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gh-145019: improve SyntaxError when match patterns bind different names #145939

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gh-145019: improve `SyntaxError` when `match` patterns bind different…
picnixz Mar 14, 2026
37d5b5c
Update Lib/test/test_syntax.py
picnixz Mar 14, 2026
5cc7b11
use correct format specified `%d` -> `%zd`
picnixz Mar 14, 2026
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27 changes: 27 additions & 0 deletions Lib/test/test_syntax.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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: pass
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'])
Comment on lines +1520 to +1530
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@hugovk hugovk Mar 30, 2026

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Suggested change
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'])
SyntaxError: alternative patterns bind different names (pattern 1 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 (pattern 1 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 (pattern 1 binds ['id'], pattern 3 binds ['ip'])

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@picnixz picnixz Apr 3, 2026

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I actually wondered whether it was better to display the list as is or use some join to have a string. How do you feel about it?

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@hugovk hugovk Apr 4, 2026

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Could you give an example?

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@picnixz picnixz Apr 4, 2026 •
edited
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Currently:l (I think)

  • pattern 1 binds ["a", "b"] but pattern k binds ["u", "v"]

Suggestion:

  1. (formatted) pattern 1 binds "a" and "b" but pattern k binds "u" and "v"
  2. (minimal) pattern k binds "u' and "v' but pattern 1 does not
  3. (minimal swapped) pattern 1 binds "a" and "b" but pattern k does not.
  4. If pattern 1 binds a and b and pattern k binds a and c, only report that patten k does not bind b (or vice-versa)

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@hugovk hugovk Apr 4, 2026

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I think either current or number 1. Current will be easier :)

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@picnixz picnixz Apr 4, 2026

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Yes current is simple and easier to parse. It may however be better if we remove common entries (unless it is already done).


Incomplete dictionary literals

>>> {1:2, 3:4, 5}
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-03-14-11-20-52.gh-issue-145019.ls2EXo.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Improve :exc:`SyntaxError` when :keyword:`match` alternative patterns bind
different names. Patch by Bénédikt Tran.
26 changes: 24 additions & 2 deletions Python/codegen.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
Expand Down Expand Up @@ -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) {
Expand All @@ -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) {
Expand Down Expand Up @@ -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) {
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@pablogsal pablogsal Apr 5, 2026

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Unless I am missing something here the condition if (PyList_GET_SIZE(control) == 0 || !mismatched_names) has a permanently dead second operand. Both paths to goto diff (lines 6309-6311 and 6321-6323) unconditionally execute mismatched_names = Py_NewRef(pc->stores) before jumping, so mismatched_names is always non-NULL at the diff: label.

Similarly, the ternary at line 6428 (mismatched_names == NULL ? no_names : mismatched_names) always evaluates to mismatched_names.

no_names = PyUnicode_FromString("no names");
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@pablogsal pablogsal Apr 5, 2026

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This guy is only substituted when the first pattern (control) has 0 names. When a later pattern binds nothing (e.g., case (a, b) | 1:), mismatched_names is a valid empty list, so the error message would read:

alternative patterns bind different names (first pattern binds ['a', 'b'], pattern 2 binds [])

Showing [] instead of no names is inconsistent with the reverse case

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@picnixz picnixz Apr 5, 2026

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Oh good catch. I should have increased test coverage. I will put more cases to exercise this path. Thanks!

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@pablogsal pablogsal Apr 5, 2026

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This is freed later before the error: label, but error: itself doesn't free it. Currently safe because the only goto error from diff: is when no_names is NULL. But this is fragile as if future code between allocation and Py_XDECREF(no_names) adds a goto error, it would leak. Other variables (mismatched_names) use the pattern of cleanup in error:. Consider moving no_names to function scope and adding Py_XDECREF(no_names) to the error: block.

if (no_names == NULL) {
goto error;
}
}
_PyCompile_Error(
c, LOC(p),
"alternative patterns bind different names "
"(first pattern binds %S, pattern %zd binds %S)",
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@hugovk hugovk Mar 30, 2026

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Nit: rather than "first pattern... pattern X...", let's be consistent.

"second/third/etc pattern" is tricky, so:

Suggested change
"(first pattern binds %S, pattern %zd binds %S)",
"(pattern 1 binds %S, pattern %zd 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;
Expand Down
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