issue #48, functions: improve Python defined function error handling

When exceptions happen in a Python defined function, the execution of
the Rule is interrupted. The error and traceback are logged on
`stdwrn` routers and the error message is forwarded back to CLIPS.

A new set of API `error_state` and `clear_error_state()` are provided
to check if errors during rule execution happened.

Signed-off-by: Matteo Cafasso <noxdafox@gmail.com>

clips/functions.py

@@ -37,6 +37,7 @@
 """
 
 import traceback
+from typing import Union
 
 import clips
 
@@ -347,6 +348,31 @@ class Functions:
     def __init__(self, env: ffi.CData):
         self._env = env
 
+    @property
+    def error_state(self) -> Union[None, CLIPSError]:
+        """Get the CLIPS environment error state.
+
+        Equivalent to the CLIPS (get-error) function.
+
+        """
+        value = clips.values.clips_udf_value(self._env)
+
+        lib.GetErrorFunction(self._env, ffi.NULL, value)
+        state = clips.values.python_value(self._env, value)
+
+        if isinstance(state, clips.Symbol):
+            return None
+        else:
+            return CLIPSError(self._env, message=state)
+
+    def clear_error_state(self):
+        """Clear the CLIPS environment error state.
+
+        Equivalent to the CLIPS (clear-error) function.
+
+        """
+        lib.ClearErrorValue(self._env)
+
     def call(self, function: str, *arguments) -> type:
         """Call the CLIPS function with the given arguments."""
         value = clips.values.clips_value(self._env)
@@ -439,7 +465,11 @@ def python_function(env: ffi.CData, context: ffi.CData, output: ffi.CData):
     try:
         ret = environment_data(env, 'user_functions')[funcname](*arguments)
     except Exception as error:
-        clips.values.clips_udf_value(env, traceback.format_exc(), value)
+        message = "[PYCODEFUN1] %r" % error
+        string = "\n".join((message, traceback.format_exc()))
+
+        lib.WriteString(env, 'stdwrn'.encode(), string.encode())
+        clips.values.clips_udf_value(env, message, value)
         lib.SetErrorValue(env, value.header)
         lib.UDFThrowError(context)
     else:

test/environment_test.py

@@ -2,9 +2,9 @@
 import unittest
 from tempfile import mkstemp
 
+from clips import CLIPSError
 from clips import Environment, Symbol, LoggingRouter, ImpliedFact, InstanceName
 
-
 DEFRULE_FACT = """
 (defrule fact-rule
    ?fact <- (test-fact)
@@ -38,6 +38,10 @@ def python_types():
     return None, True, False
 
 
+def python_error():
+    raise Exception("BOOM!")
+
+
 class TempFile:
     """Cross-platform temporary file."""
     name = None
@@ -60,6 +64,7 @@ def setUp(self):
         self.env.define_function(python_function)
         self.env.define_function(python_function,
                                  name='python-function-renamed')
+        self.env.define_function(python_error)
         self.env.define_function(python_types)
         self.env.define_function(self.python_method)
         self.env.define_function(self.python_fact_method)
@@ -93,6 +98,18 @@ def test_eval_python_function(self):
         ret = self.env.eval('(python_types)')
         self.assertEqual(ret, expected)
 
+    def test_eval_python_error(self):
+        """Errors in Python functions are correctly set."""
+        self.assertIsNone(self.env.error_state)
+
+        with self.assertRaises(CLIPSError):
+            self.env.eval('(python_error)')
+        self.assertEqual(str(self.env.error_state),
+                         "[PYCODEFUN1] Exception('BOOM!')")
+
+        self.env.clear_error_state()
+        self.assertIsNone(self.env.error_state)
+
     def test_eval_python_method(self):
         """Python method is evaluated correctly."""
         expected = [0, 1.1, "2", Symbol('three')]