MAINT: Make argument determination in NameValidator more precise.

The function was useing `'u' in case_sensitive` to detect `upper`.
Make that more precise with `case_sensitive.startswith('u').

Raise ValueError if case_sensitive has unrecognized value.

numpy/lib/_iotools.py

@@ -320,12 +320,13 @@ def __init__(self, excludelist=None, deletechars=None,
         # Process the case option .....
         if (case_sensitive is None) or (case_sensitive is True):
             self.case_converter = lambda x: x
-        elif (case_sensitive is False) or ('u' in case_sensitive):
+        elif (case_sensitive is False) or case_sensitive.startswith('u'):
             self.case_converter = lambda x: x.upper()
-        elif 'l' in case_sensitive:
+        elif case_sensitive.startswith('l'):
             self.case_converter = lambda x: x.lower()
         else:
-            self.case_converter = lambda x: x
+            msg = 'unrecognized case_sensitive value %s.' % case_sensitive
+            raise ValueError(msg)
         #
         self.replace_space = replace_space
 

numpy/lib/tests/test__iotools.py

@@ -7,7 +7,8 @@
 import numpy as np
 from numpy.compat import asbytes, asbytes_nested
 from numpy.testing import (
-    run_module_suite, TestCase, assert_, assert_equal, assert_allclose
+    run_module_suite, TestCase, assert_, assert_equal, assert_allclose,
+    assert_raises
     )
 from numpy.lib._iotools import (
     LineSplitter, NameValidator, StringConverter,
@@ -93,6 +94,9 @@ def test_case_sensitivity(self):
         test = NameValidator(case_sensitive='lower').validate(names)
         assert_equal(test, ['a', 'a_1', 'b', 'c'])
 
+        # check exceptions
+        assert_raises(ValueError, NameValidator, case_sensitive='foobar')
+
     def test_excludelist(self):
         "Test excludelist"
         names = ['dates', 'data', 'Other Data', 'mask']