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bpo-37757: Disallow PEP 572 cases that expose implementation details (GH-15131)
- drop TargetScopeError in favour of raising SyntaxError directly as per the updated PEP 572 - comprehension iteration variables are explicitly local, but named expression targets in comprehensions are nonlocal or global. Raise SyntaxError as specified in PEP 572 - named expression targets in the outermost iterable of a comprehension have an ambiguous target scope. Avoid resolving that question now by raising SyntaxError. PEP 572 originally required this only for cases where the bound name conflicts with the iteration variable in the comprehension, but CPython can't easily restrict the exception to that case (as it doesn't know the target variable names when visiting the outermost iterator expression)
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-76
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10 files changed

+191
-76
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‎Doc/c-api/exceptions.rst

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@@ -809,7 +809,6 @@ the variables:
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single: PyExc_SystemError
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single: PyExc_SystemExit
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single: PyExc_TabError
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single: PyExc_TargetScopeError
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single: PyExc_TimeoutError
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single: PyExc_TypeError
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single: PyExc_UnboundLocalError
@@ -911,8 +910,6 @@ the variables:
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+-----------------------------------------+---------------------------------+----------+
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| :c:data:`PyExc_TabError` | :exc:`TabError` | |
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+-----------------------------------------+---------------------------------+----------+
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| :c:data:`PyExc_TargetScopeError` | :exc:`TargetScopeError` | |
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+-----------------------------------------+---------------------------------+----------+
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| :c:data:`PyExc_TimeoutError` | :exc:`TimeoutError` | |
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+-----------------------------------------+---------------------------------+----------+
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| :c:data:`PyExc_TypeError` | :exc:`TypeError` | |

‎Include/pyerrors.h

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@@ -97,7 +97,6 @@ PyAPI_DATA(PyObject *) PyExc_NotImplementedError;
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PyAPI_DATA(PyObject *) PyExc_SyntaxError;
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PyAPI_DATA(PyObject *) PyExc_IndentationError;
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PyAPI_DATA(PyObject *) PyExc_TabError;
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PyAPI_DATA(PyObject *) PyExc_TargetScopeError;
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PyAPI_DATA(PyObject *) PyExc_ReferenceError;
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PyAPI_DATA(PyObject *) PyExc_SystemError;
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PyAPI_DATA(PyObject *) PyExc_SystemExit;

‎Include/symtable.h

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@@ -58,6 +58,8 @@ typedef struct _symtable_entry {
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unsigned ste_needs_class_closure : 1; /* for class scopes, true if a
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closure over __class__
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should be created */
61+
unsigned ste_comp_iter_target : 1; /* true if visiting comprehension target */
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int ste_comp_iter_expr; /* non-zero if visiting a comprehension range expression */
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int ste_lineno; /* first line of block */
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int ste_col_offset; /* offset of first line of block */
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int ste_opt_lineno; /* lineno of last exec or import * */
@@ -94,6 +96,7 @@ PyAPI_FUNC(void) PySymtable_Free(struct symtable *);
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#define DEF_FREE_CLASS 2<<5 /* free variable from class's method */
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#define DEF_IMPORT 2<<6 /* assignment occurred via import */
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#define DEF_ANNOT 2<<7 /* this name is annotated */
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#define DEF_COMP_ITER 2<<8 /* this name is a comprehension iteration variable */
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#define DEF_BOUND (DEF_LOCAL | DEF_PARAM | DEF_IMPORT)
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‎Lib/_compat_pickle.py

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@@ -128,7 +128,6 @@
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"SystemError",
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"SystemExit",
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"TabError",
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"TargetScopeError",
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"TypeError",
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"UnboundLocalError",
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"UnicodeDecodeError",

‎Lib/test/exception_hierarchy.txt

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@@ -42,7 +42,6 @@ BaseException
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| +-- NotImplementedError
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| +-- RecursionError
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+-- SyntaxError
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| +-- TargetScopeError
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| +-- IndentationError
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| +-- TabError
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+-- SystemError

