argparse: terminal width is not detected properly

Lib/test/test_argparse.py

 # Author: Steven J. Bethard <steven.bethard@gmail.com>.
 
 import codecs
 import inspect
 import os
 import shutil
 import stat
 import sys
 import textwrap
 import tempfile
@@ skipping 13 matching lines @@
             print('')
             print(repr(obj1))
             print(repr(obj2))
             print(obj1)
             print(obj2)
         super(TestCase, self).assertEqual(obj1, obj2)
 
     def setUp(self):
         # The tests assume that line wrapping occurs at 80 columns, but this
         # behaviour can be overridden by setting the COLUMNS environment
-        # variable.  To ensure that this assumption is true, unset COLUMNS.
+        # variable.  To ensure that this width is used, set COLUMNS=80.
         env = support.EnvironmentVarGuard()
-        env.unset("COLUMNS")
+        env['COLUMNS'] = '80'
         self.addCleanup(env.__exit__)
 
 
 class TempDirMixin(object):
 
     def setUp(self):
         self.temp_dir = tempfile.mkdtemp()
         self.old_dir = os.getcwd()
         os.chdir(self.temp_dir)
 
@@ skipping 4721 matching lines @@
              DeprecationWarning)):
         support.run_unittest(__name__)
     # Remove global references to avoid looking like we have refleaks.
     RFile.seen = {}
     WFile.seen = set()
 
 
 
 if __name__ == '__main__':
     test_main()