Example to demonstrate exec in Python

pyexec.py

def run(init_code, code, wrapper, tests):
    env = {}
    exec(init_code, env)

    runTests = env['runTests']

    # Now all the variables and functions defined in init_code will be available in env

    # replace the marker in wrapper. This will be done by erb template in pythonmonk
    code2 = wrapper.replace("<%= user_code %>", code)

    # execute the code in the same env, so that functions in init_code are available here
    exec(code2, env)

    exec(tests, env)

    runTests()

init_code = """

def runTests():
    # take all variables start with test_
    tests = [v for k, v in globals().items() if k.startswith("test_")]

    # simple test runner. We way want to use nosetests in production.
    for t in tests:
        try:
            t()
        except Exception, e:
            print "FAIL", t.__doc__ or t.__name
            print "error:", str(e)
        else:
            print "PASS", t.__doc__ or t.__name__

def square(x):
    return x * x
"""

code = "square(3) + square(4)"

wrapper = "x = <%= user_code %>"

tests = """
def test_x():
    "3 square + 4 square is 25."
    assert x == 25, "expected 25, found %d" % x

def test_fail():
    "just a failing test"
    assert x == 20, "expected 20, found %d" % x
"""

run(init_code, code, wrapper, tests)