‎Lib/test/test_named_expressions.py

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@@ -104,15 +104,69 @@ def test_named_expression_invalid_17(self):
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with self.assertRaisesRegex(SyntaxError, "invalid syntax"):
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exec(code, {}, {})
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107-
def test_named_expression_invalid_18(self):
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def test_named_expression_invalid_in_class_body(self):
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code = """class Foo():
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[(42, 1 + ((( j := i )))) for i in range(5)]
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"""
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with self.assertRaisesRegex(TargetScopeError,
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"named expression within a comprehension cannot be used in a class body"):
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with self.assertRaisesRegex(SyntaxError,
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"assignment expression within a comprehension cannot be used in a class body"):
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exec(code, {}, {})
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116+
def test_named_expression_invalid_rebinding_comprehension_iteration_variable(self):
117+
cases = [
118+
("Local reuse", 'i', "[i := 0 for i in range(5)]"),
119+
("Nested reuse", 'j', "[[(j := 0) for i in range(5)] for j in range(5)]"),
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("Reuse inner loop target", 'j', "[(j := 0) for i in range(5) for j in range(5)]"),
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("Unpacking reuse", 'i', "[i := 0 for i, j in [(0, 1)]]"),
122+
("Reuse in loop condition", 'i', "[i+1 for i in range(5) if (i := 0)]"),
123+
("Unreachable reuse", 'i', "[False or (i:=0) for i in range(5)]"),
124+
("Unreachable nested reuse", 'i',
125+
"[(i, j) for i in range(5) for j in range(5) if True or (i:=10)]"),
126+
]
127+
for case, target, code in cases:
128+
msg = f"assignment expression cannot rebind comprehension iteration variable '{target}'"
129+
with self.subTest(case=case):
130+
with self.assertRaisesRegex(SyntaxError, msg):
131+
exec(code, {}, {})
132+
133+
def test_named_expression_invalid_rebinding_comprehension_inner_loop(self):
134+
cases = [
135+
("Inner reuse", 'j', "[i for i in range(5) if (j := 0) for j in range(5)]"),
136+
("Inner unpacking reuse", 'j', "[i for i in range(5) if (j := 0) for j, k in [(0, 1)]]"),
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]
138+
for case, target, code in cases:
139+
msg = f"comprehension inner loop cannot rebind assignment expression target '{target}'"
140+
with self.subTest(case=case):
141+
with self.assertRaisesRegex(SyntaxError, msg):
142+
exec(code, {}) # Module scope
143+
with self.assertRaisesRegex(SyntaxError, msg):
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exec(code, {}, {}) # Class scope
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with self.assertRaisesRegex(SyntaxError, msg):
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exec(f"lambda: {code}", {}) # Function scope
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def test_named_expression_invalid_comprehension_iterable_expression(self):
149+
cases = [
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("Top level", "[i for i in (i := range(5))]"),
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("Inside tuple", "[i for i in (2, 3, i := range(5))]"),
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("Inside list", "[i for i in [2, 3, i := range(5)]]"),
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("Different name", "[i for i in (j := range(5))]"),
154+
("Lambda expression", "[i for i in (lambda:(j := range(5)))()]"),
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("Inner loop", "[i for i in range(5) for j in (i := range(5))]"),
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("Nested comprehension", "[i for i in [j for j in (k := range(5))]]"),
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("Nested comprehension condition", "[i for i in [j for j in range(5) if (j := True)]]"),
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("Nested comprehension body", "[i for i in [(j := True) for j in range(5)]]"),
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]
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msg = "assignment expression cannot be used in a comprehension iterable expression"
161+
for case, code in cases:
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with self.subTest(case=case):
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with self.assertRaisesRegex(SyntaxError, msg):
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exec(code, {}) # Module scope
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with self.assertRaisesRegex(SyntaxError, msg):
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exec(code, {}, {}) # Class scope
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with self.assertRaisesRegex(SyntaxError, msg):
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exec(f"lambda: {code}", {}) # Function scope
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class NamedExpressionAssignmentTest(unittest.TestCase):
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@@ -306,39 +360,6 @@ def test_named_expression_scope_11(self):
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self.assertEqual(res, [0, 1, 2, 3, 4])
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self.assertEqual(j, 4)
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def test_named_expression_scope_12(self):
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res = [i := i for i in range(5)]
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self.assertEqual(res, [0, 1, 2, 3, 4])
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self.assertEqual(i, 4)
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def test_named_expression_scope_13(self):
316-
res = [i := 0 for i, j in [(1, 2), (3, 4)]]
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self.assertEqual(res, [0, 0])
319-
self.assertEqual(i, 0)
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321-
def test_named_expression_scope_14(self):
322-
res = [(i := 0, j := 1) for i, j in [(1, 2), (3, 4)]]
323-
324-
self.assertEqual(res, [(0, 1), (0, 1)])
325-
self.assertEqual(i, 0)
326-
self.assertEqual(j, 1)
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328-
def test_named_expression_scope_15(self):
329-
res = [(i := i, j := j) for i, j in [(1, 2), (3, 4)]]
330-
331-
self.assertEqual(res, [(1, 2), (3, 4)])
332-
self.assertEqual(i, 3)
333-
self.assertEqual(j, 4)
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335-
def test_named_expression_scope_16(self):
336-
res = [(i := j, j := i) for i, j in [(1, 2), (3, 4)]]
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338-
self.assertEqual(res, [(2, 2), (4, 4)])
339-
self.assertEqual(i, 4)
340-
self.assertEqual(j, 4)
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342363
def test_named_expression_scope_17(self):
343364
b = 0
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res = [b := i + b for i in range(5)]
@@ -421,6 +442,33 @@ def spam():
421442

