Fix GH13139: better error message on invalid pd.eval and df.query input

Author: tserafim

doc/source/whatsnew/v0.19.1.txt

@@ -57,3 +57,4 @@ Bug Fixes
 
 - Bug in ``pd.pivot_table`` may raise ``TypeError`` or ``ValueError`` when ``index`` or ``columns``
   is not scalar and ``values`` is not specified (:issue:`14380`)
+- pd.eval('') and df.query('') now show better error message and raise ``ValueError`` (:issue: `13139`)

pandas/computation/eval.py

@@ -233,6 +233,7 @@ def eval(expr, parser='pandas', engine=None, truediv=True,
     """
     first_expr = True
     if isinstance(expr, string_types):
+        _check_expression(expr)
         exprs = [e for e in expr.splitlines() if e != '']
     else:
         exprs = [expr]

pandas/computation/tests/test_eval.py

@@ -1891,6 +1891,17 @@ def test_bad_resolver_raises():
         yield check_bad_resolver_raises, engine, parser
 
 
+def check_empty_string_raises(engine, parser):
+    tm.skip_if_no_ne(engine)
+    with tm.assertRaisesRegexp(ValueError, 'expr cannot be an empty string'):
+        pd.eval('', engine=engine, parser=parser)
+
+
+def test_empty_string_raises():
+    for engine, parser in ENGINES_PARSERS:
+        yield check_empty_string_raises, engine, parser
+
+
 def check_more_than_one_expression_raises(engine, parser):
     tm.skip_if_no_ne(engine)
     with tm.assertRaisesRegexp(SyntaxError,

pandas/tests/frame/test_query_eval.py

@@ -147,6 +147,14 @@ def test_query_non_str(self):
         with tm.assertRaisesRegexp(ValueError, msg):
             df.query(111)
 
+    def test_query_empty_string(self):
+        # GH 13139
+        df = pd.DataFrame({'A': [1, 2, 3]})
+
+        msg = "expr cannot be an empty string"
+        with tm.assertRaisesRegexp(ValueError, msg):
+            df.query('')
+
     def test_eval_resolvers_as_list(self):
         # GH 14095
         df = DataFrame(randn(10, 2), columns=list('ab'))