Collating list traits fails when only one is given - regression in traitlets 5.13.0

[krassowski]

See jupyterlab/jupyter-ai#913. Introduced in #885 which should have no side effects as it was meant to only update types.

• 952d6a7 (Publish 5.12.0) good
• 3a03835 (Use passed-in info argument for TraitType.error() #884) good
• 4f0e68b (Finish Strict Typing #885) bad

[krassowski]

The regression is caused by a change in traitlets/traitlets.py file, in Instance class, in validate method:

-    def validate(self, obj, value):
+    def validate(self, obj: t.Any, value: t.Any) -> T | None:
         assert self.klass is not None
+        if self.allow_none and value is None:
+            return value
         if isinstance(value, self.klass):  # type:ignore[arg-type]
-            return value
+            return t.cast(T, value)
         else:
             self.error(obj, value)

Nothing obviously wrong here, but apparently the Container logic depends on the error being thrown here 🤷

[krassowski]

The self.error() call was previously making the logic work correctly because the error is excepted to be raised by DeferredConfigString.get_value():

traitlets/traitlets/config/loader.py

Lines 390 to 417 in 13de53c

	class DeferredConfigString(str, DeferredConfig):
	"""Config value for loading config from a string
	Interpretation is deferred until it is loaded into the trait.
	Subclass of str for backward compatibility.
	This class is only used for values that are not listed
	in the configurable classes.
	When config is loaded, `trait.from_string` will be used.
	If an error is raised in `.from_string`,
	the original string is returned.
	.. versionadded:: 5.0
	"""
	def get_value(self, trait: TraitType[t.Any, t.Any]) -> t.Any:
	"""Get the value stored in this string"""
	s = str(self)
	try:
	return trait.from_string(s)
	except Exception:
	# exception casting from string,
	# let the original string lie.
	# this will raise a more informative error when config is loaded.
	return s

get_value() calls trait.from_string():

traitlets/traitlets/traitlets.py

Lines 3511 to 3519 in 13de53c

	def from_string(self, s: str) -> T | None:
	"""Load value from a single string"""
	if not isinstance(s, str):
	raise TraitError(f"Expected string, got {s!r}")
	try:
	test = literal_eval(s)
	except Exception:
	test = None
	return self.validate(None, test)

which calls validate().

So if we pass --ServerApp.a_list=a_value where a_list = List(allow_none=True) previously we would get a_list = ['a_value'] and now we are getting a_list = None.