[Python-checkins] [3.12] gh-130077: Properly match full soft keywords in the parser (GH-135317) (#135400)

[pablogsal]

https://github.com/python/cpython/commit/b69f3118a9e1eb00f5dd5447215787bbd7a72ad2
commit: b69f3118a9e1eb00f5dd5447215787bbd7a72ad2
branch: 3.12
author: Pablo Galindo Salgado <pablog...@gmail.com>
committer: pablogsal <pablog...@gmail.com>
date: 2025-07-09T00:40:55+01:00
summary:

[3.12] gh-130077: Properly match full soft keywords in the parser (GH-135317) 
(#135400)

(cherry picked from commit ff2b5f40c2bf5c71255caac8a743c09ba0758c02)

files:
A Misc/NEWS.d/next/Core and 
Builtins/2025-06-09-23-57-37.gh-issue-130077.MHknDB.rst
M Lib/test/test_syntax.py
M Parser/pegen.c

diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index 36a7f5a873dc8f..f2a2bc1d289861 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -322,6 +322,13 @@
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
+# But prefixes of soft keywords should
+# still raise specialized errors
+
+>>> (mat x)
+Traceback (most recent call last):
+SyntaxError: invalid syntax. Perhaps you forgot a comma?
+
 From compiler_complex_args():
 
 >>> def f(None=1):
diff --git a/Misc/NEWS.d/next/Core and 
Builtins/2025-06-09-23-57-37.gh-issue-130077.MHknDB.rst b/Misc/NEWS.d/next/Core 
and Builtins/2025-06-09-23-57-37.gh-issue-130077.MHknDB.rst
new file mode 100644
index 00000000000000..a7d02426b6fc13
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and 
Builtins/2025-06-09-23-57-37.gh-issue-130077.MHknDB.rst 
@@ -0,0 +1,2 @@
+Properly raise custom syntax errors when incorrect syntax containing names
+that are prefixes of soft keywords is encountered.  Patch by Pablo Galindo.
diff --git a/Parser/pegen.c b/Parser/pegen.c
index 5b92133d9f905b..fa1a4280019bbe 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -637,7 +637,8 @@ expr_ty _PyPegen_soft_keyword_token(Parser *p) {
     Py_ssize_t size;
     PyBytes_AsStringAndSize(t->bytes, &the_token, &size);
     for (char **keyword = p->soft_keywords; *keyword != NULL; keyword++) {
-        if (strncmp(*keyword, the_token, size) == 0) {
+        if (strlen(*keyword) == (size_t)size &&
+            strncmp(*keyword, the_token, (size_t)size) == 0) {
             return _PyPegen_name_from_token(p, t);
         }
     }

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