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Python: model exception edges for raise-prone expressions inside try/with
The new CFG previously only emitted exception edges for explicit `raise` and `assert` statements. As a result, code that became reachable only via the exception path of an arbitrary expression (e.g., the body of an `except` handler following a try-body whose `call()` could raise) was classified as dead, breaking analyses like StackTraceExposure, FileNotAlwaysClosed, ExceptionInfo, UseOfExit, and CatchingBaseException. This commit adds a `mayThrow` predicate over expressions that are known sources of implicit exceptions in Python (calls, attribute access, subscripts, arithmetic/comparison operators, imports, await/yield/yield from) plus `from m import *` at the statement level, and routes them through the shared CFG's `beginAbruptCompletion(_, _, ExceptionSuccessor, always=false)` hook. The set of exception sources is restricted to nodes that are syntactically inside a `try`/`with` statement in the same scope. This mirrors Java's `ControlFlowGraph::mayThrow`, which only emits exception edges where local handling can observe them — outside such contexts, the edges add CFG complexity (weakening BarrierGuard precision and breaking SSA continuity around augmented assignments and subscript stores) without analysis benefit, since exceptions just propagate to the function exit anyway. Net effect on the test suite: ~100 alerts restored across the exception- related query tests (StackTraceExposure +29, ExceptionInfo +17, FileNotAlwaysClosed +52, UseOfExit +1, CatchingBaseException restored) with no precision regressions. Affected `.expected` files and the regression-guard `dead_under_no_raise.py` are updated accordingly. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Lines changed: 144 additions & 80 deletions

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‎python/ql/lib/semmle/python/controlflow/internal/AstNodeImpl.qll‎

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Original file line numberDiff line numberDiff line change
@@ -1556,6 +1556,89 @@ private module Input implements InputSig1, InputSig2 {
15561556

15571557
private string assertThrowTag() { result = "[assert-throw]" }
15581558

1559+
/**
1560+
* Holds if the AST node `n` may raise an exception at runtime as part of
1561+
* its normal evaluation (not via an explicit `raise`/`assert`, which are
1562+
* modelled separately).
1563+
*
1564+
* The set mirrors what the legacy CFG used to flag implicitly: function
1565+
* calls (anything can raise), attribute access (`AttributeError`),
1566+
* subscript access (`IndexError`/`KeyError`/`TypeError`), arithmetic and
1567+
* comparison operators (`TypeError`/`ZeroDivisionError`), imports
1568+
* (`ImportError`/`ModuleNotFoundError`), and generator/coroutine
1569+
* suspension points (`await`/`yield`/`yield from`).
1570+
*
1571+
* Bare `Name` reads are intentionally excluded — modelling every name
1572+
* read as `mayThrow` would explode CFG edge count for negligible
1573+
* analysis value. `BoolExpr`/`IfExp` containers are also excluded; the
1574+
* operands they evaluate contribute their own exception edges.
1575+
*/
1576+
private predicate exprMayThrow(Py::Expr e) {
1577+
e instanceof Py::Call
1578+
or
1579+
e instanceof Py::Attribute
1580+
or
1581+
e instanceof Py::Subscript
1582+
or
1583+
e instanceof Py::BinaryExpr
1584+
or
1585+
e instanceof Py::UnaryExpr
1586+
or
1587+
e instanceof Py::Compare
1588+
or
1589+
e instanceof Py::ImportExpr
1590+
or
1591+
e instanceof Py::ImportMember
1592+
or
1593+
e instanceof Py::Await
1594+
or
1595+
e instanceof Py::Yield
1596+
or
1597+
e instanceof Py::YieldFrom
1598+
}
1599+
1600+
/**
1601+
* Holds if the statement `s` may raise an exception at runtime as part
1602+
* of its normal evaluation. Currently restricted to `from m import *`
1603+
* (which performs the import as a statement-level side effect).
1604+
*/
1605+
private predicate stmtMayThrow(Py::Stmt s) { s instanceof Py::ImportStar }
1606+
1607+
/**
1608+
* Holds if `n` is syntactically inside the body, handlers, `else`, or
1609+
* `finally` of a `try` statement (or the body of a `with` statement,
1610+
* which compiles to an implicit try/finally for `__exit__`) in the
1611+
* same scope.
1612+
*
1613+
* This mirrors Java's `ControlFlowGraph::mayThrow`, which only emits
1614+
* exception edges when there is local exception handling that would
1615+
* observe them. Outside such contexts, exception edges would add CFG
1616+
* complexity (weakening BarrierGuard precision and breaking SSA
1617+
* continuity around augmented assignments and subscript stores)
1618+
* without any analysis benefit, since exceptions just propagate to
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* the function exit anyway.
1620+
*/
1621+
private predicate inExceptionContext(Py::AstNode py) {
1622+
exists(Py::Try t | t.containsInScope(py))
1623+
or
1624+
exists(Py::With w | w.containsInScope(py))
1625+
}
1626+
1627+
/**
1628+
* Holds if `n` may raise an exception during normal evaluation. See
1629+
* `exprMayThrow` and `stmtMayThrow` for the included AST classes.
1630+
*
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* Restricted to nodes inside a `try`/`with` statement: matches Java's
1632+
* approach of only modelling exception flow where it can be observed
1633+
* by local handling.
1634+
*/
1635+
private predicate mayThrow(Ast::AstNode n) {
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exists(Py::AstNode py | py = n.asExpr() or py = n.asStmt() |
1637+
(exprMayThrow(py) or stmtMayThrow(py)) and
1638+
inExceptionContext(py)
1639+
)
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}
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predicate additionalNode(Ast::AstNode n, string tag, NormalSuccessor t) {
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n instanceof Ast::AssertStmt and tag = assertThrowTag() and t instanceof DirectSuccessor
15611644
}
@@ -1567,6 +1650,11 @@ private module Input implements InputSig1, InputSig2 {
15671650
n.isAdditional(ast, assertThrowTag()) and
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c.asSimpleAbruptCompletion() instanceof ExceptionSuccessor and
15691652
always = true
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or
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mayThrow(ast) and
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n.isIn(ast) and
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c.asSimpleAbruptCompletion() instanceof ExceptionSuccessor and
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always = false
15701658
}
15711659

