Add comprehensive tests for kwargs validation across all functions

INVESTIGATION_SUMMARY.md

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+# Investigation Notes: Kwargs Validation
+
+Looking into issue #142 - checking if functions properly reject invalid kwargs.
+
+## Background
+
+Issue #142 is about making sure functions raise TypeError for invalid keyword arguments instead of silently ignoring them. PR #143 already fixed this for hdi(), eti(), kde(), and histogram().
+
+Wanted to see if other functions validate kwargs too.
+
+## What I did
+
+Wrote some test scripts to check the main functions. My approach:
+- Tested 24+ functions with invalid kwargs like `invalid_kwarg="test"`
+- Also tried common typos (`dims` vs `dim`, `methods` vs `method`) 
+- Checked both direct calls and `.azstats` accessor methods
+
+## Results
+
+Good news - everything works! All the functions I tested properly reject invalid kwargs.
+
+Functions tested:
+- Sampling diagnostics: ess(), rhat(), mcse(), bfmi(), diagnose()
+- Summary stats: summary(), mean(), median(), mode(), ci_in_rope()
+- Density/viz: hdi(), eti(), kde(), histogram(), qds(), ecdf()
+- LOO stuff: loo(), compare()
+- Metrics: bayesian_r2(), metrics()
+- Other: thin(), weight_predictions(), bayes_factor(), psense()
+
+### How it works
+
+Functions with `**kwargs` either pass them downstream (where they get validated) or check for unexpected kwargs explicitly and raise TypeError.
+
+Two error patterns I saw:
+- `"got an unexpected keyword argument..."` (most common)
+- `"got multiple values for argument 'dims'"` (when typo conflicts with internal param)
+
+Both work fine for catching bad inputs.
+
+## The actual problem
+
+Only 4 functions have tests for kwargs validation (from PR #143). The functionality is there, but test coverage is incomplete.
+
+So I added 26 tests covering all the functions above. Tests check both direct calls and accessor methods, plus common typo scenarios.
+
+## Test results
+
+```
+tests/test_kwargs_validation.py: 26 passed
+Full test suite: 2475 passed, 2 skipped
+```
+
+## Tests added
+
+`tests/test_kwargs_validation.py` has 26 tests organized into:
+- TestKwargsValidationSamplingDiagnostics (5 tests)
+- TestKwargsValidationSummaryStatistics (5 tests)
+- TestKwargsValidationVisualizationFunctions (2 tests)
+- TestKwargsValidationLOOFunctions (2 tests)
+- TestKwargsValidationMetrics (2 tests)
+- TestKwargsValidationOtherFunctions (4 tests)
+- TestKwargsValidationAccessors (6 tests)
+
+## Why add tests if validation already works?
+
+- Prevents regressions if someone refactors the validation logic later
+- Documents expected behavior for new contributors
+- Covers edge cases and common typos
+- Matches the test coverage level I saw elsewhere in the codebase
+
+Issue #142 is technically resolved functionality-wise, but this adds the test documentation we were missing.

