Python: refactor getChild into per-class OO dispatch

Replace the single ~240-line top-level getChild predicate with one
override per AST class. AstNode declares a default

  AstNode getChild(int index) { none() }

and each subclass with children overrides it (41 classes total).
The top-level predicate becomes a one-line dispatch:

  AstNode getChild(AstNode n, int index) { result = n.getChild(index) }

No behavioral change: NewCfg evaluation-order tests still pass at the
same 22/24 baseline, and all 11 shared-CFG consistency queries still
report 0 violations on CPython.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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

@@ -143,6 +143,13 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the underlying Python `Pattern`, if this node wraps one. */
     Py::Pattern asPattern() { this = TPattern(result) }
+
+    /**
+     * Gets the child of this AST node at the specified (zero-based)
+     * index, in evaluation order. Subclasses with children override
+     * this method.
+     */
+    AstNode getChild(int index) { none() }
   }
 
   /** Gets the immediately enclosing callable that contains `node`. */
@@ -186,6 +193,8 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the last statement in this block. */
     Stmt getLastStmt() { result = TStmt(getBodyStmtList(parent, slot).getLastItem()) }
+
+    override AstNode getChild(int index) { result = this.getStmt(index) }
   }
 
   /** An expression statement. */
@@ -196,6 +205,8 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the expression in this expression statement. */
     Expr getExpr() { result = TExpr(exprStmt.getValue()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getExpr() }
   }
 
   /** An assignment statement (`x = y = expr`). */
@@ -209,6 +220,12 @@ module Ast implements AstSig<Py::Location> {
     Expr getTarget(int n) { result = TExpr(assign.getTarget(n)) }
 
     int getNumberOfTargets() { result = count(assign.getATarget()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getValue()
+      or
+      result = this.getTarget(index - 1) and index >= 1
+    }
   }
 
   /** An augmented assignment statement (`x += expr`). */
@@ -218,6 +235,8 @@ module Ast implements AstSig<Py::Location> {
     AugAssignStmt() { augAssign = this.asStmt() }
 
     Expr getOperation() { result = TExpr(augAssign.getOperation()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getOperation() }
   }
 
   /** An assignment expression / walrus operator (`x := expr`). */
@@ -229,6 +248,12 @@ module Ast implements AstSig<Py::Location> {
     Expr getValue() { result = TExpr(assignExpr.getValue()) }
 
     Expr getTarget() { result = TExpr(assignExpr.getTarget()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getValue()
+      or
+      index = 1 and result = this.getTarget()
+    }
   }
 
   /**
@@ -256,6 +281,14 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the `else` (false) branch, if any. */
     Stmt getElse() { result = TBlockStmt(ifStmt, "orelse") }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getCondition()
+      or
+      index = 1 and result = this.getThen()
+      or
+      index = 2 and result = this.getElse()
+    }
   }
 
   /** A loop statement. */
@@ -279,6 +312,14 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the `else` branch of this `while` loop, if any. */
     Stmt getElse() { result = TBlockStmt(whileStmt, "orelse") }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getCondition()
+      or
+      index = 1 and result = this.getBody()
+      or
+      index = 2 and result = this.getElse()
+    }
   }
 
   /**
@@ -317,6 +358,16 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the `else` branch of this `for` loop, if any. */
     Stmt getElse() { result = TBlockStmt(forStmt, "orelse") }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getCollection()
+      or
+      index = 1 and result = this.getVariable()
+      or
+      index = 2 and result = this.getBody()
+      or
+      index = 3 and result = this.getElse()
+    }
   }
 
   /** A `break` statement. */
@@ -342,6 +393,8 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the expression being returned, if any. */
     Expr getExpr() { result = TExpr(ret.getValue()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getExpr() }
   }
 
   /** A `raise` statement (mapped to `Throw`). */
@@ -355,6 +408,12 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the cause of this `raise`, if any. */
     Expr getCause() { result = TExpr(raise.getCause()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getExpr()
+      or
+      index = 1 and result = this.getCause()
+    }
   }
 
