Add factorial example for hypothesis

Author: gjbex

source-code/testing/Hypothesis/README.md

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+# Hypothesis
+
+Examples of using `hypothesis`, a testing library that
+uses fuzzing to find issues in your code.
+
+
+## What is it?
+
+1. `factorial.py`: implementatino of the factorial
+   function.
+1. `test_factorial.py`: hypothesis tests of the factorial
+   function
+
+
+## How to use it?
+
+Run with pytest:
+```bash
+$ pytest --hypothesis-show-statistics
+```

source-code/testing/Hypothesis/factorial.py

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+'''Module to compute the factorial of a number'''
+
+
+def fac(n: int) -> int:
+    '''Return the factorial of n
+
+    Parameters
+    ----------
+    n : int
+        The number to compute the factorial of
+
+    Returns
+    -------
+    int
+        The factorial of n
+
+    Raises
+    ------
+    ValueError
+        If n is negative
+    '''
+    if n < 0:
+        raise ValueError("fac_iter() not defined for negative values")
+    result = 1
+    for i in range(n, 1, -1):
+        result *= i
+    return result

source-code/testing/Hypothesis/test_factorial.py

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+from math import factorial as fac_math
+from hypothesis import assume, given, strategies as st
+from factorial import fac
+
+
+@given(st.integers(min_value=0, max_value=100))
+def test_fac_same(n):
+    assert fac(n) == fac_math(n)
+
+
+@given(st.integers(max_value=-1))
+def test_fac_neg(n):
+    try:
+        assert fac(n) == fac_math(n)
+    except ValueError:
+        assert n < 0