Property based testing in Ruby
Abhishek Yadav
@h6165
Property based testing
- What is it ?
- Code examples
- ??
- Profit
Property based testing
- Most testing we do in Ruby is example based
(Rspec tests are called examples) - We try to come up with examples that cover the various usage scenarios. And we verify with literal results.
- We generally don't go by the properties
- All possible cases are often not covered, obviously
# Example of examples
def add(a, b)
a + b
end
# Test examples -
expect(add(1,2)).to eq(3)
expect(add(0,0)).to eq(0)
expect(add(-1,1)).to eq(0)
# and so on...
Property based testing
That seems ok
# Pythagoras
def hypotenuse(a, b)
Math.sqrt(a**2 + b**2)
end
# Test examples -
expect(hypotenuse(3, 4)).to eq(5.0)
expect(hypotenuse(5, 7)).to eq(8.6)
expect(hypotenuse(0, 0)).to eq(0)
expect(hypotenuse(-1, -2))
.to raise_error
# and so on...
Property based testing
Here,
we desperately feel the urge to express the Pythagoras theorem
To say, that the square of the hypotenuse equals the sum of the squares of the sides
That, is a property
# Pythagoras
def hypotenuse(a, b)
Math.sqrt(a**2 + b**2)
end
# What we need -
for_all_possible_values {
h = hypotenuse(a,b)
expect(h**2).to eq(a**2 + b**2)
}
Property based testing
The square of the hypotenuse equals the sum of the squares of the sides
Property based testing
Haskells Quickcheck
- Haskell folks built a library for just this
(and published a couple of papers too) - What the library does -
- Help generate a random test data
- Help express the properties
- Shrink the failure cases
- Quickcheck - http://www.cse.chalmers.se/~rjmh/QuickCheck/manual.html
- Quickcheck has been ported to most languages
https://en.wikipedia.org/wiki/QuickCheck
Property based testing
Ruby's Quickcheck
- Ruby has a few implementations -
-
rubycheck - a direct port
https://github.com/mcandre/rubycheck -
theft -
https://rubygems.org/gems/theft -
rantly
https://github.com/rantly-rb/rantly
-
Property based testing
Ruby's Rantly
# Pythagoras
# Property Test -
it 'hypotenuse squared is sum of sides squared' do
property_of {
a = integer
b = integer
[a, b]
}
.check(200) { |a, b|
h2 = a**2 + b**2
h2x = hypotenuse(a, b) ** 2
expect(h2x).to eq(h2)
}
end
Property based testing
Ruby's Rantly
property_of {
a = integer
b = integer
[a, b]
}
Generate random test data
.check(200) { |a, b|
h2 = a**2 + b**2
h2x = hypotenuse(a, b) ** 2
expect(h2x).to eq(h2)
}
Verify our function using the test data
(with 200 different data points)
Property based testing
Ruby's Rantly
.check(200) { |a, b|
h2 = a**2 + b**2
h2x = hypotenuse(a, b) ** 2
expect(h2x).to eq(h2)
}
Does that feel a bit wrong ?
We've almost specified the implementation here
Is that acceptable ?
Why or why not ?
Property based testing
Ruby's Rantly
Demo
Property based testing
Conclusion - Example tests vs Property tests
- Property tests are more useful with algorithms, formulae etc, where -
- there are lots of test values
- there's a risk of missing edge cases
- functions are pure
- Example unit tests - more useful with business logic, where code is mostly impure
Property based testing
Conclusion - Example tests vs Property tests
That's all
Property based testing
By Abhishek Yadav
Property based testing
- 1,076