Make error message from parsing more explicit (#437)

python/podio/podio_config_reader.py

@@ -57,7 +57,7 @@ def _parse_with_regexps(string, regexps_callbacks):
       if result:
         return callback(result)
 
-    raise DefinitionError(f"'{string}' is not a valid member definition")
+    raise DefinitionError(f"'{string}' is not a valid member definition. Check syntax of the member definition.")
 
   @staticmethod
   def _full_array_conv(result):
@@ -85,18 +85,26 @@ def _bare_member_conv(result):
 
   def parse(self, string, require_description=True):
     """Parse the passed string"""
-    matchers_cbs = [
+    default_matchers_cbs = [
         (self.full_array_re, self._full_array_conv),
-        (self.member_re, self._full_member_conv)
-        ]
-
-    if not require_description:
-      matchers_cbs.extend((
-          (self.bare_array_re, self._bare_array_conv),
-          (self.bare_member_re, self._bare_member_conv)
-          ))
-
-    return self._parse_with_regexps(string, matchers_cbs)
+        (self.member_re, self._full_member_conv)]
+
+    no_desc_matchers_cbs = [
+        (self.bare_array_re, self._bare_array_conv),
+        (self.bare_member_re, self._bare_member_conv)]
+
+    if require_description:
+      try:
+        return self._parse_with_regexps(string, default_matchers_cbs)
+      except DefinitionError:
+        # check whether we could parse this if we don't require a description and
+        # provide more details in the error if we can
+        self._parse_with_regexps(string, no_desc_matchers_cbs)
+        # pylint: disable-next=raise-missing-from
+        raise DefinitionError(f"'{string}' is not a valid member definition. Description comment is missing.\n"
+                              "Correct Syntax: <type> <name> // <comment>")
+
+    return self._parse_with_regexps(string, default_matchers_cbs + no_desc_matchers_cbs)
 
 
 class ClassDefinitionValidator: