#12776,#11839: call argparse type function only once.

Before, the type function was called twice in the case where the default
was specified and the argument was given as well.  This was especially
problematic for the FileType type, as a default file would always be
opened, even if a file argument was specified on the command line.

Patch by Arnaud Fontaine, with additional test by Mike Meyer.

Lib/test/test_argparse.py

@@ -1462,6 +1462,22 @@ class TestFileTypeR(TempDirMixin, ParserTestCase):
         ('readonly', NS(x=None, spam=RFile('readonly'))),
     ]
 
+class TestFileTypeDefaults(TempDirMixin, ParserTestCase):
+    """Test that a file is not created unless the default is needed"""
+    def setUp(self):
+        super(TestFileTypeDefaults, self).setUp()
+        file = open(os.path.join(self.temp_dir, 'good'), 'w')
+        file.write('good')
+        file.close()
+
+    argument_signatures = [
+        Sig('-c', type=argparse.FileType('r'), default='no-file.txt'),
+    ]
+    # should provoke no such file error
+    failures = ['']
+    # should not provoke error because default file is created
+    successes = [('-c good', NS(c=RFile('good')))]
+
 
 class TestFileTypeRB(TempDirMixin, ParserTestCase):
     """Test the FileType option/argument type for reading files"""
@@ -4468,6 +4484,38 @@ class TestArgumentTypeError(TestCase):
         else:
             self.fail()
 
+# ================================================
+# Check that the type function is called only once
+# ================================================
+
+class TestTypeFunctionCallOnlyOnce(TestCase):
+
+    def test_type_function_call_only_once(self):
+        def spam(string_to_convert):
+            self.assertEqual(string_to_convert, 'spam!')
+            return 'foo_converted'
+
+        parser = argparse.ArgumentParser()
+        parser.add_argument('--foo', type=spam, default='bar')
+        args = parser.parse_args('--foo spam!'.split())
+        self.assertEqual(NS(foo='foo_converted'), args)
+
+# ================================================================
+# Check that the type function is called with a non-string default
+# ================================================================
+
+class TestTypeFunctionCallWithNonStringDefault(TestCase):
+
+    def test_type_function_call_with_non_string_default(self):
+        def spam(int_to_convert):
+            self.assertEqual(int_to_convert, 0)
+            return 'foo_converted'
+
+        parser = argparse.ArgumentParser()
+        parser.add_argument('--foo', type=spam, default=0)
+        args = parser.parse_args([])
+        self.assertEqual(NS(foo='foo_converted'), args)
+
 # ======================
 # parse_known_args tests
 # ======================