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

Syntax:
cmath.acos(x)
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x The number(real or complex) whose arc cosine we want.
Parameters:

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.

ExampleEdit & Run
import cmath 

print(cmath.acos(1))
print(cmath.acos(0.5))
print(cmath.acos(2 + 3j))
print(cmath.acos(0.5-2j))
print(cmath.acos(3 + 5j))
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Output:
-0j (1.0471975511965979-0j) (1.0001435424737972-1.9833870299165355j) (1.3497776911720125+1.4657153519472905j) (1.036797257200728-2.4598315216234345j) [Finished in 0.010839227586984634s]

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.