[python] How to get the value of a variable given its name in a string?

For simplicity this is a stripped down version of what I want to do:

def foo(a):
    # I want to print the value of the variable
    # the name of which is contained in a

I know how to do this in PHP:

function foo($a) {
    echo $$a;
}

global $string = "blah"; // might not need to be global but that's irrelevant
foo("string"); // prints "blah"

Any way to do this?