   /** A `with` statement. */
@@ -368,6 +427,14 @@ module Ast implements AstSig<Py::Location> {
     Expr getOptionalVars() { result = TExpr(withStmt.getOptionalVars()) }
 
     Stmt getBody() { result = TBlockStmt(withStmt, "body") }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getContextExpr()
+      or
+      index = 1 and result = this.getOptionalVars()
+      or
+      index = 2 and result = this.getBody()
+    }
   }
 
   /** An `assert` statement. */
@@ -379,6 +446,12 @@ module Ast implements AstSig<Py::Location> {
     Expr getTest() { result = TExpr(assertStmt.getTest()) }
 
     Expr getMsg() { result = TExpr(assertStmt.getMsg()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getTest()
+      or
+      index = 1 and result = this.getMsg()
+    }
   }
 
   /** A `delete` statement. */
@@ -388,6 +461,8 @@ module Ast implements AstSig<Py::Location> {
     DeleteStmt() { del = this.asStmt() }
 
     Expr getTarget(int n) { result = TExpr(del.getTarget(n)) }
+
+    override AstNode getChild(int index) { result = this.getTarget(index) }
   }
 
   /** A `try` statement. */
@@ -404,6 +479,16 @@ module Ast implements AstSig<Py::Location> {
     Stmt getFinally() { result = TBlockStmt(tryStmt, "finally") }
 
     CatchClause getCatch(int index) { result = TStmt(tryStmt.getHandler(index)) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getBody()
+      or
+      result = this.getCatch(index - 1) and index >= 1
+      or
+      index = -1 and result = this.getFinally()
+      or
+      index = -2 and result = this.getElse()
+    }
   }
 
   /**
@@ -442,6 +527,14 @@ module Ast implements AstSig<Py::Location> {
       or
       result = TBlockStmt(handler.(Py::ExceptGroupStmt), "body")
     }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getType()
+      or
+      index = 1 and result = this.getVariable()
+      or
+      index = 2 and result = this.getBody()
+    }
   }
 
   /** A `match` statement, mapped to the shared CFG's `Switch`. */
@@ -455,6 +548,12 @@ module Ast implements AstSig<Py::Location> {
     Case getCase(int index) { result = TStmt(matchStmt.getCase(index)) }
 
     Stmt getStmt(int index) { none() }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getExpr()
+      or
+      result = this.getCase(index - 1) and index >= 1
+    }
   }
 
   /** A `case` clause in a match statement. */
@@ -471,6 +570,14 @@ module Ast implements AstSig<Py::Location> {
 
     /** Holds if this case is a wildcard pattern (`case _:`). */
     predicate isWildcard() { caseStmt.getPattern() instanceof Py::MatchWildcardPattern }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getAPattern()
+      or
+      index = 1 and result = this.getGuard()
+      or
+      index = 2 and result = this.getBody()
+    }
   }
 
   /** A wildcard case (`case _:`). */
@@ -492,6 +599,14 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the false branch of this expression. */
     Expr getElse() { result = TExpr(ifExp.getOrelse()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getCondition()
+      or
+      index = 1 and result = this.getThen()
+      or
+      index = 2 and result = this.getElse()
+    }
   }
 
   /**
@@ -547,6 +662,12 @@ module Ast implements AstSig<Py::Location> {
         result = TBoolExprPair(be, i + 1)
       )
     }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getLeftOperand()
+      or
+      index = 1 and result = this.getRightOperand()
+    }
   }
 
   /** A short-circuiting logical `and` expression. */
@@ -582,6 +703,8 @@ module Ast implements AstSig<Py::Location> {
 
     /** Gets the operand of this unary expression. */
     Expr getOperand() { result = TExpr(this.asExpr().(Py::UnaryExpr).getOperand()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getOperand() }
   }
 
   /** A logical `not` expression. */
@@ -633,6 +756,12 @@ module Ast implements AstSig<Py::Location> {
     Expr getLeft() { result = TExpr(binExpr.getLeft()) }
 
     Expr getRight() { result = TExpr(binExpr.getRight()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getLeft()
+      or
+      index = 1 and result = this.getRight()
+    }
   }
 
