The attached patch changes lru_cache to use if/else instead of try/except.
This has 2 effects:
1) it avoids chained exceptions and makes the error messages clearer;
2) it probably makes lru_cache a bit faster since building and catching exceptions is expensive. It also gets rid of the KeyError=KeyError optimization/hack;

For a couple of examples of 1) see #12749 (msg142059, msg142063), or #13169 (msg145469).  See also related issue #6210.