fix: FLOW2.reach() handles mixed-type conditional parameters (#903)

[immu4989]

Why are these changes needed?

FLOW2.reach in flaml/tune/searcher/flow2.py builds a numeric delta array by subtracting two incumbents element-wise:

delta = np.array([incumbent1[key] - incumbent2.get(key, np.inf) for key in self._tunable_keys])

When a search space contains a conditional tune.choice whose branches yield values of different types — e.g. one branch returns the literal string 'None' and another returns an int from tune.qrandint (the exact pattern from #903) — two FLOW2 instances can hold incumbents whose values are different types for the same key. The subtraction then raises:

TypeError: unsupported operand type(s) for -: 'float' and 'str'

This was reported by @bbudescu in #903 (2023-01); @sonichi's suggested fix in that thread is to wrap the line in try/except TypeError and return False. This PR implements exactly that. Returning False is the right semantics here — a string-vs-numeric incumbent comparison is structurally not "within step" of each other, so reporting "cannot reach" is correct.

Verified

• Repro from Conditional parameter flow2 crash #903 reproduces cleanly on current main via FLOW2.reach() (TypeError: unsupported operand type(s) for -: 'float' and 'str').
• After the fix, reach() returns False for the mixed-type case (no crash).
• A focused regression test was added in test/tune/test_searcher.py::test_flow2_reach_mixed_type_incumbents — verified to fail on main without the fix and pass with it.
• Existing tests in test/tune/test_searcher.py continue to pass.
• pre-commit run --files flaml/tune/searcher/flow2.py test/tune/test_searcher.py — all hooks pass.

Related issue number

Closes #903

Checks

• I've used pre-commit to lint the changes in this PR (note the same in integrated in our CI checks).
• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[Copilot]

Pull request overview

This PR fixes a crash in FLOW2.reach() when conditional/choice parameters produce incumbents with mixed types (e.g., numeric vs string), by treating such comparisons as “not reachable” instead of raising a TypeError. This improves robustness of the FLOW2 local search logic in flaml/tune/searcher/flow2.py for hierarchical/conditional search spaces.

Changes:

• Wrap the incumbent delta computation in FLOW2.reach() with a try/except TypeError and return False on mixed-type subtraction.
• Add a regression test ensuring reach() returns False (and does not crash) for mixed-type incumbents.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
flaml/tune/searcher/flow2.py	Prevents FLOW2.reach() from crashing on mixed-type incumbent comparisons by returning False on TypeError.
test/tune/test_searcher.py	Adds a regression test covering the mixed-type incumbent case reported in #903.

[thinkall]

Thank you very much for the PR, @immu4989 !