol ending with ?:

expect :empty? do

To expect it’s negation, expect the same symbol beginning with not_:

expect :not_nil? do
  [1, 2, 3]

This is the same as

expect true do


expect false do
  [1, 2, 3].empty?

but provides much clearer failure messages. It also makes the expectation’s intent a lot clearer.

Queries By Proxy

There’s also a way to make the expectations of query methods explicit by invoking methods on the result of the block. For example, to check that the even elements of the Array [1, 2, 3] include 1 you could write

expect result.to.include? 1 do
  [1, 2, 3].reject{ |e| e.even? }

You could likewise check that the result doesn’t include 2:

expect result.not.to.include? 2 do
  [1, 2, 3].reject{ |e| e.even? }

This is the same as (and executes a little bit slower than) writing

expect false do
  [1, 2, 3].reject{ |e| e.even? }.include? 2

but provides much clearer failure messages. Given that these two last examples would fail, you’d get a message saying “[1, 2, 3]#include?(2)” instead of the terser “true≠false”. It also clearly separates the actual expectation from the set-up.

The keyword for this kind of expectations is result. This may be followed by any of the methods

  • #not

  • #to

  • #be

  • #have

or any other method you will want to call on the result. The methods #to, #be, and #have do nothing except improve readability. The #not method inverts the expectation.

Literal Literals

If you need to literally check against any of the types of objects otherwise treated specially, that is, any instances of

  • Module

  • Range

  • Regexp

  • Exception

  • Symbol, given that it ends with ?

you can do so by wrapping it in literal(…):

expect literal(:empty?) do

You almost never need to do this, as, for all but symbols, instances will match accordingly as well.

Expectations on Behavior

We expect our objects to be on their best behavior. Lookout allows you to make sure that they are.

Reception expectations let us verify that a method is called in the way that we expect it to be:

expect mock.to.receive.to_str(without_arguments){ '123' } do |o|

Here, #mock creates a mock object, an object that doesn’t respond to anything unless you tell it to. We tell it to expect to receive a call to #to_str without arguments and have #to_str return '123' when called. The mock object is then passed in to the block so that the expectations placed upon it can be fulfilled.

Sometimes we only want to make sure that a method is called in the way that we expect it to be, but we don’t care if any other methods are called on the object. A stub object, created with #stub, expects any method and returns a stub object that, again, expects any method, and thus fits the bill.

expect stub.to.receive.to_str(without_arguments){ '123' } do |o|
  o.to_str if o.convertable?

You don’t have to use a mock object to verify that a method is called:

expect Object.to.receive.name do

As you have figured out by now, the expected method call is set up by calling #receive after #to. #Receive is followed by a call to the method to expect with any expected arguments. The body of the expected method can be given as the block to the method. Finally, an expected invocation count may follow the method. Let’s look at this formal specification in more detail.

The expected method arguments may be given in a variety of ways. Let’s introduce them by giving some examples:

expect mock.to.receive.a do |m|

Here, the method #a must be called with any number of arguments. It may be called any number of times, but it must be called at least once.

If a method must receive exactly one argument, you can use Object, as the same matching rules apply for arguments as they do for state expectations:

expect mock.to.receive.a(Object) do |m|
  m.a 0

If a method must receive a specific argument, you can use that argument:

expect mock.to.receive.a(1..2) do |m|
  m.a 1

Again, the same matching rules apply for arguments as they do for state expectations, so the previous example expects a call to #a with 1, 2, or the Range 1..2 as an argument on m.

If a method must be invoked without any arguments you can use without_arguments:

expect mock.to.receive.a(without_arguments) do |m|

You can of course use both Object and actual arguments:

expect mock.to.receive.a(Object, 2, Object) do |m|
  m.a nil, 2, '3'

The body of the expected method may be given as the block. Here, calling #a on m will give the result 1:

expect mock.to.receive.a{ 1 } do |m|
  raise 'not 1' unless m.a == 1

If no body has been given, the result will be a stub object.

