Effective Testing with RSpec 3, From Writing Specs to Running Them

Notes from Effective Testing with RSpec 3, chapter 2.

Chapter two walked us through creating a spec to guide the development of our class. This time we explored the useful ways RSpec runs tests. We learned about structuring specs with tags and context blocks. Then filtering what specs are run.


We can assign meta data to an example block using hash value syntax. Rspec can then filter these tests to only run with or without them.

In ‘Identifying Slow Examples’ we used profile to identify the last two examples as our slowest tests. We can mark these tests as ‘slow’ using the code below:

RSpec.describe 'The sleep() method' do
  it ('can sleep for 0.1 second') { sleep 0.1 }
  it ('can sleep for 0.2 second') { sleep 0.2 }
  it ('can sleep for 0.3 second') { sleep 0.3 }
  it 'can sleep for 0.4 second', slow: true do; sleep 0.4; end
  it 'can sleep for 0.5 second', slow: true do; sleep 0.5; end

We can then run just the ‘slow’ tests, or exclude them using command line options or configurations:

# exclude slow tests from our run
rspec --tag ~slow

# only run the slow tests
rspec --tag slow

# Configure Rspec to auto exclude tests marked as slow
RSpec.configure do |config|
  config.filter_run_excluding slow: true

Effective Testing walks us through configuring RSpec with `filter_run_when_matching`, but there are many filters we can use to configure RSpec.

In addition to slow, we can use meta-tags to filter

  • Required: Features that have contractual obligation to be available. You may never use TDD to ensure a Partners link is available on your front page. You may want a set of tests for ease-of-mind on your business relationships.
  • Components & Stories: Tests that cover a general idea or the next milestone.
  • Smoke tests: quick check to ensure application is functional.

Prepare Tests with Pending

Another way of filtering specs is with pending. Pending is way of categorizing specs that are not yet ready. It allows us to sit down and completely think out the attributes of a class.

  RSpec.describe 'blog post' do
    it 'has a title'
    it 'has a subtitle'
    it 'has content'

This is easy to knockout and easy to read. I don’t have to know how I am going to accomplish this behavior but get’s things out of my head. Making me less likely to forget the big picture.

Pending goes further into detail. We can mark a test with pending and provide a helpful message.

it​ ​'is light in color'​ ​do​
  pending ​'Color not implemented yet'​
  expect​(coffee.color).to be(​:light​)

#Rspec output
1) A cup of coffee with milk is light in color
  # Color not implemented yet
​  Failure/Error: expect(coffee.color).to be(:light)”

#Rspec output once completed
1) A cup of coffee with milk is light in color FIXED
  Expected pending ’Color not implemented yet’ to fail. No error was raised.
​   # ./spec/coffee_spec.rb:42”

# Excerpt From: Myron Marston, Ian Dees. “Effective Testing with RSpec 3.” iBooks. 

If we forget to remove pending after completing a feature, RSpec will remind us to clean up our test. The authors point out that this is handy for bugs. An unexpected failing test can be marked as ‘pending’ with the issue tracker in the description.

Example Filter & Dry Run

In ‘Running Just What You Need’ we learn about the −−example or -e flag. This is exciting because combined with −−dry-run we can easily explore our specs as documentation. In the last chapter, the authors pointed out that let safeguards us from memoization gotchas. Curios about what let is expected to do I can quickly find out.

rspec --example let --dry-run
  raises an error when referenced from `before(:all)`
  yields the example
  raises a useful error when called without a block
  caches a nil value
  caches the value
  raises an error when attempting to define a reserved method name
  generates an instance method
  raises an error when referenced from `after(:all)`
  does not pass the block up the ancestor chain
  when the declaration uses `return`
    can get past a conditional `return` statement
    can exit the let declaration early
  when overriding let in a nested context
    can use `super` to reference the parent context value
  when included modules have hooks that define memoized helpers
    allows memoized helpers to override methods in previously included modules

47 examples are returned. This is more than we wanted, but it’s much easier to scan 47 examples instead of thousands. It’s also easier to read as it is just the authors intentions. There are no stack traces, profiling, or meta-data.

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