Start with python match statement (#1801)

* Start with python match statement

* fix bug, add test

* More testing

* Add implicit break

* Review feedback

* nullable MatchSingleton: comment and handling

cpg-core/src/main/kotlin/de/fraunhofer/aisec/cpg/graph/statements/SwitchStatement.kt

@@ -36,7 +36,7 @@ import org.neo4j.ogm.annotation.Relationship
 
 /**
  * Represents a Java or C++ switch statement of the `switch (selector) {...}` that can include case
- * and default statements. Break statements break out of the switch and labeled breaks in JAva are
+ * and default statements. Break statements break out of the switch and labeled breaks in Java are
  * handled properly.
  */
 class SwitchStatement : Statement(), BranchingNode {
@@ -51,7 +51,7 @@ class SwitchStatement : Statement(), BranchingNode {
 
     @Relationship(value = "SELECTOR_DECLARATION")
     var selectorDeclarationEdge = astOptionalEdgeOf<Declaration>()
-    /** C++ allows to use a declaration instead of a expression as selector */
+    /** C++ allows to use a declaration instead of an expression as selector */
     var selectorDeclaration by unwrapping(SwitchStatement::selectorDeclarationEdge)
 
     @Relationship(value = "STATEMENT") var statementEdge = astOptionalEdgeOf<Statement>()

cpg-language-python/src/main/kotlin/de/fraunhofer/aisec/cpg/frontends/python/ExpressionHandler.kt

@@ -237,19 +237,24 @@ class ExpressionHandler(frontend: PythonLanguageFrontend) :
      * where the first element in [nodes] is the lhs of the root of the tree of binary operators.
      * The last operands are further down the tree.
      */
-    private fun joinListWithBinOp(
+    internal fun joinListWithBinOp(
         operatorCode: String,
         nodes: List<Expression>,
-        rawNode: Python.AST.AST? = null
+        rawNode: Python.AST.AST? = null,
+        isImplicit: Boolean = true
     ): BinaryOperator {
-        val lastTwo = newBinaryOperator(operatorCode, rawNode = rawNode)
-        lastTwo.rhs = nodes.last()
-        lastTwo.lhs = nodes[nodes.size - 2]
+        val lastTwo =
+            newBinaryOperator(operatorCode = operatorCode, rawNode = rawNode).apply {
+                rhs = nodes.last()
+                lhs = nodes[nodes.size - 2]
+                this.isImplicit = isImplicit
+            }
         return nodes.subList(0, nodes.size - 2).foldRight(lastTwo) { newVal, start ->
-            val nextValue = newBinaryOperator(operatorCode)
-            nextValue.rhs = start
-            nextValue.lhs = newVal
-            nextValue
+            newBinaryOperator(operatorCode = operatorCode, rawNode = rawNode).apply {
+                rhs = start
+                lhs = newVal
+                this.isImplicit = isImplicit
+            }
         }
     }
 
@@ -297,18 +302,12 @@ class ExpressionHandler(frontend: PythonLanguageFrontend) :
                 rawNode = node
             )
         } else {
-            // Start with the last two operands, then keep prepending the previous ones until the
-            // list is finished.
-            val lastTwo = newBinaryOperator(op, rawNode = node)
-            lastTwo.rhs = handle(node.values.last())
-            lastTwo.lhs = handle(node.values[node.values.size - 2])
-            return node.values.subList(0, node.values.size - 2).foldRight(lastTwo) { newVal, start
-                ->
-                val nextValue = newBinaryOperator(op, rawNode = node)
-                nextValue.rhs = start
-                nextValue.lhs = handle(newVal)
-                nextValue
-            }
+            joinListWithBinOp(
+                operatorCode = op,
+                nodes = node.values.map(::handle),
+                rawNode = node,
+                isImplicit = true
+            )
         }
     }
 

cpg-language-python/src/main/kotlin/de/fraunhofer/aisec/cpg/frontends/python/Python.kt

@@ -1153,7 +1153,12 @@ interface Python {
          * ```
          */
         class MatchSingleton(pyObject: PyObject) : BasePattern(pyObject) {
-            val value: Any by lazy { "value" of pyObject }
+            /**
+             * [value] is not optional. We have to make it nullable though because the value will be
+             * set to `null` if the case matches on `None`. This is known behavior of jep (similar
+             * to literals/constants).
+             */
+            val value: Any? by lazy { "value" of pyObject }
         }
 
         /**

cpg-language-python/src/main/kotlin/de/fraunhofer/aisec/cpg/frontends/python/StatementHandler.kt

@@ -77,15 +77,134 @@ class StatementHandler(frontend: PythonLanguageFrontend) :
             is Python.AST.Global -> handleGlobal(node)
             is Python.AST.Nonlocal -> handleNonLocal(node)
             is Python.AST.Raise -> handleRaise(node)
-            is Python.AST.Match,
+            is Python.AST.Match -> handleMatch(node)
             is Python.AST.TryStar ->
                 newProblemExpression(
-                    "The statement of class ${node.javaClass} is not supported yet",
+                    problem = "The statement of class ${node.javaClass} is not supported yet",
                     rawNode = node
                 )
         }
     }
 
