Lookout is a unit testing framework for Ruby¹ 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

• Setting up only one expectation per test • Not setting expectations on non-public APIs • Test isolation

This is done by

• Only allowing one expectation to be set per test • Providing no (additional) way of accessing private state • Providing no setup and tear-down methods, nor a method of providing test


Other important points are

• Putting the expected outcome of a test in focus with the steps of the

calculation of the actual result only as a secondary concern

• A focus on code readability by providing no mechanism for describing an

expectation other than the code in the expectation itself

• A unified syntax for setting up both state-based and behavior-based


The way Lookout works has been heavily influenced by expectations², by
{Jay Fields}³.  The code base was once also heavily based on expectations,
based at Subversion {revision 76}⁴.  A lot has happened since then and all of
the work past that revision are due to {Nikolai Weibull}⁵.

¹ Ruby: ruby-lang.org/ ² Expectations: expectations.rubyforge.org/ ³ Jay Fields’s blog: blog.jayfields.com/ ⁴ Lookout revision 76:


⁵ Nikolai Weibull’s home page: disu.se/

§ Installation

Install Lookout with

  % gem install lookout

§ Usage

  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›.

§ Regexps

    ‹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 /…/›.)

§ Modules

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

      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›.

§ Booleans

    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›.

§ IO

    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.

§ Warnings

    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

§ Errors

    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

      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

      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

  •   ‹#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

      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

§ Integration

Lookout can be used from Rake¹.  Simply install Lookout-Rake²:

  % 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

¹ Read more about Rake at rake.rubyforge.org/ ² Get information on Lookout-Rake at disu.se/software/lookout-rake/


Lookout comes with an API¹ that let’s you create things such as new
expected values, difference reports for your types, and so on.

¹ See disu.se/software/lookout/api/

§ 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}¹ for Emacs².  Diffs are generated for Strings,
Arrays, Hashes, and I/O.

¹ Read up on Compilation mode for Emacs at www.emacswiki.org/emacs/CompilationMode ² Visit The GNU Foundation’s Emacs’ software page at www.gnu.org/software/emacs/

§ External Design

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

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

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}¹ 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}² 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-rack³ is another example of a library providing test helper
methods (well, one method, actually) that are very useful in testing web
applications that use Rack⁴.

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.

¹ Read the Wikipedia entry for Resource Acquisition Is Initialization at


² Read how 37signals had problems with slow Test::Unit tests at


³ Visit the Lookout-rack home page at


⁴ Visit the Rack Rubyforge project page at


§ Internal Design

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

•   As few external dependencies as possible
•   As few internal dependencies as possible
•   Internal extensibility provides external extensibility
•   As fast load times as possible
•   As high a ratio of value objects to mutable objects as possible
•   Each object must have a simple, obvious name
•   Use mix-ins, not inheritance for shared behavior
•   As few responsibilities per object as possible
•   Optimizing for speed can only be done when you have all the facts

§ 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}¹, according
to {Wolfram Alpha}², 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”.

¹ See the Wolfram Alpha calculation at www.wolframalpha.com/input/?i=days+between+march+17%2C+2010+and+november+26%2C+2010 ² Check out the Wolfram Alpha computational knowledge engine at www.wolframalpha.com/

§ 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

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.

§ Naming

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.)

§ News

§ 3.0.0

    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

    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.

§ Financing

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 [email protected]¹.  Thanks!  Your support won’t go unnoticed!

¹ Send a donation:


§ Reporting Bugs

  Please report any bugs that you encounter to the {issue tracker}¹.

¹ See https://github.com/now/lookout/issues

§ Contributors

  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}¹ if you do.

•   Nikolai Weibull

¹ Add an issue to the Lookout issue tracker at github.com/now/lookout/issues

§ Licensing

Lookout is free software: you may redistribute it and/or modify it under
the terms of the {GNU Lesser General Public License, version 3}¹ or later²,
as published by the {Free Software Foundation}³.

¹ See disu.se/licenses/lgpl-3.0/ ² See gnu.org/licenses/ ³ See fsf.org/