To take a block, grab a block parameter and #call it:

expect mock.to.receive.a{ |&b| b.call(1) } do |m|
  j = 0
  m.a{ |i| j = i }
  raise 'not 1' unless j == 1

To simulate an #each-like method, #call the block several times.

Invocation count expectations can be set if the default expectation of “at least once” isn’t good enough. The following expectations are possible

  • #at_most_once

  • #once

  • #at_least_once

  • #twice

And, for a given N,

  • #at_most(N)

  • #exactly(N)

  • #at_least(N)

Utilities: Stubs

Method stubs are another useful thing to have in a unit testing framework. Sometimes you need to override a method that does something a test shouldn’t do, like access and alter bank accounts. We can override – stub out – a method by using the #stub method. Let’s assume that we have an Account class that has two methods, #slips and #total. #Slips retrieves the bank slips that keep track of your deposits to the Account from a database. #Total sums the #slips. In the following test we want to make sure that #total does what it should do without accessing the database. We therefore stub out #slips and make it return something that we can easily control.

expect 6 do |m|
         def slips
           raise 'database not available'

         def total
           slips.reduce(0){ |m, n| m.to_i + n.to_i }
       }.new, :slips => [1, 2, 3]){ |account| account.total }

To make it easy to create objects with a set of stubbed methods there’s also a convenience method:

expect 3 do
  s = stub(:a => 1, :b => 2)
  s.a + s.b

This short-hand notation can also be used for the expected value:

expect stub(:a => 1, :b => 2).to.receive.a do |o|
  o.a + o.b

and also works for mock objects:

expect mock(:a => 2, :b => 2).to.receive.a do |o|
  o.a + o.b

Blocks are also allowed when defining stub methods:

expect 3 do
  s = stub(:a => proc{ |a, b| a + b })
  s.a(1, 2)

If need be, we can stub out a specific method on an object:

expect 'def' do
  stub('abc', :to_str => 'def'){ |a| a.to_str }

The stub is active during the execution of the block.

Overriding Constants

Sometimes you need to override the value of a constant during the execution of some code. Use #with_const to do just that:

expect 'hello' do
  with_const 'A::B::C', 'hello' do

Here, the constant A::B::C is set to 'hello' during the execution of the block. None of the constants A, B, and C need to exist for this to work. If a constant doesn’t exist it’s created and set to a new, empty, Module. The value of A::B::C, if any, is restored after the block returns and any constants that didn’t previously exist are removed.

Overriding Environment Variables

Another thing you often need to control in your tests is the value of environment variables. Depending on such global values is, of course, not a good practice, but is often unavoidable when working with external libraries. #With_env allows you to override the value of environment variables during the execution of a block by giving it a Hash of key/value pairs where the key is the name of the environment variable and the value is the value that it should have during the execution of that block:

expect 'hello' do
  with_env 'INTRO' => 'hello' do

Any overridden values are restored and any keys that weren’t previously a part of the environment are removed when the block returns.

Overriding Globals

You may also want to override the value of a global temporarily:

expect 'hello' do
  with_global :$stdout, StringIO.new do
    print 'hello'

You thus provide the name of the global and a value that it should take during the execution of a block of code. The block gets passed the overridden value, should you need it:

expect true do
  with_global :$stdout, StringIO.new do |overridden|
    $stdout != overridden


Lookout can be used from Rake. Simply install Lookout-Rake:

% gem install lookout-rake

and add the following code to your Rakefile

require 'lookout-rake-3.0'


Make sure to read up on using Lookout-Rake for further benefits and customization.


Lookout comes with an API that let’s you create things such as new expected values, difference reports for your types, and so on.

Interface Design

The default output of Lookout can Spartanly be described as Spartan. If no errors or failures occur, no output is generated. This is unconventional, as unit testing frameworks tend to dump a lot of information on the user, concerning things such as progress, test count summaries, and flamboyantly colored text telling you that your tests passed. None of this output is needed. Your tests should run fast enough to not require progress reports. The lack of output provides you with the same amount of information as reporting success. Test count summaries are only useful if you’re worried that your tests aren’t being run, but if you worry about that, then providing such output doesn’t really help. Testing your tests requires something beyond reporting some arbitrary count that you would have to verify by hand anyway.

