gh-145019: improve SyntaxError when match patterns bind different names

Author: picnixz

Lib/test/test_syntax.py

@@ -1502,6 +1502,33 @@
    Traceback (most recent call last):
    SyntaxError: invalid syntax
 
+Ensure that alternative patterns bind the same names
+
+   >>> match 1:
+   ...     case x | 1: pass
+   Traceback (most recent call last):
+   SyntaxError: name capture 'x' makes remaining patterns unreachable
+
+   >>> match 1:
+   ...     case x | y: pass
+   Traceback (most recent call last):
+   SyntaxError: name capture 'x' makes remaining patterns unreachable
+
+   >>> match 1:
+   ...     case 1 | x: ...
+   Traceback (most recent call last):
+   SyntaxError: alternative patterns bind different names (first pattern binds no names, pattern 2 binds ['x'])
+
+   >>> match 1:
+   ...     case ("user", {"id": id}) | ("admin", {"name": name}): pass
+   Traceback (most recent call last):
+   SyntaxError: alternative patterns bind different names (first pattern binds ['id'], pattern 2 binds ['name'])
+
+   >>> match 1:
+   ...     case ("user", {"id": id}) | ("admin", {"id": id}) | ("other", {"ip": ip}): pass
+   Traceback (most recent call last):
+   SyntaxError: alternative patterns bind different names (first pattern binds ['id'], pattern 3 binds ['ip'])
+
 Incomplete dictionary literals
 
    >>> {1:2, 3:4, 5}

Misc/NEWS.d/next/Core_and_Builtins/2026-03-14-11-20-52.gh-issue-145019.ls2EXo.rst

@@ -0,0 +1,2 @@
+Improve :exc:`SyntaxError` when :keyword:`match` alternative patterns bind
+different names. Patch by Bénédikt Tran.

Python/codegen.c

@@ -6270,6 +6270,8 @@ codegen_pattern_or(compiler *c, pattern_ty p, pattern_context *pc)
     NEW_JUMP_TARGET_LABEL(c, end);
     Py_ssize_t size = asdl_seq_LEN(p->v.MatchOr.patterns);
     assert(size > 1);
+    PyObject *mismatched_names = NULL;
+    Py_ssize_t mismatch_index = 0;
     // We're going to be messing with pc. Keep the original info handy:
     pattern_context old_pc = *pc;
     Py_INCREF(pc->stores);
@@ -6304,6 +6306,8 @@ codegen_pattern_or(compiler *c, pattern_ty p, pattern_context *pc)
             control = Py_NewRef(pc->stores);
         }
         else if (nstores != PyList_GET_SIZE(control)) {
+            mismatch_index = i;
+            mismatched_names = Py_NewRef(pc->stores);
             goto diff;
         }
         else if (nstores) {
@@ -6314,6 +6318,8 @@ codegen_pattern_or(compiler *c, pattern_ty p, pattern_context *pc)
                 Py_ssize_t istores = PySequence_Index(pc->stores, name);
                 if (istores < 0) {
                     PyErr_Clear();
+                    mismatch_index = i;
+                    mismatched_names = Py_NewRef(pc->stores);
                     goto diff;
                 }
                 if (icontrol != istores) {
@@ -6405,10 +6411,26 @@ codegen_pattern_or(compiler *c, pattern_ty p, pattern_context *pc)
     // Pop the copy of the subject:
     ADDOP(c, LOC(p), POP_TOP);
     return SUCCESS;
-diff:
-    _PyCompile_Error(c, LOC(p), "alternative patterns bind different names");
+diff:;
+    PyObject *no_names = NULL;
+    if (PyList_GET_SIZE(control) == 0 || !mismatched_names) {
+        no_names = PyUnicode_FromString("no names");
+        if (no_names == NULL) {
+            goto error;
+        }
+    }
+    _PyCompile_Error(
+        c, LOC(p),
+        "alternative patterns bind different names "
+        "(first pattern binds %S, pattern %d binds %S)",
+        PyList_GET_SIZE(control) == 0 ? no_names : control,
+        mismatch_index + 1,
+        mismatched_names == NULL ? no_names : mismatched_names
+    );
+    Py_XDECREF(no_names);
 error:
     PyMem_Free(old_pc.fail_pop);
+    Py_XDECREF(mismatched_names);
     Py_DECREF(old_pc.stores);
     Py_XDECREF(control);
     return ERROR;