I've been using sympy to automate some derivative computation and I loved it. I found the evalf[1] function to be really helpful as it helps bridge the gap between the symbolic and numeric world.

[1] http://docs.sympy.org/dev/modules/evalf.html

Good point. I think you could use the standard python eval() instead of defining f. something like:

    f = 'x**3+x-1'
    x = 4
    eval(f)

It should work fine.

That is a very good option. Thanks!

I was hoping for a sympy tutorial when I clicked

I am sure that can be arranged in the future, if one doesn't exist already.