When errors or failures do occur, however, the relevant information is output in a format that can easily be parsed by an 'errorformat' for Vim or with Compilation Mode for Emacs. Diffs are generated for Strings, Arrays, Hashes, and I/O.

External Design

Let’s now look at some of the points made in the introduction in greater detail.

Lookout only allows you to set one expectation per test. If you’re testing behavior with a reception expectation, then only one method-invocation expectation can be set. If you’re testing state, then only one result can be verified. It may seem like this would cause unnecessary duplication between tests. While this is certainly a possibility, when you actually begin to try to avoid such duplication you find that you often do so by improving your interfaces. This kind of restriction tends to encourage the use of value objects, which are easy to test, and more focused objects, which require simpler tests, as they have less behavior to test, per method. By keeping your interfaces focused you’re also keeping your tests focused.

Keeping your tests focused improves, in itself, test isolation, but let’s look at something that hinders it: setup and tear-down methods. Most unit testing frameworks encourage test fragmentation by providing setup and tear-down methods.

Setup methods create objects and, perhaps, just their behavior for a set of tests. This means that you have to look in two places to figure out what’s being done in a test. This may work fine for few methods with simple set-ups, but makes things complicated when the number of tests increases and the set-up is complex. Often, each test further adjusts the previously set-up object before performing any verifications, further complicating the process of figuring out what state an object has in a given test.

Tear-down methods clean up after tests, perhaps by removing records from a database or deleting files from the file-system.

The duplication that setup methods and tear-down methods hope to remove is better avoided by improving your interfaces. This can be done by providing better set-up methods for your objects and using idioms such as Resource Acquisition Is Initialization for guaranteed clean-up, test or no test.

By not using setup and tear-down methods we keep everything pertinent to a test in the test itself, thus improving test isolation. (You also won’t slow down your tests by keeping unnecessary state.)

Most unit test frameworks also allow you to create arbitrary test helper methods. Lookout doesn’t. The same rationale as that that has been crystallized in the preceding paragraphs applies. If you need helpers you’re interface isn’t good enough. It really is as simple as that.

To clarify: there’s nothing inherently wrong with test helper methods, but they should be general enough that they reside in their own library. The support for mocks in Lookout is provided through a set of test helper methods that make it easier to create mocks than it would have been without them. Lookout-Rack is another example of a library providing test helper methods (well, one method, actually) that are very useful in testing web applications that use Rack.

A final point at which some unit test frameworks try to fragment tests further is documentation. These frameworks provide ways of describing the whats and hows of what’s being tested, the rationale being that this will provide documentation of both the test and the code being tested. Describing how a stack data structure is meant to work is a common example. A stack is, however, a rather simple data structure, so such a description provides little, if any, additional information that can’t be extracted from the implementation and its tests themselves. The implementation and its tests is, in fact, its own best documentation. Taking the points made in the previous paragraphs into account, we should already have simple, self-describing, interfaces that have easily understood tests associated with them. Rationales for the use of a given data structure or system-design design documentation is better suited in separate documentation focused at describing exactly those issues.

Internal Design

The internal design of Lookout has had a couple of goals.

External Dependencies

Lookout used to depend on Mocha for mocks and stubs. While benchmarking I noticed that a method in Mocha was taking up more than 300 percent of the runtime. It turned out that Mocha’s method for cleaning up back-traces generated when a mock failed was doing something incredibly stupid:

backtrace.reject{ |l| Regexp.new(@lib).match(File.expand_path(l)) }

Here @lib is a String containing the path to the lib sub-directory in the Mocha installation directory. I reported it, provided a patch five days later, then waited. Nothing happened. 254 days later, according to Wolfram Alpha, half of my patch was, apparently – I say “apparently”, as I received no notification – applied. By that time I had replaced the whole mocking-and-stubbing subsystem and dropped the dependency.

