Add hypothesis testing

requirements.txt

@@ -1,3 +1,4 @@
 pandas==2.*
 requests==2.*
-pytest
+pytest
+hypothesis==6.*

tests/test_hypothesis.py

@@ -0,0 +1,40 @@
+"""
+Property based testing
+Testing module using hypothesis
+"""
+from hypothesis import given, reject, settings
+from hypothesis.strategies import floats, integers
+
+from sunblock import fahrenheit_to_celsius
+
+# In property based testing, we encode assumptions or properties of the
+# system that should always be true, regardless of parameter values a
+# function takes. Often this reveals edge cases that are not handled in
+# the code, which might not have been caught by example-based testing if
+# our list of examples was not exhaustive enough.
+
+# Here we check that no matter what temperature in degrees fahrenheit we
+# are given, we never return a value less than absolute zero (0 K) which
+# is equivalent to -273.15 degrees celsius.
+
+@given(floats())
+def test_fahrenheit_to_celsius(temp):
+    try:
+        assert fahrenheit_to_celsius(temp) > -273.15  # absolute zero
+    except ValueError:
+        reject()
+
+# hypothesis can also be used to intentionally find counter-examples
+# here we are trying to find an example where the numerical values of a
+# temperature in celsius and fahrenheit are the same.
+
+# This should fail as there is a counterexample (-40 F = -40 C) but
+# hypothesis may not find it. Try increasing the max_examples setting to
+# make hypothesis stumble upon the correct answer!
+@given(integers(min_value=-450))
+@settings(max_examples=100)
+def test_fahrenheit_differs_from_celsius(temp):
+    try:
+        assert fahrenheit_to_celsius(temp) != temp
+    except ValueError:
+        reject()