   /** A call expression (`func(args...)`). */
@@ -654,6 +783,15 @@ module Ast implements AstSig<Py::Location> {
     }
 
     int getNumberOfNamedArgs() { result = count(call.getANamedArg()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getFunc()
+      or
+      result = this.getPositionalArg(index - 1) and index >= 1
+      or
+      result = this.getKeywordValue(index - 1 - this.getNumberOfPositionalArgs()) and
+      index >= 1 + this.getNumberOfPositionalArgs()
+    }
   }
 
   /** A subscript expression (`obj[index]`). */
@@ -665,6 +803,12 @@ module Ast implements AstSig<Py::Location> {
     Expr getObject() { result = TExpr(sub.getObject()) }
 
     Expr getIndex() { result = TExpr(sub.getIndex()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getObject()
+      or
+      index = 1 and result = this.getIndex()
+    }
   }
 
   /** An attribute access (`obj.name`). */
@@ -674,6 +818,8 @@ module Ast implements AstSig<Py::Location> {
     AttributeExpr() { attr = this.asExpr() }
 
     Expr getObject() { result = TExpr(attr.getObject()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getObject() }
   }
 
   /** A tuple literal. */
@@ -683,6 +829,8 @@ module Ast implements AstSig<Py::Location> {
     TupleExpr() { tuple = this.asExpr() }
 
     Expr getElt(int n) { result = TExpr(tuple.getElt(n)) }
+
+    override AstNode getChild(int index) { result = this.getElt(index) }
   }
 
   /** A list literal. */
@@ -692,6 +840,8 @@ module Ast implements AstSig<Py::Location> {
     ListExpr() { list = this.asExpr() }
 
     Expr getElt(int n) { result = TExpr(list.getElt(n)) }
+
+    override AstNode getChild(int index) { result = this.getElt(index) }
   }
 
   /** A set literal. */
@@ -701,6 +851,8 @@ module Ast implements AstSig<Py::Location> {
     SetExpr() { set = this.asExpr() }
 
     Expr getElt(int n) { result = TExpr(set.getElt(n)) }
+
+    override AstNode getChild(int index) { result = this.getElt(index) }
   }
 
   /** A dict literal. */
@@ -718,6 +870,14 @@ module Ast implements AstSig<Py::Location> {
     Expr getValue(int n) { result = TExpr(dict.getItem(n).(Py::KeyValuePair).getValue()) }
 
     int getNumberOfItems() { result = count(dict.getAnItem()) }
+
+    override AstNode getChild(int index) {
+      exists(int item |
+        index = 2 * item and result = this.getKey(item)
+        or
+        index = 2 * item + 1 and result = this.getValue(item)
+      )
+    }
   }
 
   /** A unary expression other than `not` (e.g., `-x`, `+x`, `~x`). */
@@ -727,6 +887,8 @@ module Ast implements AstSig<Py::Location> {
     ArithUnaryExpr() { unaryExpr = this.asExpr() and not unaryExpr.getOp() instanceof Py::Not }
 
     Expr getOperand() { result = TExpr(unaryExpr.getOperand()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getOperand() }
   }
 
   /**
@@ -748,6 +910,8 @@ module Ast implements AstSig<Py::Location> {
     }
 
     Expr getIterable() { result = TExpr(iterable) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getIterable() }
   }
 
   /** A comparison expression (`a < b`, `a < b < c`, etc.). */
@@ -759,6 +923,12 @@ module Ast implements AstSig<Py::Location> {
     Expr getLeft() { result = TExpr(cmp.getLeft()) }
 
     Expr getComparator(int n) { result = TExpr(cmp.getComparator(n)) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getLeft()
+      or
+      result = this.getComparator(index - 1) and index >= 1
+    }
   }
 
   /** A slice expression (`start:stop:step`). */
@@ -772,6 +942,14 @@ module Ast implements AstSig<Py::Location> {
     Expr getStop() { result = TExpr(slice.getStop()) }
 
