Here is a preliminary implementation of PEP 463, minus mandatory parentheses and with the most straightforward precedence rule: equal to if-expr -- which means this:

A if C else B except E: D

is parsed as

A if C else (B except E: D)

(because of associativity, not precedence) and not as

(A if C else B) except E: D

as suggested by the PEP with the hand-wavy words 'between lambda and if/else in precedence'. The latter is possible but means a little more hoop-jumping in the grammar.

The tests included are rudimentary (at the end of test_grammar.py) and could probably do with some fleshing out, partially as tests and partially to try out how the precedence rules work out in practice.