Fix unbound local with bad engine (#16511)

(cherry picked from commit 9b0ea41)

Author: jtratner

doc/source/whatsnew/v0.20.2.txt

@@ -39,6 +39,8 @@ Bug Fixes
 
 - Bug in using ``pathlib.Path`` or ``py.path.local`` objects with io functions (:issue:`16291`)
 - Bug in ``DataFrame.update()`` with ``overwrite=False`` and ``NaN values`` (:issue:`15593`)
+- Passing an invalid engine to :func:`read_csv` now raises an informative
+  ``ValueError`` rather than ``UnboundLocalError``. (:issue:`16511`)
 - Fixed a compatibility issue with IPython 6.0's tab completion showing deprecation warnings on Categoricals (:issue:`16409`)
 
 Conversion

pandas/io/parsers.py

@@ -969,6 +969,10 @@ def _make_engine(self, engine='c'):
                 klass = PythonParser
             elif engine == 'python-fwf':
                 klass = FixedWidthFieldParser
+            else:
+                raise ValueError('Unknown engine: {engine} (valid options are'
+                                 ' "c", "python", or' ' "python-fwf")'.format(
+                                     engine=engine))
             self._engine = klass(self.f, **self.options)
 
     def _failover_to_python(self):

pandas/tests/io/test_common.py

@@ -141,3 +141,10 @@ def test_next(self):
             assert next_line.strip() == line.strip()
 
         pytest.raises(StopIteration, next, wrapper)
+
+    def test_unknown_engine(self):
+        with tm.ensure_clean() as path:
+            df = tm.makeDataFrame()
+            df.to_csv(path)
+            with tm.assert_raises_regex(ValueError, 'Unknown engine'):
+                read_csv(path, engine='pyt')