+    /**
+     * Translates a pattern which can be used by a `match_case`. There are various options available
+     * and all of them are translated to traditional comparisons and logical expressions which could
+     * also be seen in the condition of an if-statement.
+     */
+    private fun handlePattern(node: Python.AST.BasePattern, subject: String): Expression {
+        return when (node) {
+            is Python.AST.MatchValue ->
+                newBinaryOperator(operatorCode = "==", rawNode = node).implicit().apply {
+                    this.lhs = newReference(name = subject)
+                    this.rhs = frontend.expressionHandler.handle(ctx = node.value)
+                }
+            is Python.AST.MatchSingleton ->
+                newBinaryOperator(operatorCode = "===", rawNode = node).implicit().apply {
+                    this.lhs = newReference(name = subject)
+                    this.rhs =
+                        when (val value = node.value) {
+                            is Python.AST.BaseExpr -> frontend.expressionHandler.handle(ctx = value)
+                            null -> newLiteral(value = null, rawNode = node)
+                            else ->
+                                newProblemExpression(
+                                    problem =
+                                        "Can't handle ${value::class} in value of Python.AST.MatchSingleton yet"
+                                )
+                        }
+                }
+            is Python.AST.MatchOr ->
+                frontend.expressionHandler.joinListWithBinOp(
+                    operatorCode = "or",
+                    nodes = node.patterns.map { handlePattern(node = it, subject = subject) },
+                    rawNode = node,
+                    isImplicit = false
+                )
+            is Python.AST.MatchSequence,
+            is Python.AST.MatchMapping,
+            is Python.AST.MatchClass,
+            is Python.AST.MatchStar,
+            is Python.AST.MatchAs ->
+                newProblemExpression(
+                    problem = "Cannot handle of type ${node::class} yet",
+                    rawNode = node
+                )
+            else ->
+                newProblemExpression(
+                    problem = "Cannot handle of type ${node::class} yet",
+                    rawNode = node
+                )
+        }
+    }
+
+    /**
+     * Translates a [`match_case`](https://docs.python.org/3/library/ast.html#ast.match_case) to a
+     * [Block] which holds the [CaseStatement] and then all other statements of the
+     * [Python.AST.match_case.body].
+     *
+     * The [CaseStatement] is generated by the [Python.AST.match_case.pattern] and, if available,
+     * [Python.AST.match_case.guard]. A `guard` is modeled with an `AND` BinaryOperator in the
+     * [CaseStatement.caseExpression]. Its `lhs` is the normal pattern and the `rhs` is the guard.
+     * This is in line with [PEP 634](https://peps.python.org/pep-0634/).
+     */
+    private fun handleMatchCase(node: Python.AST.match_case, subject: String): List<Statement> {
+        val statements = mutableListOf<Statement>()
+        // First, we add the CaseStatement. A `MatchAs` without a `pattern` implies
+        // it's a default statement.
+        // We have to handle this here since we do not want to generate the CaseStatement in this
+        // case.
+        val pattern = node.pattern
+        val guard = node.guard
+        statements +=
+            if (pattern is Python.AST.MatchAs && pattern.pattern == null) {
+                newDefaultStatement(rawNode = pattern)
+            } else if (guard != null) {
+                newCaseStatement(rawNode = node).apply {
+                    this.caseExpression =
+                        newBinaryOperator(operatorCode = "and")
+                            .implicit(
+                                code = frontend.codeOf(astNode = node),
+                                location = frontend.locationOf(astNode = node)
+                            )
+                            .apply {
+                                this.lhs = handlePattern(node = node.pattern, subject = subject)
+                                this.rhs = frontend.expressionHandler.handle(ctx = guard)
+                            }
+                }
+            } else {
+                newCaseStatement(rawNode = node).apply {
+                    this.caseExpression = handlePattern(node = node.pattern, subject = subject)
+                }
+            }
+        // Now, we add the remaining body.
+        statements += node.body.map(::handle)
+        // Currently, the EOG pass requires a break statement to work as expected. For this reason,
+        // we insert an implicit break statement at the end of the block.
+        statements +=
+            newBreakStatement()
+                .implicit(
+                    code = frontend.codeOf(astNode = node),
+                    location = frontend.locationOf(astNode = node)
+                )
+        return statements
+    }
+
+    /**
+     * Translates a Python [`Match`](https://docs.python.org/3/library/ast.html#ast.Match) into a
+     * [SwitchStatement].
+     */
+    private fun handleMatch(node: Python.AST.Match): SwitchStatement =
+        newSwitchStatement(rawNode = node).apply {
+            val subject = frontend.expressionHandler.handle(ctx = node.subject)
+            this.selector = subject
+
+            this.statement =
+                node.cases.fold(initial = newBlock().implicit()) { block, case ->
+                    block.statements +=
+                        handleMatchCase(node = case, subject = subject.name.localName)
+                    block
+                }
+        }
+
     /**
      * Translates a Python [`Raise`](https://docs.python.org/3/library/ast.html#ast.Raise) into a
      * [ThrowExpression].