def METHOD_NAME(self):
    md = TMultiDict()
    md["foo"] = "bar"
    assert md.fields == (("foo", "bar"),)
    md["foo"] = "baz"
    assert md.fields == (("foo", "baz"),)
    md["bar"] = "bam"
    assert md.fields == (("foo", "baz"), ("bar", "bam"))