fx.py

import parser
import math

class Function:
    def __init__(self, expr):
        self.code = parser.expr(expr).compile()
    def __call__(self, **kwargs):
        return eval(self.code, {'pi': math.pi, 'e': math.e, 'cos': math.cos}, kwargs)

f = Function('2*x')
assert f(x=2) == 4
assert f(x=-2) == -4
g = Function('3*x+2*y+5*z')
assert g(x=1, y=1, z=1) == 10
h = Function('cos(3*pi+x)')
assert h(x=1) == math.cos((3*math.pi)+1)