BUG: allow single element bool queries

Author: cpcloud

doc/source/release.rst

@@ -170,6 +170,8 @@ Bug Fixes
     a datetimelike (:issue:`6152`)
   - Fixed a stack overflow bug in ``query``/``eval`` during lexicographic
     string comparisons (:issue:`6155`).
+  - Fixed a bug in ``query`` where the index of a single-element ``Series`` was
+    being thrown away (:issue:`6148`).
 
 pandas 0.13.0
 -------------

pandas/computation/align.py

@@ -81,16 +81,12 @@ def wrapper(terms):
             return _align_core_single_unary_op(terms[0])
 
         term_values = (term.value for term in terms)
+
         # only scalars or indexes
         if all(isinstance(term.value, pd.Index) or term.isscalar for term in
                terms):
             return np.result_type(*term_values), None
 
-        # single element ndarrays
-        all_has_size = all(hasattr(term.value, 'size') for term in terms)
-        if all_has_size and all(term.value.size == 1 for term in terms):
-            return np.result_type(*term_values), None
-
         # no pandas objects
         if not _any_pandas_objects(terms):
             return np.result_type(*term_values), None

pandas/tests/test_frame.py

@@ -12860,6 +12860,20 @@ def test_query_lex_compare_strings(self):
         for parser, engine in product(PARSERS, ENGINES):
             yield self.check_query_lex_compare_strings, parser, engine
 
+    def check_query_single_element_booleans(self, parser, engine):
+        tm.skip_if_no_ne(engine)
+        columns = 'bid', 'bidsize', 'ask', 'asksize'
+        data = np.random.randint(2, size=(1, len(columns))).astype(bool)
+        df = DataFrame(data, columns=columns)
+        res = df.query('bid & ask', engine=engine, parser=parser)
+        expected = df[df.bid & df.ask]
+        assert_frame_equal(res, expected)
+
+    def test_query_single_element_booleans(self):
+        for parser, engine in product(PARSERS, ENGINES):
+            yield self.check_query_single_element_booleans, parser, engine
+
+
 class TestDataFrameEvalNumExprPandas(tm.TestCase):
 
     @classmethod