The Code works right but there is something wrong

Question

This is my code, when I run it gives me the right answer but also it gives me 'None':

My Code:

def art() :
    a = input('enter a values')
    b = eval(a)

    print(b)    
print(art())

Output:

enter a values3 + 4
7
None

Does anyone know why is that and how to remove it? Thank you.