For statements, use exec(string) (Python 2/3) or exec string (Python 2):

>>> mycode="print "hello world""
>>> exec(mycode)
Hello world

When you need the value of an expression, use eval(string):

>>> x = eval("2+2")
>>> x

However, the first step should be to ask yourself if you really need to. Executing code should generally be the position of last resort: It’s slow, ugly and dangerous if it can contain user-entered code. You should always look at alternatives first, such as higher order functions, to see if these can better meet your needs.