Hi, Since a decision on PEP 634 is imminent, I'd like to reiterate some concerns that I voiced last year. I am worried about the semantics and implementation of PEP 634. I don't want to comment on the merits of pattern matching in general, or the proposed syntax in PEP 634 (or PEP 640 or PEP 642). Semantics --------- 1. PEP 634 eschews the object model, in favour of adhoc instance checks, length checks and attribute accesses. This is in contrast to almost all of the the rest of the language, which uses special (dunder) methods: All operators, subscripting, attribute lookup, iteration, calls, tests, object creation, conversions, and the with statement AFAICT, no justification is given for this. Why can't pattern matching use the object model? PEP 343 (the "with" statement) added the __enter__ and __exit__ methods to the object model, and that works very well. 2. PEP 634 deliberately introduces a large area of undefined behaviour into Python. https://www.python.org/dev/peps/pep-0634/#side-effects-and-undefined-behavio... Python has, in general, been strict about not having undefined behaviour. Having defined semantics means that you can reason about programs, even non-idiomatic ones. [This is not unique to Python, it is really only C and C++ that have areas of undefined behaviour] I can see no good reason for adding undefined behaviour. It doesn't help anyone. The lack of precise semantics makes programs harder to understand, and it makes the language harder to implement. If the semantics aren't specified, then the implementation becomes the specification. This bakes bugs into the language and makes it harder to maintain, as bug-for-bug compatibility must be maintained. 3. Performance PEP 634 says that each pattern must be checked in turn. That means that multiple redundant checks must be performed on (for example) a sequence if there are several mapping patterns. This is unnecessarily slow. Implementation -------------- My main concern with the implementation is that it does too much work into the interpreter. Much of that work can and should be done in the compiler. For example, deep in the implementation of the MATCH_CLASS instruction is the following comment: https://github.com/brandtbucher/cpython/blob/patma/Python/ceval.c#L981 Such complex control flow should be handled during compilation, rather than in the interpreter. Giant instructions like MATCH_CLASS are likely to have odd corner cases, and may well have a negative impact on the performance of the rest of the language. It is a lot easier to reason about a sequence of simple bytecodes, than one giant one with context-dependent behaviour. We have spent quite a lot of effort over the last few years streamlining the interpreter. Adding these extremely complex instructions would be a big backward step. Cheers, Mark.