Honor falsey callable processors in cdist and cpdist

src/rapidfuzz/process_cpp_impl.pyx

@@ -1671,8 +1671,8 @@ cdef inline vector[PyObjectWrapper] preprocess_py(queries, processor) except *:
     cdef int64_t queries_len = <int64_t>len(queries)
     proc_queries.reserve(queries_len)
 
-    # processor None/False
-    if not processor:
+    # Preserve the legacy False sentinel without truth-testing callables.
+    if processor is None or processor is False:
         for query in queries:
             proc_queries.emplace_back(<PyObject*>query)
     # processor has to be called through python
@@ -1691,8 +1691,8 @@ cdef inline vector[RF_StringWrapper] preprocess(const RF_ScorerFlags* scorer_fla
     flags = scorer_flags.flags
     proc_queries.reserve(queries_len)
 
-    # No processor
-    if not processor:
+    # Preserve the legacy False sentinel without truth-testing callables.
+    if processor is None or processor is False:
         for query in queries:
             conv_sequence_with_none(query, &proc_str)
             if proc_str.data == NULL:

src/rapidfuzz/process_py.py

@@ -516,6 +516,8 @@ def cdist(
 
     _ = workers, score_hint
     scorer_kwargs = scorer_kwargs or {}
+    if processor is False:
+        processor = None
     dtype = _dtype_to_type_num(dtype, scorer, scorer_kwargs)
     results = np.zeros((len(queries), len(choices)), dtype=dtype)
 
@@ -551,7 +553,7 @@ def cdist(
                 results[i, j] = results[j, i] = score
     else:
         for i, query in enumerate(queries):
-            proc_query = processor(query) if (processor and not is_none(query)) else query
+            proc_query = processor(query) if (processor is not None and not is_none(query)) else query
             for j, choice in enumerate(proc_choices):
                 score = (
                     scorer(
@@ -651,14 +653,16 @@ def cpdist(
 
     _ = workers, score_hint
     scorer_kwargs = scorer_kwargs or {}
+    if processor is False:
+        processor = None
     dtype = _dtype_to_type_num(dtype, scorer, scorer_kwargs)
     results = np.zeros((len_queries,), dtype=dtype)
 
     setupPandas()
 
     for i, (query, choice) in enumerate(zip(queries, choices)):
-        proc_query = processor(query) if (processor and not is_none(query)) else query
-        proc_choice = processor(choice) if (processor and not is_none(choice)) else choice
+        proc_query = processor(query) if (processor is not None and not is_none(query)) else query
+        proc_choice = processor(choice) if (processor is not None and not is_none(choice)) else choice
         score = scorer(
             proc_query,
             proc_choice,

tests/test_process.py

@@ -772,3 +772,68 @@ def test_cpdist_integral_dtype_rounding():
     assert float_result % 1 >= 0.5
     # Check if the rounded float result is equal to the integer result
     assert round(float_result) == int_result
+
+
+class _FalseBoolProcessor:
+    def __call__(self, value):
+        return value.lower()
+
+    def __bool__(self):
+        return False
+
+
+class _EmptyProcessor:
+    def __call__(self, value):
+        return value.lower()
+
+    def __len__(self):
+        return 0
+
+
+class _NoTruthTestProcessor:
+    def __call__(self, value):
+        return value.lower()
+
+    def __bool__(self):
+        message = "processor truth value must not be tested"
+        raise AssertionError(message)
+
+
+@pytest.mark.parametrize("module", [process_py, process_cpp])
+@pytest.mark.parametrize("method", ["cdist", "cpdist"])
+@pytest.mark.parametrize("processor_type", [_FalseBoolProcessor, _EmptyProcessor, _NoTruthTestProcessor])
+@pytest.mark.parametrize("custom_scorer", [False, True])
+def test_pairwise_processor_truth_value(module, method, processor_type, custom_scorer):
+    pytest.importorskip("numpy")
+
+    def scorer(first, second, **kwargs):
+        return fuzz.ratio(first, second, **kwargs)
+
+    result = getattr(module, method)(
+        ["A"], ["a"], processor=processor_type(), scorer=scorer if custom_scorer else fuzz.ratio
+    )
+    assert result.item() == 100
+
+
+@pytest.mark.parametrize("module", [process_py, process_cpp])
+@pytest.mark.parametrize("processor_type", [_FalseBoolProcessor, _EmptyProcessor, _NoTruthTestProcessor])
+def test_cdist_symmetric_processor_truth_value(module, processor_type):
+    np = pytest.importorskip("numpy")
+
+    values = ["A", "a", None]
+    result = module.cdist(values, values, processor=processor_type())
+    np.testing.assert_array_equal(result, [[100, 100, 0], [100, 100, 0], [0, 0, 0]])
+
+
+@pytest.mark.parametrize("module", [process_py, process_cpp])
+@pytest.mark.parametrize("method", ["cdist", "cpdist"])
+@pytest.mark.parametrize("processor", [None, False, str.lower])
+@pytest.mark.parametrize("custom_scorer", [False, True])
+def test_pairwise_processor_compatibility(module, method, processor, custom_scorer):
+    pytest.importorskip("numpy")
+
+    def scorer(first, second, **kwargs):
+        return fuzz.ratio(first, second, **kwargs)
+
+    result = getattr(module, method)(["A"], ["a"], processor=processor, scorer=scorer if custom_scorer else fuzz.ratio)
+    assert result.item() == (100 if processor is str.lower else 0)