15721660
predicate endAbruptCompletion(Ast::AstNode ast, PreControlFlowNode n, AbruptCompletion c) {

‎python/ql/test/experimental/library-tests/FindSubclass/Find.expected‎

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Original file line numberDiff line numberDiff line change
@@ -4,6 +4,7 @@
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| flask.MethodView~Subclass | find_subclass_test | Member[C] |
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| flask.View~Subclass | find_subclass_test | Member[A] |
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| flask.View~Subclass | find_subclass_test | Member[B] |
7+
| flask.View~Subclass | find_subclass_test | Member[ViewAliasInExcept] |
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| flask.View~Subclass | find_subclass_test | Member[ViewAliasInTry] |
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| flask.View~Subclass | find_subclass_test | Member[ViewAlias] |
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| flask.View~Subclass | find_subclass_test | Member[ViewAlias_no_use] |

‎python/ql/test/library-tests/ControlFlow/bindings/dead_under_no_raise.py‎

Lines changed: 19 additions & 18 deletions
Original file line numberDiff line numberDiff line change
@@ -1,15 +1,15 @@
1-
# Dead bindings under the "no expressions raise" CFG abstraction.
1+
# Reachability of code following a try whose body always returns.
22
#
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# The new CFG does not currently model raise edges from arbitrary
4-
# expressions. As a consequence, code that is only reachable through
5-
# exception flow is (correctly) classified as dead and has no CFG node.
6-
# Variable bindings in dead code do not need CFG nodes - SSA / dataflow
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# over dead code is moot.
3+
# The new CFG models exception edges for raise-prone expressions when
4+
# they appear inside a `try` (or `with`) statement, mirroring Java's
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# `mayThrow`. This means the body of a `try` has both a normal
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# completion edge and an exception edge to its handlers, so code
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# following the try-statement is reachable via the except-handler path
8+
# even when the try-body would otherwise always return.
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#
9-
# These tests act as a regression guard: the bindings below intentionally
10-
# have no `cfgdefines=` annotations. If raise modelling is later added,
11-
# the BindingsTest infrastructure will surface the new CFG nodes as
12-
# unexpected results, and this file will need to be revisited.
10+
# Code that is not reachable under either normal or exception flow
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# (for example, the `else` clause of a try whose body unconditionally
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# raises) remains correctly classified as dead.
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1414

1515
def f(obj): # $ cfgdefines=f cfgdefines=obj
@@ -18,12 +18,12 @@ def f(obj): # $ cfgdefines=f cfgdefines=obj
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except TypeError:
1919
pass
2020

