bpo-37150: Throw ValueError if FileType class object was passed in add_argument (GH-13805)

There is a possibility that someone (like me) accidentally will omit parentheses with `FileType` arguments after `FileType`, and parser will contain wrong file until someone will try to use it. 

Example:
```python
parser = argparse.ArgumentParser()
parser.add_argument('-x', type=argparse.FileType)
```


https://bugs.python.org/issue37150

1 files changed, 18 insertions, 0 deletions

diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 9d68f40571..bcf2cc9b26 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -1619,6 +1619,24 @@ class TestFileTypeOpenArgs(TestCase):
                 m.assert_called_with('foo', *args)
 
 
+class TestFileTypeMissingInitialization(TestCase):
+    """
+    Test that add_argument throws an error if FileType class
+    object was passed instead of instance of FileType
+    """
+
+    def test(self):
+        parser = argparse.ArgumentParser()
+        with self.assertRaises(ValueError) as cm:
+            parser.add_argument('-x', type=argparse.FileType)
+
+        self.assertEqual(
+            '%r is a FileType class object, instance of it must be passed'
+            % (argparse.FileType,),
+            str(cm.exception)
+        )
+
+
 class TestTypeCallable(ParserTestCase):
     """Test some callables as option/argument types"""