You signed in with another tab or window. Reload to refresh your session.You signed out in another tab or window. Reload to refresh your session.You switched accounts on another tab or window. Reload to refresh your session.Dismiss alert
This draft PR contains a couple of pattern matching improvements I'm experimenting with. Most significantly, it implements behavior that several people requested during the PEP-634 feedback process: no more name bindings for partial matches.
(One caveat: all names are bound *before* evaluating the corresponding guard. If the guard fails, these bindings won't be rolled back before matching the next case.)
In short, this is accomplished by storing captured objects on the stack underneath the more immediately important stuff, popping them all on failure or storing them all only once the *entire* pattern for that case matches. The control flow needs to be greatly simplified in order to make this work correctly, so the patch replaces the current "push bools and jump if false a bunch of times" flow with something much more efficient: jumping straight into a run of POP_TOPs on failure. We already do something very similar to this in pattern_helper_sequence_unpack (the comments in the current code explain it well).
Parts of the new code are a bit more complex than before (especially or-patterns, which need to do a bit of juggling on the stack to reorder captured items and avoid popping important data on failure), and other parts can certainly still be improved... but I personally consider it a win to have more intuitive partial match behavior with no performance penalty.
I thought I'd loop a few of you in on it to hear your thoughts before progressing much further.
Since the feature freeze is coming up (and this changes the bytecode), I'd like to open this up for review now.
It probably shouldn't actually be merged before the AST changes in bpo-43892, though. There will be quite a few conflicts that need resolving, but I'd rather integrate the AST changes into this PR than force Nick to integrate these deeper changes into his branch.