Writing tests with `gets` and `puts` in rspec

What's in this repo

I have trial & error for a bit, and figured out a way to test gets and puts without using StringIO / dependency injection, or overwriting global var $stdin. Here I summarize my findings with some sample code.

(Basically it's just the way that Clare introduced to us in her wiki page)

How to run this repo

(to have a try on the user interaction)

ruby lib/greeting.rb

(to run the test)

bundle exec rspec

To test `gets` (getting user input from console)

Assume you have a class Greeting, in which you calls gets in method #ask_for_user_name
In spec code, make an instance of Greeting. Then, use expect ... to receive(:gets) ... and_return('a_string') ... to let rspec mock the user input. After that, calls the method that use gets.

e.g.

greeting = Greeting.new
expect(greeting).to receive(:gets).and_return('a name')
greeting.ask_for_user_name

If we replace expect with allow in above, rspec will still mock the user input, but the test can pass even if gets was not called.

To test `puts`

Assume you have a class, in which you calls puts in some method
In spec code, make an instance of your class. Then, use expect { instance.method_call }.to output('some_string').to_stdout to make rspec check that method_call does print out some_string to the console.

e.g.

greeting = Greeting.new
expected_msg = "Hello, my friend!\n"
expect { greeting.say_hello }.to output(expected_msg).to_stdout

Note that rspec normally will try to compare strictly with "\n" (linebreaks). It might helps to put some rspec matcher a_string_starting_with, a_string_matching etc. around your string so that your test can have some flexibility.

Environment

The code in this repo was confirmed to run as expected on a machine with below config:

MacOS ver 12.1
ruby ver 2.6.9 (installed via rbenv)
bundler ver 2.3.6

Reference

Clare's wiki: https://clare-wiki.herokuapp.com/pages/coding/lang/oo/ruby/Ruby#testing-command-line-inputs-and-outputs-stdin-stdout-stderr

kapppa-joe/test_with_gets_and_puts

Writing tests with gets and puts in rspec