First working implementation. To be tests much more.

src/pyappengine/__init__.py

@@ -1,10 +1,12 @@
 #!/usr/bin/python3
 
 import os
+import traceback
 from configparser import ConfigParser
 import logging
 from systemd import journal
 import inspect
+from enum import Enum, auto
 from importlib import import_module
 from threading import Thread, Lock
 
@@ -21,6 +23,34 @@ def is_number(s):
     return False
 
 
+class AEErrs(Enum):
+    PARSE_ERROR = -32700
+    INVALID_REQUEST = -32600
+    METH_NOT_FOUND = -32601
+    INVALID_PARAMS = -32602
+    INTERNAL_ERROR = -32000
+
+    def __str__(self) -> str:
+        return ERROR_MESSAGES[self.value]
+
+ERROR_MESSAGES = {
+    AEErrs.PARSE_ERROR: """Invalid JSON was received by the server.
+                        An error occurred on the server while parsing the JSON text.""",
+    AEErrs.INVALID_REQUEST: "The request sent is not a valid object.",
+    AEErrs.METH_NOT_FOUND: "The method does not exist / is not available.",
+    AEErrs.INVALID_PARAMS: "Invalid method parameter(s).",
+    AEErrs.INTERNAL_ERROR: "Internal error.",
+}
+
+
+class AppEngineException(Exception):
+    def __init__(self, error: AEErrs, mesg=None) -> None:
+        if mesg is None:
+            self.mesg = str(error)
+        super().__init__(self.mesg)
+        self.value = error.value
+
+
 class Commands(Thread):
     defmod = None
     precmd = "cmd_"
@@ -81,7 +111,7 @@ class Commands(Thread):
 
     def _execute_command(self, method: str, *args, **kwargs) -> tuple:
         success = False
-        ret = "function not found"
+        ret = (AEErrs.INTERNAL_ERROR.value, "function not found")
         if hasattr(self, self.precmd + method) and inspect.ismethod(
             getattr(self, self.precmd + method)
         ):
@@ -99,17 +129,27 @@ class Commands(Thread):
                         method, self.name
                     )
                 )
-                ret = "param error"
+                ret = (
+                    AEErrs.INVALID_PARAMS.value,
+                    ERROR_MESSAGES[AEErrs.INVALID_PARAMS],
+                )
+            except AppEngineException as e:
+                self.log.error(
+                    'function "{}" of module "{}" returned an error with code {} and message "{}"'.format(
+                        method, self.name, e.value, e.mesg
+                    )
+                )
+                ret = (e.value, e.mesg)
             except:
                 self.log.error(
-                    'function "{}" of module "{}" crashed'.format(method, self.name)
+                    'function "{}" of module "{}" crashed without other notif'.format(
+                        method, self.name
+                    )
                 )
-                ret = "function crash"
+                ret = (AEErrs.INTERNAL_ERROR.value, traceback.format_exc())
             self.lock.release()
 
-        if ret is None:
-            ret = ""
-        return success, ret.strip()
+        return success, ret
 
     def execute_command(self, module: str, method: str, *args, **kwargs):
         # isolate the module called