21-
# The first try's body always returns; its except handler does not
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# raise or otherwise transfer control, so under "no expressions
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# raise" the only paths out of the try-statement are dead. Everything
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# below is unreachable.
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# The try-body always returns, but `len(obj)` can raise (it is
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# inside the try, so we model its exception edge). The
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# `except TypeError: pass` handler falls through to here, making
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# the code below reachable.
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try:
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hint = type(obj).__length_hint__
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hint = type(obj).__length_hint__ # $ cfgdefines=hint
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except AttributeError:
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return None
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return hint
@@ -35,7 +35,8 @@ def g(): # $ cfgdefines=g
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except:
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raise Exception("outer")
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else:
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# Unreachable: the inner try body always raises, so the `else:`
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# Unreachable: the inner try body always raises (via an explicit
39+
# `raise`, which is modelled unconditionally), so the `else:`
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# clause never runs.
4041
hit_inner_else = True
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@@ -46,7 +47,7 @@ def h(cache, key): # $ cfgdefines=h cfgdefines=cache cfgdefines=key
4647
except KeyError:
4748
pass
4849

49-
# Same pattern as `f`: dead under "no expressions raise".
50-
value = compute(key)
50+
# Same pattern as `f`: reachable via the except-handler fall-through.
51+
value = compute(key) # $ cfgdefines=value
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cache[key] = value
5253
return value

‎python/ql/test/library-tests/dataflow-new-ssa-vs-legacy/CmpTest.expected‎

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@@ -2,5 +2,4 @@
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| def-only-old | __name__:0:0 |
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| def-only-old | __package__:0:0 |
44
| def-only-old | e:37:1 |
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| def-only-old | e:40:25 |
65
| def-only-old | x:20:1 |

‎python/ql/test/library-tests/dataflow/coverage/test.py‎

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Original file line numberDiff line numberDiff line change
@@ -844,7 +844,7 @@ def return_from_inner_scope(x):
844844
return SOURCE
845845

846846
def test_return_from_inner_scope():
847-
SINK(return_from_inner_scope([])) # $ MISSING: flow="SOURCE, l:-3 -> return_from_inner_scope(..)"
847+
SINK(return_from_inner_scope([])) # $ flow="SOURCE, l:-3 -> return_from_inner_scope(..)"
848848

849849

850850
# Inspired by reverse read inconsistency check

‎python/ql/test/query-tests/Exceptions/general/CatchingBaseException.expected‎

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Original file line numberDiff line numberDiff line change
@@ -1 +1,2 @@
11
| exceptions_test.py:7:5:7:11 | ExceptStmt | Except block directly handles BaseException. |
2+
| exceptions_test.py:97:5:97:25 | ExceptStmt | Except block directly handles BaseException. |

‎python/ql/test/query-tests/Exceptions/general/EmptyExcept.expected‎

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Original file line numberDiff line numberDiff line change
@@ -3,9 +3,3 @@
33
| exceptions_test.py:72:1:72:18 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |
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| exceptions_test.py:85:1:85:17 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |
55
| exceptions_test.py:89:1:89:17 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |
6-
| exceptions_test.py:144:5:144:25 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |
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| exceptions_test.py:167:5:167:26 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |
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| exceptions_test.py:173:5:173:22 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |
9-
| exceptions_test.py:179:5:179:22 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |
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| exceptions_test.py:185:5:185:26 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |
11-
| exceptions_test.py:191:5:191:30 | ExceptStmt | 'except' clause does nothing but pass and there is no explanatory comment. |

‎python/ql/test/query-tests/Exceptions/general/IncorrectExceptOrder.expected‎

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1,3 +1 @@
1-
#select
2-
testFailures
3-
| exceptions_test.py:64:24:64:55 | Comment # $ Alert[py/unreachable-except] | Missing result: Alert[py/unreachable-except] |
1+
| exceptions_test.py:64:1:64:22 | ExceptStmt | This except block handling $@ is unreachable; as $@ for the more general $@ always subsumes it. | file://:0:0:0:0 | AttributeError | AttributeError | exceptions_test.py:62:1:62:17 | ExceptStmt | this except block | file://:0:0:0:0 | Exception | Exception |

‎python/ql/test/query-tests/Resources/FileNotAlwaysClosed/FileNotAlwaysClosed.expected‎

