allow from_string to be specified via metadata

traitlets/config/loader.py

@@ -408,7 +408,10 @@ def get_value(self, trait):
         s = str(self)
         try:
             return trait.from_string(s)
-        except Exception:
+        except Exception as e:
+            warnings.warn(
+                f"Error calling {trait.name}.from_string({s!r}): {e}", RuntimeWarning
+            )
             # exception casting from string,
             # let the original string lie.
             # this will raise a more informative error when config is loaded.
@@ -435,22 +438,28 @@ class DeferredConfigList(list, DeferredConfig):
     """
     def get_value(self, trait):
         """Get the value stored in this string"""
-        if hasattr(trait, "from_string_list"):
+        if getattr(trait, "from_string_list", None) is not None:
             src = list(self)
             cast = trait.from_string_list
+            method_name = "from_string_list"
         else:
             # only allow one item
             if len(self) > 1:
                 raise ValueError(f"{trait.name} only accepts one value, got {len(self)}: {list(self)}")
             src = self[0]
             cast = trait.from_string
+            method_name = "from_string"
 
         try:
             return cast(src)
-        except Exception:
+        except Exception as e:
             # exception casting from string,
             # let the original value lie.
             # this will raise a more informative error when config is loaded.
+            warnings.warn(
+                f"Error calling {trait.name}.{method_name}({src!r}): {e}",
+                RuntimeWarning,
+            )
             return src
 
     def __repr__(self):

traitlets/config/tests/test_application.py

@@ -12,6 +12,7 @@
 import logging
 import os
 import sys
+from binascii import a2b_hex
 from io import StringIO
 from tempfile import TemporaryDirectory
 from unittest import TestCase
@@ -629,6 +630,25 @@ def test_loaded_config_files(self):
             self.assertEqual(app.running, False)
 
 
+def test_custom_from_string():
+    path_items = sys.path[:5]
+
+    path_str = os.pathsep.join(path_items)
+
+    def from_path_string(s):
+        return s.split(os.pathsep)
+
+    class App(Application):
+        path = List().tag(config=True, from_string=from_path_string)
+        hex = Bytes().tag(config=True, from_string=a2b_hex)
+        aliases = {"path": "App.path", "hex": "App.hex"}
+
+    app = App()
+    app.parse_command_line(["--path", path_str, "--hex", "a1b2"])
+    assert app.path == path_items
+    assert app.hex == b"\xa1\xb2"
+
+
 def test_cli_multi_scalar(caplog):
     class App(Application):
         aliases = {"opt": "App.opt"}

traitlets/traitlets.py

@@ -720,12 +720,29 @@ def tag(self, **metadata):
 
         >>> Int(0).tag(config=True, sync=True)
         """
-        maybe_constructor_keywords = set(metadata.keys()).intersection({'help','allow_none', 'read_only', 'default_value'})
+        maybe_constructor_keywords = set(metadata.keys()).intersection(
+            {"help", "allow_none", "read_only", "default_value"}
+        )
         if maybe_constructor_keywords:
             warn('The following attributes are set in using `tag`, but seem to be constructor keywords arguments: %s '%
                     maybe_constructor_keywords, UserWarning, stacklevel=2)
 
         self.metadata.update(metadata)
+
+        # allow from_string to be overridden via metadata
+        if self.metadata.get("from_string"):
+            self.from_string = self.metadata["from_string"]
+            if (not self.metadata.get("from_string_list")) and getattr(
+                self, "from_string_list", None
+            ):
+                self.from_string_list = None
+                # from_string overridden, from_string_list inherited and not overridden
+                # ensure inherited from_string_list does not take priority over our new from_string
+                # by removing it
+
+        if self.metadata.get("from_string_list"):
+            self.from_string_list = self.metadata["from_string_list"]
+
         return self
 
     def default_value_repr(self):