BUG: Fix uninformative error on bad CSV engine name

Previously had an UnboundLocalError - no fun!

Author: jtratner

doc/source/whatsnew/v0.21.0.txt

@@ -83,6 +83,8 @@ Performance Improvements
 Bug Fixes
 ~~~~~~~~~
 
+- Passing an invalid engine to `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

@@ -1,6 +1,9 @@
 # -*- coding: utf-8 -*-
 
 import os
+import tempfile
+
+import pytest
 
 import pandas.util.testing as tm
 
@@ -99,3 +102,14 @@ 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 tempfile.NamedTemporaryFile() as fp:
+            df = tm.makeDataFrame()
+            df.to_csv(fp.name)
+            fp.flush()
+            with pytest.raises(ValueError) as exc_info:
+                read_csv(fp.name, engine='pyt')
+            exc_info.match('Unknown engine')