Thank you for this PEP! Pattern matching is really exciting.
As the PEP mentions and the thread evidences, the current dot syntax for
the “constant value pattern” is a tricky point. Given this, I thought I’d
throw another suggestion into the bikeshed.
Use percent placeholder to indicate lookup (or even eval) semantics for a
given name. For example:
```
FOO = 1
value = 0
match value:
case %(FOO): # This would not be matched
...
case BAR: # This would be matched
...
```
I like this syntax because it’s reminiscent of named substitution in
percent formatted strings. It suggests a substitution / placeholder
metaphor that is quite fitting. It has the benefit of not introducing a new
symbol into Python and being explicit and hard to miss, including in nested
contexts.
Note, it seems like it would also be technically possible to use curly
braces (the more idiomatic means of named substitution in Python 3):
```
case {FOO}: …
```
The main downside of this is that it could look like some sort of singleton
“set pattern” (note that the PEP only supports “sequence patterns” and
“mapping patterns”).
(But set patterns maybe don’t quite make sense + if your set pattern had
multiple elements you’d still get a SyntaxError. For other examples where
something in Python looks like a set literal but isn’t, refer to `{}` and
f-strings, so it’s maybe not the biggest stretch)
Both of these suggestions could also allow us more flexibility for constant
value patterns, since currently there isn't a good way to match against a
constant expression. For example, we could extend this syntax to allow us
to express:
```
case %(2 ** 10): ...
# or
case {get_default()}: ...
```
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