github
diff --git a/‎python/ql/lib/semmle/python/controlflow/internal/AstNodeImpl.qll‎
Lines changed: 88 additions & 0 deletions b/‎python/ql/lib/semmle/python/controlflow/internal/AstNodeImpl.qll‎
Lines changed: 88 additions & 0 deletions
diff --git a/‎python/ql/test/library-tests/ControlFlow/bindings/dead_under_no_raise.py‎
Lines changed: 19 additions & 18 deletions b/‎python/ql/test/library-tests/ControlFlow/bindings/dead_under_no_raise.py‎
Lines changed: 19 additions & 18 deletions
@@ -1553,6 +1553,89 @@ private module Input implements InputSig1, InputSig2 {
 
   private string assertThrowTag() { result = "[assert-throw]" }
 
+  /**
+   * Holds if the AST node `n` may raise an exception at runtime as part of
+   * its normal evaluation (not via an explicit `raise`/`assert`, which are
+   * modelled separately).
+   *
+   * The set mirrors what the legacy CFG used to flag implicitly: function
+   * calls (anything can raise), attribute access (`AttributeError`),
+   * subscript access (`IndexError`/`KeyError`/`TypeError`), arithmetic and
+   * comparison operators (`TypeError`/`ZeroDivisionError`), imports
+   * (`ImportError`/`ModuleNotFoundError`), and generator/coroutine
+   * suspension points (`await`/`yield`/`yield from`).
+   *
+   * Bare `Name` reads are intentionally excluded — modelling every name
+   * read as `mayThrow` would explode CFG edge count for negligible
+   * analysis value. `BoolExpr`/`IfExp` containers are also excluded; the
+   * operands they evaluate contribute their own exception edges.
+   */
+  private predicate exprMayThrow(Py::Expr e) {
+    e instanceof Py::Call
+    or
+    e instanceof Py::Attribute
+    or
+    e instanceof Py::Subscript
+    or
+    e instanceof Py::BinaryExpr
+    or
+    e instanceof Py::UnaryExpr
+    or
+    e instanceof Py::Compare
+    or
+    e instanceof Py::ImportExpr
+    or
+    e instanceof Py::ImportMember
+    or
+    e instanceof Py::Await
+    or
+    e instanceof Py::Yield
+    or
+    e instanceof Py::YieldFrom
+  }
+
+  /**
+   * Holds if the statement `s` may raise an exception at runtime as part
+   * of its normal evaluation. Currently restricted to `from m import *`
+   * (which performs the import as a statement-level side effect).
+   */
+  private predicate stmtMayThrow(Py::Stmt s) { s instanceof Py::ImportStar }
+
+  /**
+   * Holds if `n` is syntactically inside the body, handlers, `else`, or
+   * `finally` of a `try` statement (or the body of a `with` statement,
+   * which compiles to an implicit try/finally for `__exit__`) in the
+   * same scope.
+   *
+   * This mirrors Java's `ControlFlowGraph::mayThrow`, which only emits
+   * exception edges when there is local exception handling that would
+   * observe them. Outside such contexts, exception edges would add CFG
+   * complexity (weakening BarrierGuard precision and breaking SSA
+   * continuity around augmented assignments and subscript stores)
+   * without any analysis benefit, since exceptions just propagate to
+   * the function exit anyway.
+   */
+  private predicate inExceptionContext(Py::AstNode py) {
+    exists(Py::Try t | t.containsInScope(py))
+    or
+    exists(Py::With w | w.containsInScope(py))
+  }
+
+  /**
+   * Holds if `n` may raise an exception during normal evaluation. See
+   * `exprMayThrow` and `stmtMayThrow` for the included AST classes.
+   *
+   * Restricted to nodes inside a `try`/`with` statement: matches Java's
+   * approach of only modelling exception flow where it can be observed
+   * by local handling.
+   */
+  private predicate mayThrow(Ast::AstNode n) {
+    exists(Py::AstNode py | py = n.asExpr() or py = n.asStmt() |
+      (exprMayThrow(py) or stmtMayThrow(py)) and
+      inExceptionContext(py)
+    )
+  }
+
   predicate additionalNode(Ast::AstNode n, string tag, NormalSuccessor t) {
     n instanceof Ast::AssertStmt and tag = assertThrowTag() and t instanceof DirectSuccessor
   }
@@ -1564,6 +1647,11 @@ private module Input implements InputSig1, InputSig2 {
     n.isAdditional(ast, assertThrowTag()) and
     c.asSimpleAbruptCompletion() instanceof ExceptionSuccessor and
     always = true
+    or
+    mayThrow(ast) and
+    n.isIn(ast) and
+    c.asSimpleAbruptCompletion() instanceof ExceptionSuccessor and
+    always = false
   }
 
   predicate endAbruptCompletion(Ast::AstNode ast, PreControlFlowNode n, AbruptCompletion c) {
 
@@ -1,15 +1,15 @@
-# Dead bindings under the "no expressions raise" CFG abstraction.
+# Reachability of code following a try whose body always returns.
 #
-# The new CFG does not currently model raise edges from arbitrary
-# expressions. As a consequence, code that is only reachable through
-# exception flow is (correctly) classified as dead and has no CFG node.
-# Variable bindings in dead code do not need CFG nodes - SSA / dataflow
-# over dead code is moot.
+# The new CFG models exception edges for raise-prone expressions when
+# they appear inside a `try` (or `with`) statement, mirroring Java's
+# `mayThrow`. This means the body of a `try` has both a normal
+# completion edge and an exception edge to its handlers, so code
+# following the try-statement is reachable via the except-handler path
+# even when the try-body would otherwise always return.
 #
-# These tests act as a regression guard: the bindings below intentionally
-# have no `cfgdefines=` annotations. If raise modelling is later added,
-# the BindingsTest infrastructure will surface the new CFG nodes as
-# unexpected results, and this file will need to be revisited.
+# Code that is not reachable under either normal or exception flow
+# (for example, the `else` clause of a try whose body unconditionally
+# raises) remains correctly classified as dead.
 
 
 def f(obj):  # $ cfgdefines=f cfgdefines=obj
@@ -18,12 +18,12 @@ def f(obj):  # $ cfgdefines=f cfgdefines=obj
     except TypeError:
         pass
 
-    # The first try's body always returns; its except handler does not
-    # raise or otherwise transfer control, so under "no expressions
-    # raise" the only paths out of the try-statement are dead. Everything
-    # below is unreachable.
+    # The try-body always returns, but `len(obj)` can raise (it is
+    # inside the try, so we model its exception edge). The
+    # `except TypeError: pass` handler falls through to here, making
+    # the code below reachable.
     try:
-        hint = type(obj).__length_hint__
+        hint = type(obj).__length_hint__  # $ cfgdefines=hint
     except AttributeError:
         return None
     return hint
@@ -35,7 +35,8 @@ def g():  # $ cfgdefines=g
     except:
         raise Exception("outer")
     else:
-        # Unreachable: the inner try body always raises, so the `else:`
+        # Unreachable: the inner try body always raises (via an explicit
+        # `raise`, which is modelled unconditionally), so the `else:`
         # clause never runs.
         hit_inner_else = True
 
@@ -46,7 +47,7 @@ def h(cache, key):  # $ cfgdefines=h cfgdefines=cache cfgdefines=key
     except KeyError:
         pass
 
-    # Same pattern as `f`: dead under "no expressions raise".
-    value = compute(key)
+    # Same pattern as `f`: reachable via the except-handler fall-through.
+    value = compute(key)  # $ cfgdefines=value
     cache[key] = value
     return value