[dask] Add unit tests that signatures are the same between Dask and s…

…cikit-learn estimators (#3911)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

* [dask] Add unit tests that signatures are the same between Dask and scikit-learn estimators (fixes #3907)

tests/python_package_test/test_dask.py

@@ -976,3 +976,31 @@ def test_dask_classes_and_sklearn_equivalents_have_identical_constructors_except
     assert dask_spec.defaults[:-1] == sklearn_spec.defaults
     assert dask_spec.args[-1] == 'client'
     assert dask_spec.defaults[-1] is None
+
+
+@pytest.mark.parametrize(
+    "methods",
+    [
+        (lgb.DaskLGBMClassifier.fit, lgb.LGBMClassifier.fit),
+        (lgb.DaskLGBMClassifier.predict, lgb.LGBMClassifier.predict),
+        (lgb.DaskLGBMClassifier.predict_proba, lgb.LGBMClassifier.predict_proba),
+        (lgb.DaskLGBMRegressor.fit, lgb.LGBMRegressor.fit),
+        (lgb.DaskLGBMRegressor.predict, lgb.LGBMRegressor.predict),
+        (lgb.DaskLGBMRanker.fit, lgb.LGBMRanker.fit),
+        (lgb.DaskLGBMRanker.predict, lgb.LGBMRanker.predict)
+    ]
+)
+def test_dask_methods_and_sklearn_equivalents_have_similar_signatures(methods):
+    dask_spec = inspect.getfullargspec(methods[0])
+    sklearn_spec = inspect.getfullargspec(methods[1])
+    dask_params = inspect.signature(methods[0]).parameters
+    sklearn_params = inspect.signature(methods[1]).parameters
+    assert dask_spec.args == sklearn_spec.args[:len(dask_spec.args)]
+    assert dask_spec.varargs == sklearn_spec.varargs
+    if sklearn_spec.varkw:
+        assert dask_spec.varkw == sklearn_spec.varkw[:len(dask_spec.varkw)]
+    assert dask_spec.kwonlyargs == sklearn_spec.kwonlyargs
+    assert dask_spec.kwonlydefaults == sklearn_spec.kwonlydefaults
+    for param in dask_spec.args:
+        error_msg = f"param '{param}' has different default values in the methods"
+        assert dask_params[param].default == sklearn_params[param].default, error_msg