Writing Minitest clone in 30 minutes

by Masafumi Okura

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Writing Minitest clone in 30 minutes RubyConfTw 2023 OKURA Masafumi, 2023-12-16

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Minitest

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Have you used it?

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Features

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Features of Minitest • Just Ruby • No DSL • Fast • Supports spec style testing • describe, it, etc. • Clean and readable

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Features of Minitest • Just Ruby • No DSL • Fast • Supports spec style testing • describe, it, etc. • Clean and readable

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Readable test code

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What about internal code?

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Let’s battle!

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Lines of code

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Minitest:

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2000!

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RSpec:

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7800!

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… only for rspec-core gem

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Much smaller codebase

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Question: Can I write my own Minitest?

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Let’s do it!

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Me • Name: OKURA Masafumi (େ૔խ࢙) • From: Tokyo, Japan • Work: Freelance web developer (Rails) • Activities: Kaigi on Rails, Alba, etc. • Hobbies: Coffee, learning languages including Chinese • Development environment: Neovim, iTerm2, zsh

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About this talk

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Live coding

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Goal: Writing something similar to Minitest

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… in 30 minutes

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Using my own dev env

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Let’s go!

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Wrapping up

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What Minitest does • `ruby sample_test.rb` evaluates test fi le • Instance methods including `setup` are de fi ned, no class methods are de fi ned • All instance methods whose name start with `test_` are recognized as test methods • Inside test methods there are assertion methods • When assertion methods are executed it stores results, and when the result is failure the execution stops • It produces some outputs like "2 runs, 3 assertions, 0 failures"

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Problems • There's no explicit step to execute tests • There's no explicit step to create an instance of the test class • Not all methods are test methods • Tests whose name do not start with `test_` are simply helper methods • It needs to collection results

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Solutions • We can use àt_exit` hook to execute test • Alternatively, we can use ÒbjectSpace` module to get all classes with `Test` suf fi x, but this is much more complicated • Inside àt_exit` hook we can create an instance of the test class • We can use `method_added` hook to check the name and if it matches `test_` pre fi x it's marked as a test method. • We can store results in an instance variable

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Summary

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Takeaways

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at_exit

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at_exit • Executed when Ruby interpreter exits • Not suitable for a large business logic, but convenient for smaller tasks • https://docs.ruby-lang.org/en/master/Kernel.html#method-i-at_exit

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class_eval

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some_class.class_eval(&block) • Executes given block as if it’s in the class • The argument can be a String instead of a block, but block form is often preferred • Useful if we want to do a lot of thing for that class • https://docs.ruby-lang.org/en/master/Module.html#method-i- class_eval

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inherited

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def self.inherited(subclass) • Can be used when subclass needs some behavior • `subclass` argument is a `Class` object, so we can call `class_eval`, `include` or `extend` on it • Don’t forget to call `super` • https://docs.ruby-lang.org/en/master/Class.html#method-i- inherited

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method_added

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def self.method_added(name) • Lesser known, less frequently used • Can be used when we want to categorize methods, treat some methods as special ones, and customize each method • When customize, it’s often used with `method` and `de fi ne_method` method • https://docs.ruby-lang.org/en/master/Module.html#method-i- method_added

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Designing

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Designing a framework/library • Be familiar with the features the language gives us • In Ruby, there are varieties of hooks • Think of usages, rather than implementations • Autorun is a optional but useful feature, so we want it • Consider where to put what data • In more complex case, more data would be stored in its own place