Additionally (probably should have separate PR) the _checkLevel was full of holes.
First, it's allowing any Integer which if you're claiming to "check" things is rather silly. At least bounds-check it to GE to NOTSET and LE to CRITICAL (or MAX or something handy in that regard).

Second, why is a string representation of an Integer a problem that needs to force the caller to fix his code? Same with lowercase of a recognized value. "Be liberal in what you accept" would seem to apply here. If it can be trivially reduced to an integer or upcased() to get a match then just do it, and send the corrected value back to the caller.