     Expr getStep() { result = TExpr(slice.getStep()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getStart()
+      or
+      index = 1 and result = this.getStop()
+      or
+      index = 2 and result = this.getStep()
+    }
   }
 
   /** A starred expression (`*x`). */
@@ -781,6 +959,8 @@ module Ast implements AstSig<Py::Location> {
     StarredExpr() { starred = this.asExpr() }
 
     Expr getValue() { result = TExpr(starred.getValue()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getValue() }
   }
 
   /** A formatted string literal (`f"...{expr}..."`). */
@@ -790,6 +970,8 @@ module Ast implements AstSig<Py::Location> {
     FstringExpr() { fstring = this.asExpr() }
 
     Expr getValue(int n) { result = TExpr(fstring.getValue(n)) }
+
+    override AstNode getChild(int index) { result = this.getValue(index) }
   }
 
   /** A formatted value inside an f-string (`{expr}` or `{expr:spec}`). */
@@ -801,6 +983,12 @@ module Ast implements AstSig<Py::Location> {
     Expr getValue() { result = TExpr(fv.getValue()) }
 
     Expr getFormatSpec() { result = TExpr(fv.getFormatSpec()) }
+
+    override AstNode getChild(int index) {
+      index = 0 and result = this.getValue()
+      or
+      index = 1 and result = this.getFormatSpec()
+    }
   }
 
   /** A `yield` expression. */
@@ -810,6 +998,8 @@ module Ast implements AstSig<Py::Location> {
     YieldExpr() { yield = this.asExpr() }
 
     Expr getValue() { result = TExpr(yield.getValue()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getValue() }
   }
 
   /** A `yield from` expression. */
@@ -819,6 +1009,8 @@ module Ast implements AstSig<Py::Location> {
     YieldFromExpr() { yieldFrom = this.asExpr() }
 
     Expr getValue() { result = TExpr(yieldFrom.getValue()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getValue() }
   }
 
   /** An `await` expression. */
@@ -828,6 +1020,8 @@ module Ast implements AstSig<Py::Location> {
     AwaitExpr() { await = this.asExpr() }
 
     Expr getValue() { result = TExpr(await.getValue()) }
+
+    override AstNode getChild(int index) { index = 0 and result = this.getValue() }
   }
 
   /** A class definition expression (has base classes evaluated at definition time). */
@@ -837,6 +1031,8 @@ module Ast implements AstSig<Py::Location> {
     ClassDefExpr() { classExpr = this.asExpr() }
 
     Expr getBase(int n) { result = TExpr(classExpr.getBase(n)) }
+
+    override AstNode getChild(int index) { result = this.getBase(index) }
   }
 
   /** A function definition expression (has default args evaluated at definition time). */
@@ -863,6 +1059,12 @@ module Ast implements AstSig<Py::Location> {
     }
 
     int getNumberOfDefaults() { result = count(funcExpr.getArgs().getADefault()) }
+
+    override AstNode getChild(int index) {
+      result = this.getDefault(index)
+      or
+      result = this.getKwDefault(index - this.getNumberOfDefaults())
+    }
   }
 
   /** A lambda expression (has default args evaluated at definition time). */
@@ -884,248 +1086,16 @@ module Ast implements AstSig<Py::Location> {
     }
 
     int getNumberOfDefaults() { result = count(lambda.getArgs().getADefault()) }
+
+    override AstNode getChild(int index) {
+      result = this.getDefault(index)
+      or
+      result = this.getKwDefault(index - this.getNumberOfDefaults())
+    }
   }
 
