On Wed, Jun 24, 2020 at 4:05 AM Greg Ewing <greg.ewing@canterbury.ac.nz> wrote:
On 24/06/20 11:57 am, Guido van Rossum wrote:
> Matched key-value pairs must already be present in the mapping, and not
> created
> on-the-fly by ``__missing__`` or ``__getitem__``.  For example,
> ``collections.defaultdict`` instances will only match patterns with keys
> that
> were already present when the ``match`` block was entered.

Does that mean the pattern matching logic is in cahoots with
collections.defaultdict? What if you want to match against
your own defaultdict-like type?

IIUC the pattern matching uses either .get(key, <sentinel>) or .__contains__(key) followed by .__getitem__(key). Neither of those will auto-add the item to a defaultdict (and the Mapping protocol supports both). @Brandt: what does your implementation currently do? Do you think we need to specify this in the PEP?

--Guido van Rossum (python.org/~guido)