## Python, math, gamma_recursive.py

``````"""
Gamma function is a very useful tool in math and physics.
It helps calculating complex integral in a convenient way.
Python's Standard Library math.gamma() function overflows around gamma(171.624).
"""
from math import pi, sqrt

def gamma(num: float) -> float:
"""
Calculates the value of Gamma function of num
where num is either an integer (1, 2, 3..) or a half-integer (0.5, 1.5, 2.5 ...).
Implemented using recursion
Examples:
>>> from math import isclose, gamma as math_gamma
>>> gamma(0.5)
1.7724538509055159
>>> gamma(2)
1.0
>>> gamma(3.5)
3.3233509704478426
>>> gamma(171.5)
9.483367566824795e+307
>>> all(isclose(gamma(num), math_gamma(num)) for num in (0.5, 2, 3.5, 171.5))
True
>>> gamma(0)
Traceback (most recent call last):
...
ValueError: math domain error
>>> gamma(-1.1)
Traceback (most recent call last):
...
ValueError: math domain error
>>> gamma(-4)
Traceback (most recent call last):
...
ValueError: math domain error
>>> gamma(172)
Traceback (most recent call last):
...
OverflowError: math range error
>>> gamma(1.1)
Traceback (most recent call last):
...
NotImplementedError: num must be an integer or a half-integer
"""
if num <= 0:
raise ValueError("math domain error")
if num > 171.5:
raise OverflowError("math range error")
elif num - int(num) not in (0, 0.5):
raise NotImplementedError("num must be an integer or a half-integer")
elif num == 0.5:
return sqrt(pi)
else:
return 1.0 if num == 1 else (num - 1) * gamma(num - 1)

def test_gamma() -> None:
"""
>>> test_gamma()
"""
assert gamma(0.5) == sqrt(pi)
assert gamma(1) == 1.0
assert gamma(2) == 1.0

if __name__ == "__main__":
from doctest import testmod

testmod()
num = 1.0
while num:
num = float(input("Gamma of: "))
print(f"gamma({num}) = {gamma(num)}")
print("\nEnter 0 to exit...")
``````