Fix lookup_default leaking UNSET sentinel instead of None#1
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Context.lookup_default() returned the internal UNSET sentinel when no default was found, which leaked an implementation detail to external callers—particularly subclasses of Context that override the method and check `if default is not None`. Introduce a private _lookup_default() that preserves the UNSET return for internal use (get_default and consume_value need to distinguish "no default" from "default is None"), and make the public lookup_default() convert UNSET to None before returning. Fixes pallets#3145 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
Fixes pallets#3145
Context.lookup_default()was returning the internalUNSETsentinel object instead ofNonewhen no default was found. This was a regression introduced in 8.3.0 that broke subclasses ofContextwhich overridelookup_default()and rely on the documented return type (Any | None)._lookup_default()method that preserves theUNSETreturn value for internal use byParameter.get_default()andParameter.consume_value(), which need to distinguish "no default" from "default is None".lookup_default()delegate to_lookup_default()and convertUNSETtoNonebefore returning, matching the documented return type and pre-8.3.0 behavior.Test plan
test_lookup_default_returns_none_not_sentinel-- verifieslookup_default()returnsNone(notUNSET) in all "no default found" scenariostest_lookup_default_subclass_no_sentinel_leak-- reproduces the exact pattern from the issue (subclass callingsuper().lookup_default()and checkingis not None)test_basic.pyis a pre-existing pytest compatibility issue unrelated to this change)