fix(strategies): use is not None for strategy presence checks (#2307)

[jbbqqf]

Summary

Three remaining if strategy: truthiness checks in pandera/strategies/pandas_strategies.py test for a chained parent strategy. Hypothesis strategies implement __bool__ to raise HypothesisWarning ("bool(from_dtype(...)) is always True, did you mean to draw a value?"), so the truthy form is incorrect (and silently emits the warning) when a real strategy is supplied. Switch the three sites to the explicit is not None form, matching the convention already used elsewhere in the same module.

Resolves #2307.

Context

The reporter (#2307) cited two sites at lines 832 and 1055 that have already been migrated to is None / is not None. Three remained:

• pandera/strategies/pandas_strategies.py:1244 — series_strategy entry-point
• pandera/strategies/pandas_strategies.py:1571 — dataframe_strategy entry-point
• pandera/strategies/pandas_strategies.py:1843 — multiindex_strategy entry-point

These are base-strategy entry-points that raise BaseStrategyOnlyError when a parent strategy is supplied (it's a usage error to chain a parent onto a base strategy). The fix is one-token-per-site: if strategy: → if strategy is not None:.

Changes

• pandera/strategies/pandas_strategies.py: three if strategy: → if strategy is not None: rewrites, plus a one-shot comment at the first site explaining why (so a future contributor doesn't "simplify" it back).
• tests/strategies/test_strategies.py: structural regression test that scans the module source and asserts no if strategy: lines remain. The test fails on origin/main and passes on this branch.

A behavioural test would only fire when a caller passes a parent strategy (which is itself a usage error) — the structural test is a closer fit for the actual contract this PR enforces.

Reproduce BEFORE/AFTER yourself (copy-paste)

# --- one-time setup ---
git clone https://github.com/unionai-oss/pandera.git /tmp/repro && cd /tmp/repro

# --- BEFORE (origin/main) ---
git checkout origin/main
grep -nE "^\s*if strategy:\s*$" pandera/strategies/pandas_strategies.py | wc -l
# Expected: 3 (three remaining truthiness checks)

# --- AFTER (this PR) ---
git fetch https://github.com/jbbqqf/pandera.git bugfix/2307-strategy-is-not-none
git checkout FETCH_HEAD
grep -nE "^\s*if strategy:\s*$" pandera/strategies/pandas_strategies.py | wc -l
# Expected: 0 (all three migrated to ``if strategy is not None:``)

What I ran locally

• pytest tests/strategies/test_strategies.py::test_strategy_truthiness_check_uses_is_not_none -v → 1/1 passed.
• Verified the same test fails on origin/main (3 remaining matches reported).

The pre-existing with pytest.raises(BaseStrategyOnlyError) cases at lines 532, 692, 731 of tests/strategies/test_strategies.py exercise the non-None code path and continue to pass — is not None is logically equivalent to truthiness for the only valid input shape (a real SearchStrategy instance).

Edge cases tested

#	Scenario	Expected	Verified by
1	All three migrated sites use is not None	structural scan returns 0 matches	new regression test
2	Existing parent-strategy callers still raise BaseStrategyOnlyError	unchanged behaviour	pre-existing tests at lines 532, 692, 731
3	Default-argument call (strategy=None) doesn't raise	no warning, no error	pre-existing strategy-draw tests

Risk / blast radius

Three identical edits in one module. The new condition is logically equivalent for all observed inputs (None and a SearchStrategy subclass), the latter being the only thing that ever reaches these branches.

---

PR drafted with assistance from Claude Code. The change was reviewed manually against pandera's strategies module and against the HypothesisWarning quoted in #2307.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.51%. Comparing base (378b2de) to head (e027fa3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2328   +/-   ##
=======================================
  Coverage   83.51%   83.51%           
=======================================
  Files         190      190           
  Lines       16613    16613           
=======================================
  Hits        13875    13875           
  Misses       2738     2738           

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