tests/test_kwargs_validation.py

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+"""
+Comprehensive tests for kwargs validation across arviz-stats functions.
+
+These tests ensure that functions properly raise TypeError when given
+unexpected keyword arguments, preventing silent failures from typos.
+
+Related to issue #142 and PR #143.
+"""
+
+# pylint: disable=redefined-outer-name, no-self-use, unexpected-keyword-arg
+import pytest
+from arviz_base import load_arviz_data
+
+
+class TestKwargsValidationSamplingDiagnostics:
+    """Test kwargs validation for sampling diagnostic functions."""
+
+    @pytest.fixture(scope="class")
+    def idata(self):
+        """Load test data."""
+        return load_arviz_data("non_centered_eight")
+
+    def test_ess_rejects_invalid_kwargs(self, idata):
+        """Test that ess() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.ess(idata.posterior, invalid_kwarg="test")
+
+        # Also test common typos
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.ess(idata.posterior, methods="bulk")  # should be 'method'
+
+    def test_rhat_rejects_invalid_kwargs(self, idata):
+        """Test that rhat() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.rhat(idata.posterior, invalid_kwarg="test")
+
+        # Test common typo
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.rhat(idata.posterior, methods="rank")  # should be 'method'
+
+    def test_mcse_rejects_invalid_kwargs(self, idata):
+        """Test that mcse() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.mcse(idata.posterior, invalid_kwarg="test")
+
+        # Test common typo
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.mcse(idata.posterior, methods="mean")  # should be 'method'
+
+    def test_bfmi_rejects_invalid_kwargs(self, idata):
+        """Test that bfmi() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.bfmi(idata, invalid_kwarg="test")
+
+    def test_diagnose_rejects_invalid_kwargs(self, idata):
+        """Test that diagnose() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.diagnose(idata, invalid_kwarg="test")
+
+
+class TestKwargsValidationSummaryStatistics:
+    """Test kwargs validation for summary statistic functions."""
+
+    @pytest.fixture(scope="class")
+    def idata(self):
+        """Load test data."""
+        return load_arviz_data("non_centered_eight")
+
+    def test_summary_rejects_invalid_kwargs(self, idata):
+        """Test that summary() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.summary(idata, invalid_kwarg="test")
+
+        # Test common typo
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.summary(idata, sample_dim=["chain", "draw"])  # should be 'sample_dims'
+
+    def test_mean_rejects_invalid_kwargs(self, idata):
+        """Test that mean() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.mean(idata.posterior, invalid_kwarg="test")
+
+        # Test common typo (errors with "multiple values" due to internal dims variable)
+        with pytest.raises(TypeError):
+            azs.mean(idata.posterior, dims="draw")  # should be 'dim'
+
+    def test_median_rejects_invalid_kwargs(self, idata):
+        """Test that median() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.median(idata.posterior, invalid_kwarg="test")
+
+        # Test common typo (errors with "multiple values" due to internal dims variable)
+        with pytest.raises(TypeError):
+            azs.median(idata.posterior, dims="draw")  # should be 'dim'
+
+    def test_mode_rejects_invalid_kwargs(self, idata):
+        """Test that mode() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.mode(idata.posterior, invalid_kwarg="test")
+
+        # Test common typo (errors with "multiple values" due to internal dims variable)
+        with pytest.raises(TypeError):
+            azs.mode(idata.posterior, dims="draw")  # should be 'dim'
+
+    def test_ci_in_rope_rejects_invalid_kwargs(self, idata):
+        """Test that ci_in_rope() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.ci_in_rope(idata.posterior["mu"], rope=(-1, 1), invalid_kwarg="test")
+
+
+class TestKwargsValidationVisualizationFunctions:
+    """Test kwargs validation for visualization/density functions."""
+
+    @pytest.fixture(scope="class")
+    def idata(self):
+        """Load test data."""
+        return load_arviz_data("non_centered_eight")
+
+    def test_qds_rejects_invalid_kwargs(self, idata):
+        """Test that qds() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.qds(idata.posterior["mu"], invalid_kwarg="test")
+
+    def test_ecdf_rejects_invalid_kwargs(self, idata):
+        """Test that ecdf() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.ecdf(idata.posterior["mu"], invalid_kwarg="test")
+
+
+class TestKwargsValidationLOOFunctions:
+    """Test kwargs validation for LOO/model comparison functions."""
+
+    @pytest.fixture(scope="class")
+    def idata(self):
+        """Load test data."""
+        return load_arviz_data("non_centered_eight")
+
+    def test_loo_rejects_invalid_kwargs(self, idata):
+        """Test that loo() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.loo(idata, invalid_kwarg="test")
+
+    def test_compare_rejects_invalid_kwargs(self, idata):
+        """Test that compare() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.compare({"model": idata}, invalid_kwarg="test")
+
+
+class TestKwargsValidationMetrics:
+    """Test kwargs validation for metric functions."""
+
+    @pytest.fixture(scope="class")
+    def idata(self):
+        """Load test data."""
+        return load_arviz_data("non_centered_eight")
+
+    def test_bayesian_r2_rejects_invalid_kwargs(self, idata):
+        """Test that bayesian_r2() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.bayesian_r2(idata, invalid_kwarg="test")
+
+    def test_metrics_rejects_invalid_kwargs(self, idata):
+        """Test that metrics() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.metrics(idata, invalid_kwarg="test")
+
+
+class TestKwargsValidationOtherFunctions:
+    """Test kwargs validation for other utility functions."""
+
+    @pytest.fixture(scope="class")
+    def idata(self):
+        """Load test data."""
+        return load_arviz_data("non_centered_eight")
+
+    def test_thin_rejects_invalid_kwargs(self, idata):
+        """Test that thin() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.thin(idata, factor=2, invalid_kwarg="test")
+
+    def test_weight_predictions_rejects_invalid_kwargs(self, idata):
+        """Test that weight_predictions() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.weight_predictions(idata.posterior_predictive, invalid_kwarg="test")
+
+    def test_bayes_factor_rejects_invalid_kwargs(self, idata):
+        """Test that bayes_factor() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.bayes_factor(idata.posterior, var_names="mu", invalid_kwarg="test")
+
+    def test_psense_rejects_invalid_kwargs(self, idata):
+        """Test that psense() raises TypeError for invalid kwargs."""
+        import arviz_stats as azs
+
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            azs.psense(idata, var_name="mu", invalid_kwarg="test")
+
+
+class TestKwargsValidationAccessors:
+    """Test kwargs validation for accessor methods."""
+
+    @pytest.fixture(scope="class")
+    def data_array(self):
+        """Load test data and extract DataArray."""
+        idata = load_arviz_data("non_centered_eight")
+        return idata.posterior["mu"]
+
+    def test_accessor_ess_rejects_invalid_kwargs(self, data_array):
+        """Test that .azstats.ess() raises TypeError for invalid kwargs."""
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            data_array.azstats.ess(invalid_kwarg="test")
+
+    def test_accessor_rhat_rejects_invalid_kwargs(self, data_array):
+        """Test that .azstats.rhat() raises TypeError for invalid kwargs."""
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            data_array.azstats.rhat(invalid_kwarg="test")
+
+    def test_accessor_mcse_rejects_invalid_kwargs(self, data_array):
+        """Test that .azstats.mcse() raises TypeError for invalid kwargs."""
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            data_array.azstats.mcse(invalid_kwarg="test")
+
+    def test_accessor_mean_rejects_invalid_kwargs(self, data_array):
+        """Test that .azstats.mean() raises TypeError for invalid kwargs."""
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            data_array.azstats.mean(invalid_kwarg="test")
+
+    def test_accessor_median_rejects_invalid_kwargs(self, data_array):
+        """Test that .azstats.median() raises TypeError for invalid kwargs."""
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            data_array.azstats.median(invalid_kwarg="test")
+
+    def test_accessor_mode_rejects_invalid_kwargs(self, data_array):
+        """Test that .azstats.mode() raises TypeError for invalid kwargs."""
+        with pytest.raises(TypeError, match=".*unexpected keyword argument.*"):
+            data_array.azstats.mode(invalid_kwarg="test")