Turning String into Numerical Equation

[Brian Kazian]

Here's my problem, and hopefully someone can help me figure out if there is 
a good way to do this.

I am writing a program that allows the user to enter an equation in a text 
field using pre-existing variables.  They then enter numerical values for 
these variables, or can tell the program to randomize the values used within 
a certain bounds.  My problem is taking in this equation they have written 
in the text field and converting it into an equation Python can calculate.

The only way I know of to do this is by parsing it, which could get pretty 
nasty with all of the different mathematical rules.  Does anyone have a 
better idea than parsing to compute an equation from a string 
representation?

Thanks so much!

Brian Kazian