import sympy as sym

a, b, x = sym.symbols ('a, b, x')
expr = a*sym.sin(x) + b*sym.cos(x)
print(expr)

fun_f = sym.lambdify ([a, b, x], expr)
val_fun_f = fun_f(a=2, b=1, x=0.5)
print ('val_fun_f =', val_fun_f )