def test_redefine_variables_illegal():
"""Variables can only be defined once.
Setting a variable in an environment where it is already defined should result
in an appropriate error.
"""
env = Environment({"foo": 1})
with assert_raises_regexp(LispError, "already defined"):
env.set("foo", 2)
def test_set_changes_environment_in_place():
"""When calling `set` the environment should be updated"""
env = Environment()
env.set("foo", 2)
assert_equals(2, env.lookup("foo"))