Add context_variable to CEL Env

Context_variable is a struct type that defines top-level variables
that can be directly referenced by a CEL policy.

PiperOrigin-RevId: 933417419

Author: dmitriplotnikov

cel_expr_python/cel.pyi

@@ -13,6 +13,8 @@ class CelExtensionBase:
   def __init__(self, name: str) -> None: ...
 
 class EnvConfig:
+  @property
+  def context_type(self) -> str: ...
   def to_yaml(self) -> str: ...
 
 class ExpressionContainer:

cel_expr_python/cel_env_test.py

@@ -98,6 +98,52 @@ def test_invalid_yaml(self):
         str(e.exception),
     )
 
+  def test_parse_context_variable_config(self):
+    config = cel.NewEnvConfigFromYaml("""
+      context_variable:
+        type_name: "cel.expr.conformance.proto2.TestAllTypes"
+    """)
+    self.assertEqual(
+        config.context_type, "cel.expr.conformance.proto2.TestAllTypes"
+    )
+
+  def test_parse_context_variable_config_alternative_syntax(self):
+    config = cel.NewEnvConfigFromYaml("""
+      context_variable:
+        type: "cel.expr.conformance.proto2.TestAllTypes"
+    """)
+    self.assertEqual(
+        config.context_type, "cel.expr.conformance.proto2.TestAllTypes"
+    )
+
+  def test_parse_context_variable_malformed(self):
+    with self.assertRaisesRegex(
+        Exception, "Node 'context_variable' is not a map"
+    ):
+      cel.NewEnvConfigFromYaml("context_variable: 123")
+
+  def test_parse_context_variable_malformed2(self):
+    with self.assertRaisesRegex(
+        Exception, "Node 'context_variable' does not have a valid type"
+    ):
+      cel.NewEnvConfigFromYaml("""
+        context_variable:
+          type:
+            foo: bar
+      """)
+
+  def test_context_variable_basic(self):
+    config = cel.NewEnvConfigFromYaml("""
+      context_variable:
+        type_name: "cel.expr.conformance.proto2.TestAllTypes"
+    """)
+    env = cel.NewEnv(config=config)
+    ast = env.compile("single_int32 > 10")
+    self.assertIsNotNone(ast)
+
+    with self.assertRaises(Exception):
+      env.compile("non_existent_field > 10")
+
   def test_config_export_container(self):
     env: cel.Env = cel.NewEnv(container="test.container")
     yaml: str = env.config().to_yaml()

cel_expr_python/py_cel_env_config.cc

@@ -33,6 +33,8 @@ void PyCelEnvConfig::DefinePythonBindings(pybind11::module& m) {
   m.def("NewEnvConfigFromYaml", &PyCelEnvConfig::FromYaml, py::arg("yaml"));
 
   cel_class.def("to_yaml", &PyCelEnvConfig::ToYaml);
+  cel_class.def_property_readonly("context_type",
+                                  &PyCelEnvConfig::GetContextType);
 }
 
 PyCelEnvConfig PyCelEnvConfig::FromYaml(std::string yaml) {

cel_expr_python/py_cel_env_config.h

@@ -34,6 +34,7 @@ class PyCelEnvConfig {
   std::string ToYaml() const;
 
   const cel::Config& GetConfig() const { return config_; }
+  std::string GetContextType() const { return config_.GetContextType(); }
 
  private:
   cel::Config config_;