Add soft keywords (GH-20370)

These are like keywords but they only work in context; they are not reserved except when there is an exact match.

This would enable things like match statements without reserving `match` (which would be bad for the `re.match()` function and probably lots of other places).

Automerge-Triggered-By: @gvanrossum

2 files changed, 25 insertions, 0 deletions

diff --git a/Parser/pegen/pegen.c b/Parser/pegen/pegen.c
index cd87a9f..ee30c2c 100644
--- a/Parser/pegen/pegen.c
+++ b/Parser/pegen/pegen.c
@@ -753,6 +753,30 @@ _PyPegen_expect_token(Parser *p, int type)
     return t;
 }
 
+expr_ty
+_PyPegen_expect_soft_keyword(Parser *p, const char *keyword)
+{
+    if (p->mark == p->fill) {
+        if (_PyPegen_fill_token(p) < 0) {
+            p->error_indicator = 1;
+            return NULL;
+        }
+    }
+    Token *t = p->tokens[p->mark];
+    if (t->type != NAME) {
+        return NULL;
+    }
+    char* s = PyBytes_AsString(t->bytes);
+    if (!s) {
+        return NULL;
+    }
+    if (strcmp(s, keyword) != 0) {
+        return NULL;
+    }
+    expr_ty res = _PyPegen_name_token(p);
+    return res;
+}
+
 Token *
 _PyPegen_get_last_nonnwhitespace_token(Parser *p)
 {
diff --git a/Parser/pegen/pegen.h b/Parser/pegen/pegen.h
index bd3056e..9507d99 100644
--- a/Parser/pegen/pegen.h
+++ b/Parser/pegen/pegen.h
@@ -122,6 +122,7 @@ int _PyPegen_lookahead_with_int(int, Token *(func)(Parser *, int), Parser *, int
 int _PyPegen_lookahead(int, void *(func)(Parser *), Parser *);
 
 Token *_PyPegen_expect_token(Parser *p, int type);
+expr_ty _PyPegen_expect_soft_keyword(Parser *p, const char *keyword);
 Token *_PyPegen_get_last_nonnwhitespace_token(Parser *);
 int _PyPegen_fill_token(Parser *p);
 expr_ty _PyPegen_name_token(Parser *p);