You can use the eval function to evaluate mathematical expressions in strings.
For example, if you have a string with the content (4*5) + 21, you can eval it and get the result.
>>> s = "(4*5) + 22" >>> eval(s) 42
Eval follows Python rules for evaluating mathematical expressions for cases in which parenthesis are not provided, etc. Be very careful when using eval though as it can be a source of huge security loopholes and bugs.