BUG: Fix uninformative error on bad CSV engine name

Previously had an UnboundLocalError - no fun!

doc/source/whatsnew/v0.20.2.txt

@@ -38,6 +38,9 @@ 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`)
+
 
 
 Conversion

pandas/io/parsers.py

@@ -969,6 +969,9 @@ def _make_engine(self, engine='c'):
                 klass = PythonParser
             elif engine == 'python-fwf':
                 klass = FixedWidthFieldParser
+            else:
+                raise ValueError('Unknown engine: %r (valid are "c", "python",'
+                                 ' or "python-fwf")' % engine)
             self._engine = klass(self.f, **self.options)
 
     def _failover_to_python(self):

pandas/tests/io/parser/test_parsers.py

@@ -2,6 +2,8 @@
 
 import os
 
+import pytest
+
 import pandas.util.testing as tm
 
 from pandas import read_csv, read_table
@@ -99,3 +101,13 @@ def read_table(self, *args, **kwds):
         kwds = kwds.copy()
         kwds['engine'] = self.engine
         return read_table(*args, **kwds)
+
+
+class TestParameterValidation(object):
+    def test_unknown_engine(self):
+        with tm.ensure_clean() as path:
+            df = tm.makeDataFrame()
+            df.to_csv(path)
+            with pytest.raises(ValueError) as exc_info:
+                read_csv(path, engine='pyt')
+            exc_info.match('Unknown engine')