422443
self.assertEqual(ns["a"], 20)
423444

445+
def test_named_expression_variable_reuse_in_comprehensions(self):
446+
# The compiler is expected to raise syntax error for comprehension
447+
# iteration variables, but should be fine with rebinding of other
448+
# names (e.g. globals, nonlocals, other assignment expressions)
449+
450+
# The cases are all defined to produce the same expected result
451+
# Each comprehension is checked at both function scope and module scope
452+
rebinding = "[x := i for i in range(3) if (x := i) or not x]"
453+
filter_ref = "[x := i for i in range(3) if x or not x]"
454+
body_ref = "[x for i in range(3) if (x := i) or not x]"
455+
nested_ref = "[j for i in range(3) if x or not x for j in range(3) if (x := i)][:-3]"
456+
cases = [
457+
("Rebind global", f"x = 1; result = {rebinding}"),
458+
("Rebind nonlocal", f"result, x = (lambda x=1: ({rebinding}, x))()"),
459+
("Filter global", f"x = 1; result = {filter_ref}"),
460+
("Filter nonlocal", f"result, x = (lambda x=1: ({filter_ref}, x))()"),
461+
("Body global", f"x = 1; result = {body_ref}"),
462+
("Body nonlocal", f"result, x = (lambda x=1: ({body_ref}, x))()"),
463+
("Nested global", f"x = 1; result = {nested_ref}"),
464+
("Nested nonlocal", f"result, x = (lambda x=1: ({nested_ref}, x))()"),
465+
]
466+
for case, code in cases:
467+
with self.subTest(case=case):
468+
ns = {}
469+
exec(code, ns)
470+
self.assertEqual(ns["x"], 2)
471+
self.assertEqual(ns["result"], [0, 1, 2])
424472

425473
if __name__ == "__main__":
426474
unittest.main()

‎Misc/NEWS.d/next/Core and Builtins/2019-08-05-14-22-59.bpo-37757.lRv5HX.rst

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:pep:`572`: As described in the PEP, assignment expressions now raise
2+
:exc:`SyntaxError` when their interaction with comprehension scoping results
3+
in an ambiguous target scope.
4+
5+
The ``TargetScopeError`` subclass originally proposed by the PEP has been
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removed in favour of just raising regular syntax errors for the disallowed
7+
cases.

‎Objects/exceptions.c

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@@ -1521,13 +1521,6 @@ MiddlingExtendsException(PyExc_SyntaxError, IndentationError, SyntaxError,
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"Improper indentation.");
15221522

15231523

1524-
/*
1525-
* TargetScopeError extends SyntaxError
1526-
*/
1527-
MiddlingExtendsException(PyExc_SyntaxError, TargetScopeError, SyntaxError,
1528-
"Improper scope target.");
1529-
1530-
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/*
15321525
* TabError extends IndentationError
15331526
*/
@@ -2539,7 +2532,6 @@ _PyExc_Init(void)
25392532
PRE_INIT(AttributeError);
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PRE_INIT(SyntaxError);
25412534
PRE_INIT(IndentationError);
2542-
PRE_INIT(TargetScopeError);
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PRE_INIT(TabError);
25442536
PRE_INIT(LookupError);
25452537
PRE_INIT(IndexError);
@@ -2680,7 +2672,6 @@ _PyBuiltins_AddExceptions(PyObject *bltinmod)
26802672
POST_INIT(AttributeError);
26812673
POST_INIT(SyntaxError);
26822674
POST_INIT(IndentationError);
2683-
POST_INIT(TargetScopeError);
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POST_INIT(TabError);
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POST_INIT(LookupError);
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POST_INIT(IndexError);

‎PC/python3.def

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@@ -235,7 +235,6 @@ EXPORTS
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PyExc_SystemError=python39.PyExc_SystemError DATA
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PyExc_SystemExit=python39.PyExc_SystemExit DATA
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PyExc_TabError=python39.PyExc_TabError DATA
238-
PyExc_TargetScopeError=python39.PyExc_TargetScopeError DATA
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PyExc_TimeoutError=python39.PyExc_TimeoutError DATA
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PyExc_TypeError=python39.PyExc_TypeError DATA
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PyExc_UnboundLocalError=python39.PyExc_UnboundLocalError DATA

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