Add more coverage for scenarios that probably should not happen, unless there is a gap in support

Author: sfc-gh-jkew

modin/core/storage_formats/pandas/query_compiler_caster.py

@@ -51,7 +51,7 @@ def __init__(self):
         self._qc_cls_set = set()
         self._result_type = None
 
-    def add_query_compiler(self, query_compiler: BaseQueryCompiler):
+    def add_query_compiler(self, query_compiler):
         """
         Add a query compiler to be considered for casting.
 
@@ -83,7 +83,7 @@ def calculate(self):
         if self._result_type is not None:
             return self._result_type
         if len(self._qc_cls_set) == 1:
-            return self._qc_cls_set.pop()
+            return list(self._qc_cls_set)[0]
         if len(self._qc_cls_set) == 0:
             raise ValueError("No query compilers registered")
 
@@ -133,7 +133,10 @@ def result_data_cls(self):
             DataFrame object associated with the preferred query compiler.
         """
         qc_type = self.calculate()
-        return self._compiler_class_to_data_class[qc_type]
+        if qc_type in self._compiler_class_to_data_class:
+            return self._compiler_class_to_data_class[qc_type]
+        else:
+            return None
 
 
 class QueryCompilerCaster:

modin/tests/pandas/native_df_interoperability/test_compiler_caster.py

@@ -14,6 +14,9 @@
 import pandas
 import pytest
 
+from modin.core.storage_formats.pandas.query_compiler_caster import (
+    QueryCompilerCasterCalculator,
+)
 import modin.pandas as pd
 from modin.core.storage_formats.base.query_compiler import QCCoercionCost
 from modin.core.storage_formats.pandas.native_query_compiler import NativeQueryCompiler
@@ -249,3 +252,17 @@ def test_default_to_caller(default_df, default2_df):
     assert type(df3) is type(default_df)  # should stay on caller
     df3 = default2_df.concat(axis=1, other=default_df)
     assert type(df3) is type(default2_df)  # should stay on caller
+
+
+def test_no_qc_data_to_calculate():
+    calculator = QueryCompilerCasterCalculator()
+    calculator.add_query_compiler(ClusterQC)
+    result = calculator.calculate()
+    assert result is ClusterQC
+    assert calculator.result_data_cls() is None
+
+
+def test_no_qc_to_calculate():
+    calculator = QueryCompilerCasterCalculator()
+    with pytest.raises(ValueError):
+        calculator.calculate()