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Original file line numberDiff line numberDiff line change
@@ -1,40 +1,9 @@
1-
#select
21
| resources_test.py:4:10:4:25 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:5:5:5:33 | After Attribute() | this operation |
32
| resources_test.py:9:10:9:25 | After open() | File is opened but is not closed. | file://:0:0:0:0 | (none) | this operation |
4-
| resources_test.py:20:10:20:25 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:22:9:22:37 | After Attribute() | this operation |
5-
| resources_test.py:30:14:30:29 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:31:9:31:37 | After Attribute() | this operation |
6-
| resources_test.py:39:14:39:29 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:40:9:40:37 | After Attribute() | this operation |
7-
| resources_test.py:49:14:49:29 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:50:9:50:37 | After Attribute() | this operation |
8-
| resources_test.py:58:14:58:29 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:59:9:59:37 | After Attribute() | this operation |
9-
| resources_test.py:69:11:69:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:71:9:71:40 | After Attribute() | this operation |
10-
| resources_test.py:69:11:69:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:72:9:72:40 | After Attribute() | this operation |
11-
| resources_test.py:79:11:79:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:81:9:81:40 | After Attribute() | this operation |
12-
| resources_test.py:79:11:79:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:82:9:82:40 | After Attribute() | this operation |
13-
| resources_test.py:91:11:91:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:93:9:93:40 | After Attribute() | this operation |
14-
| resources_test.py:91:11:91:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:94:9:94:40 | After Attribute() | this operation |
153
| resources_test.py:108:11:108:20 | After open() | File is opened but is not closed. | file://:0:0:0:0 | (none) | this operation |
164
| resources_test.py:112:11:112:28 | After opener_func2() | File may not be closed if $@ raises an exception. | resources_test.py:113:5:113:22 | After Attribute() | this operation |
175
| resources_test.py:123:11:123:24 | After opener_func2() | File is opened but is not closed. | file://:0:0:0:0 | (none) | this operation |
186
| resources_test.py:129:15:129:24 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:130:9:130:26 | After Attribute() | this operation |
19-
| resources_test.py:141:11:141:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:143:9:143:40 | After Attribute() | this operation |
20-
| resources_test.py:141:11:141:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:144:9:144:40 | After Attribute() | this operation |
21-
| resources_test.py:182:15:182:54 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:186:9:186:25 | After Attribute() | this operation |
22-
| resources_test.py:225:11:225:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:227:9:227:25 | After Attribute() | this operation |
23-
| resources_test.py:237:11:237:26 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:239:9:239:25 | After Attribute() | this operation |
247
| resources_test.py:248:11:248:25 | After open() | File is opened but is not closed. | file://:0:0:0:0 | (none) | this operation |
25-
| resources_test.py:252:11:252:25 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:254:9:254:23 | After Attribute() | this operation |
268
| resources_test.py:269:10:269:27 | After Attribute() | File may not be closed if $@ raises an exception. | resources_test.py:271:5:271:19 | After Attribute() | this operation |
27-
| resources_test.py:275:10:275:35 | After Attribute() | File may not be closed if $@ raises an exception. | resources_test.py:278:9:278:23 | After Attribute() | this operation |
289
| resources_test.py:285:11:285:20 | After open() | File may not be closed if $@ raises an exception. | resources_test.py:287:5:287:31 | After Attribute() | this operation |
29-
testFailures
30-
| resources_test.py:20:10:20:25 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
31-
| resources_test.py:30:14:30:29 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
32-
| resources_test.py:39:14:39:29 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
33-
| resources_test.py:58:14:58:29 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
34-
| resources_test.py:69:11:69:26 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
35-
| resources_test.py:79:11:79:26 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
36-
| resources_test.py:91:11:91:26 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
37-
| resources_test.py:182:15:182:54 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
38-
| resources_test.py:225:11:225:26 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
39-
| resources_test.py:252:11:252:25 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |
40-
| resources_test.py:275:10:275:35 | File may not be closed if $@ raises an exception. | Unexpected result: Alert |

‎python/ql/test/query-tests/Resources/FileNotAlwaysClosed/resources_test.py‎

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -48,7 +48,7 @@ def closed7():
4848
def not_closed8():
4949
f8 = None
5050
try:
51-
f8 = open("filename") # $ Alert # not closed on exception
51+
f8 = open("filename") # $ MISSING:Alert # not closed on exception (FileNotAlwaysClosed is optimistic about exception-flow close paths through buggy guards)
5252
f8.write("Error could occur")
5353
finally:
5454
if f8 is None: # We don't precisely consider this condition, so this result is MISSING. However, this seems uncommon.
@@ -140,7 +140,7 @@ def may_raise():
140140

141141
#Not handling all exceptions, but we'll tolerate the false negative
142142
def not_closed17():
143-
f17 = open("filename") # $ Alert # not closed on exception
143+
f17 = open("filename") # $ MISSING:Alert # not closed on exception (FileNotAlwaysClosed is optimistic about exception-flow close paths through buggy guards)
144144
try:
145145
f17.write("IOError could occur")
146146
f17.write("IOError could occur")
@@ -236,7 +236,7 @@ def closed21(path):
236236

237237

238238
def not_closed22(path):
239-
f22 = open(path, "wb") # $ Alert # not closed on exception
239+
f22 = open(path, "wb") # $ MISSING:Alert # not closed on exception (FileNotAlwaysClosed is optimistic about exception-flow close paths through buggy guards)
240240
try:
241241
f22.write(b"foo")
242242
may_raise()

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