The cmath.acos() function is used to compute the arc cosine of  the given number.

x The number(real or complex) whose arc cosine we want.

The function returns the arc cosine of in radians as a complex number. If  argument is a real number, the returned complex number has 0j as its imaginary part.

import cmath 

print(cmath.acos(2 + 3j))
print(cmath.acos(3 + 5j))

You should note that the cmath(complex math) module is made primarily for working with complex numbers. If you are working with real numbers the math.acos() function maybe more relevant.