   /** Gets the child of `n` at the specified (zero-based) index. */
-  AstNode getChild(AstNode n, int index) {
-    // BlockStmt: indexed statements
-    result = n.(BlockStmt).getStmt(index)
-    or
-    // IfStmt: condition (0), then (1), else (2)
-    exists(IfStmt ifStmt | ifStmt = n |
-      index = 0 and result = ifStmt.getCondition()
-      or
-      index = 1 and result = ifStmt.getThen()
-      or
-      index = 2 and result = ifStmt.getElse()
-    )
-    or
-    // ExprStmt: the expression (0)
-    index = 0 and result = n.(ExprStmt).getExpr()
-    or
-    // Assign: value (0), targets (1..n)
-    exists(AssignStmt a | a = n |
-      index = 0 and result = a.getValue()
-      or
-      result = a.getTarget(index - 1) and index >= 1
-    )
-    or
-    // AugAssign: the operation (0)
-    index = 0 and result = n.(AugAssignStmt).getOperation()
-    or
-    // Walrus (`x := expr`): value (0), target (1)
-    exists(NamedExpr ne | ne = n |
-      index = 0 and result = ne.getValue()
-      or
-      index = 1 and result = ne.getTarget()
-    )
-    or
-    // WhileStmt: condition (0), body (1), orelse (2)
-    exists(WhileStmt w | w = n |
-      index = 0 and result = w.getCondition()
-      or
-      index = 1 and result = w.getBody()
-      or
-      index = 2 and result = w.getElse()
-    )
-    or
-    // ForeachStmt: collection (0), variable (1), body (2), orelse (3)
-    exists(ForeachStmt f | f = n |
-      index = 0 and result = f.getCollection()
-      or
-      index = 1 and result = f.getVariable()
-      or
-      index = 2 and result = f.getBody()
-      or
-      index = 3 and result = f.getElse()
-    )
-    or
-    // ReturnStmt: the value (0)
-    index = 0 and result = n.(ReturnStmt).getExpr()
-    or
-    // AssertStmt: test (0), message (1)
-    exists(AssertStmt a | a = n |
-      index = 0 and result = a.getTest()
-      or
-      index = 1 and result = a.getMsg()
-    )
-    or
-    // DeleteStmt: targets left to right
-    result = n.(DeleteStmt).getTarget(index)
-    or
-    // WithStmt: context expr (0), optional vars (1), body (2)
-    exists(WithStmt w | w = n |
-      index = 0 and result = w.getContextExpr()
-      or
-      index = 1 and result = w.getOptionalVars()
-      or
-      index = 2 and result = w.getBody()
-    )
-    or
-    // Throw (raise): exception (0), cause (1)
-    exists(Throw r | r = n |
-      index = 0 and result = r.getExpr()
-      or
-      index = 1 and result = r.getCause()
-    )
-    or
-    // TryStmt: body (0), handlers (1..n), else (-2), finally (-1)
-    exists(TryStmt t | t = n |
-      index = 0 and result = t.getBody()
-      or
-      result = t.getCatch(index - 1) and index >= 1
-      or
-      index = -1 and result = t.getFinally()
-      or
-      index = -2 and result = t.getElse()
-    )
-    or
-    // Switch (match): subject (0), cases (1..n)
-    exists(Switch m | m = n |
-      index = 0 and result = m.getExpr()
-      or
-      result = m.getCase(index - 1) and index >= 1
-    )
-    or
-    // Case: pattern (0), guard (1), body (2)
-    exists(Case c | c = n |
-      index = 0 and result = c.getAPattern()
-      or
-      index = 1 and result = c.getGuard()
-      or
-      index = 2 and result = c.getBody()
-    )
-    or
-    // CatchClause (except handler): type (0), name (1), body (2)
-    exists(CatchClause h | h = n |
-      index = 0 and result = h.getType()
-      or
-      index = 1 and result = h.getVariable()
-      or
-      index = 2 and result = h.getBody()
-    )
-    or
-    // ConditionalExpr (IfExp): condition (0), then (1), else (2)
-    exists(ConditionalExpr ie | ie = n |
-      index = 0 and result = ie.getCondition()
-      or
-      index = 1 and result = ie.getThen()
-      or
-      index = 2 and result = ie.getElse()
-    )