Many Ruby developers claim that Ruby and its gems are too fast-moving for normal package-managing systems to keep up. This is testament to the fact that this isn’t the case and that the real problem is instead related to sloppy practices.

Please note that I don’t want to single out the Mocha library nor its developers. I only want to provide an example where relying on external dependencies can be “considered harmful”.

Internal Dependencies

Lookout has been designed so as to keep each subsystem independent of any other. The diff subsystem is, for example, completely decoupled from any other part of the system as a whole and could be moved into its own library at a time where that would be of interest to anyone. What’s perhaps more interesting is that the diff subsystem is itself very modular. The data passes through a set of filters that depends on what kind of diff has been requested, each filter yielding modified data as it receives it. If you want to read some rather functional Ruby I can highly recommend looking at the code in the lib/lookout/diff directory.

This lookout on the design of the library also makes it easy to extend Lookout. Lookout-rack was, for example, written in about four hours and about 5 of those 240 minutes were spent on setting up the interface between the two.

Optimizing For Speed

The following paragraph is perhaps a bit personal, but might be interesting nonetheless.

I’ve always worried about speed. The original Expectations library used extend a lot to add new behavior to objects. Expectations, for example, used to hold the result of their execution (what we now term “evaluation”) by being extended by a module representing success, failure, or error. For the longest time I used this same method, worrying about the increased performance cost that creating new objects for results would incur. I finally came to a point where I felt that the code was so simple and clean that rewriting this part of the code for a benchmark wouldn’t take more than perhaps ten minutes. Well, ten minutes later I had my results and they confirmed that creating new objects wasn’t harming performance. I was very pleased.


I hate low lines (underscores). I try to avoid them in method names and I always avoid them in file names. Since the current “best practice” in the Ruby community is to put BeginEndStorage in a file called begin_end_storage.rb, I only name constants using a single noun. This has had the added benefit that classes seem to have acquired less behavior, as using a single noun doesn’t allow you to tack on additional behavior without questioning if it’s really appropriate to do so, given the rather limited range of interpretation for that noun. It also seems to encourage the creation of value objects, as something named Range feels a lot more like a value than BeginEndStorage. (To reach object-oriented-programming Nirvana you must achieve complete value.)



The xml expectation has been dropped. It wasn’t documented, didn’t suit very many use cases, and can be better implemented by an external library.

The arg argument matcher for mock method arguments has been removed, as it didn’t provide any benefit over using Object.

The #yield and #each methods on stub and mock methods have been removed. They were slightly weird and their use case can be implemented using block parameters instead.

The stub method inside expect blocks now stubs out the methods during the execution of a provided block instead of during the execution of the whole except block.

When a mock method is called too many times, this is reported immediately, with a full backtrace. This makes it easier to pin down what’s wrong with the code.

Query expectations were added.

Explicit query expectations were added.

Fluent boolean expectations, for example, expect nil.to.be.nil? have been replaced by query expectations (expect :nil? do nil end) and explicit query expectations (expect result.to.be.nil? do nil end). This was done to discourage creating objects as the expected value and creating objects that change during the course of the test.

The literal expectation was added.

Equality (#==) is now checked before “caseity” (#===) for modules, ranges, and regular expressions to match the documentation.


Currently, most of my time is spent at my day job and in my rather busy private life. Please motivate me to spend time on this piece of software by donating some of your money to this project. Yeah, I realize that requesting money to develop software is a bit, well, capitalistic of me. But please realize that I live in a capitalistic society and I need money to have other people give me the things that I need to continue living under the rules of said society. So, if you feel that this piece of software has helped you out enough to warrant a reward, please PayPal a donation to now@disu.se. Thanks! Your support won’t go unnoticed!

Reporting Bugs

Please report any bugs that you encounter to the issue tracker.


Contributors to the original expectations codebase are mentioned there. We hope no one on that list feels left out of this list. Please let us know if you do.


Lookout is free software: you may redistribute it and/or modify it under the terms of the GNU Lesser General Public License, version 3 or later, as published by the Free Software Foundation.