Fix container from_string collation for a single allow_none value (#908)

[apoorvdarshan]

Fixes #908.

Root cause

Container.from_string (traitlets/traitlets.py) turns a value that fails literal_eval into None, then passes it to validate:

try:
    test = literal_eval(s)
except Exception:
    test = None
return self.validate(None, test)

Before #885, Instance.validate(None, None) always raised (None is not a list/set/tuple), so DeferredConfigString.get_value / DeferredConfigList.get_value caught the error and let the original string "lie", which is later collated into a single-item list.

PR #885 added an allow_none short-circuit to Instance.validate:

if self.allow_none and value is None:
    return value

For an allow_none=True container this now accepts the substituted None instead of raising, so from_string returns None and the collation fallback never happens. A single --Class.trait=value therefore resolves to None instead of ['value'].

(The regression was bisected to #885 in the issue, and the mechanism was identified by @krassowski there.)

Reproduction

from traitlets.config import Application, Config
from traitlets.config.loader import DeferredConfigString
from traitlets import List

class MyApp(Application):
    a_list = List(allow_none=True).tag(config=True)

config = Config()
config.MyApp.a_list = DeferredConfigString("a_value")
app = MyApp()
app.update_config(config)
print(repr(app.a_list))

Observed on current main (wrong):

None

Expected / after this fix (and the behaviour on 5.12.0, before #885):

['a_value']

Fix

In Container.from_string, when literal_eval fails, validate the original string instead of substituting None:

try:
    test = literal_eval(s)
except Exception:
    test = s
return self.validate(None, test)

A bare string is not a container, so validate raises exactly as it did before #885 (independent of allow_none), and DeferredConfig.get_value falls back to collating the value into a single-item list. This is localized to the except branch and does not touch the allow_none handling in Instance.validate; the successful-parse path (including the "None" convention already covered by from_string_list) is unchanged.

Tests

Added to tests/test_traitlets.py:

• test_container_from_string_single_value_allow_none (parametrized over List, Set, Tuple): asserts from_string("a_value") on an allow_none=True container raises TraitError rather than silently returning None.
• test_list_allow_none_single_cli_value_is_collated: end-to-end check that a single deferred config value resolves to ['a_value'].

Both tests fail on unmodified main (the end-to-end one fails with assert None == ['a_value']) and pass with the fix. The full existing suite passes (591 passed, 1 skipped), and ruff check, ruff format --check, and mypy are clean on the changed files.

Disclosure: prepared with AI assistance; reviewed and verified locally.

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