-    or
-    // Call: func (0), positional args (1..n), keyword values (n+1..n+k)
-    exists(CallExpr call | call = n |
-      index = 0 and result = call.getFunc()
-      or
-      result = call.getPositionalArg(index - 1) and index >= 1
-      or
-      result = call.getKeywordValue(index - 1 - call.getNumberOfPositionalArgs()) and
-      index >= 1 + call.getNumberOfPositionalArgs()
-    )
-    or
-    // Python BinaryExpr (arithmetic, bitwise, matmul, etc.): left (0), right (1)
-    exists(ArithBinaryExpr be | be = n |
-      index = 0 and result = be.getLeft()
-      or
-      index = 1 and result = be.getRight()
-    )
-    or
-    // Subscript (obj[index]): object (0), index (1)
-    exists(SubscriptExpr sub | sub = n |
-      index = 0 and result = sub.getObject()
-      or
-      index = 1 and result = sub.getIndex()
-    )
-    or
-    // Attribute (obj.name): object (0)
-    index = 0 and result = n.(AttributeExpr).getObject()
-    or
-    // Comprehension/generator: iterable (0)
-    index = 0 and result = n.(Comprehension).getIterable()
-    or
-    // Tuple, List, Set: elements left to right
-    result = n.(TupleExpr).getElt(index)
-    or
-    result = n.(ListExpr).getElt(index)
-    or
-    result = n.(SetExpr).getElt(index)
-    or
-    // Dict: key(0), value(0), key(1), value(1), ...
-    exists(DictExpr d, int item | d = n |
-      index = 2 * item and result = d.getKey(item)
-      or
-      index = 2 * item + 1 and result = d.getValue(item)
-    )
-    or
-    // Arithmetic unary (-x, +x, ~x): operand (0)
-    index = 0 and result = n.(ArithUnaryExpr).getOperand()
-    or
-    // Compare (a < b < c): left (0), comparators (1..n)
-    exists(CompareExpr cmp | cmp = n |
-      index = 0 and result = cmp.getLeft()
-      or
-      result = cmp.getComparator(index - 1) and index >= 1
-    )
-    or
-    // Slice (start:stop:step): start (0), stop (1), step (2)
-    exists(SliceExpr sl | sl = n |
-      index = 0 and result = sl.getStart()
-      or
-      index = 1 and result = sl.getStop()
-      or
-      index = 2 and result = sl.getStep()
-    )
-    or
-    // Starred (*x): value (0)
-    index = 0 and result = n.(StarredExpr).getValue()
-    or
-    // Fstring: values left to right
-    result = n.(FstringExpr).getValue(index)
-    or
-    // FormattedValue ({expr} or {expr:spec}): value (0), format spec (1)
-    exists(FormattedValueExpr fv | fv = n |
-      index = 0 and result = fv.getValue()
-      or
-      index = 1 and result = fv.getFormatSpec()
-    )
-    or
-    // Yield: value (0)
-    index = 0 and result = n.(YieldExpr).getValue()
-    or
-    // YieldFrom: value (0)
-    index = 0 and result = n.(YieldFromExpr).getValue()
-    or
-    // Await: value (0)
-    index = 0 and result = n.(AwaitExpr).getValue()
-    or
-    // ClassExpr: base classes left to right
-    result = n.(ClassDefExpr).getBase(index)
-    or
-    // FunctionExpr: defaults left to right, then kw defaults
-    exists(FunctionDefExpr fe | fe = n |
-      result = fe.getDefault(index)
-      or
-      result = fe.getKwDefault(index - fe.getNumberOfDefaults())
-    )
-    or
-    // Lambda: defaults left to right, then kw defaults
-    exists(LambdaExpr lam | lam = n |
-      result = lam.getDefault(index)
-      or
-      result = lam.getKwDefault(index - lam.getNumberOfDefaults())
-    )
-    or
-    // LogicalNotExpr: operand (0)
-    index = 0 and result = n.(LogicalNotExpr).getOperand()
-    or
-    // BinaryExpr (`and`/`or`): left (0), right (1)
-    exists(BinaryExpr be | be = n |
-      index = 0 and result = be.getLeftOperand()
-      or
-      index = 1 and result = be.getRightOperand()
-    )
-  }
+  AstNode getChild(AstNode n, int index) { result = n.getChild(index) }
 }
 
 private module Cfg0 = Make0<Py::Location, Ast>;