#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.

1 files changed, 14 insertions, 7 deletions

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 0ee8c08..f77c0c2 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1714,10 +1714,7 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
             if action.dest is not SUPPRESS:
                 if not hasattr(namespace, action.dest):
                     if action.default is not SUPPRESS:
-                        default = action.default
-                        if isinstance(action.default, str):
-                            default = self._get_value(action, default)
-                        setattr(namespace, action.dest, default)
+                        setattr(namespace, action.dest, action.default)
 
         # add any parser defaults that aren't present
         for dest in self._defaults:
@@ -1945,12 +1942,22 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         if positionals:
             self.error(_('too few arguments'))
 
-        # make sure all required actions were present
+        # make sure all required actions were present, and convert defaults.
         for action in self._actions:
-            if action.required:
-                if action not in seen_actions:
+            if action not in seen_actions:
+                if action.required:
                     name = _get_action_name(action)
                     self.error(_('argument %s is required') % name)
+                else:
+                    # Convert action default now instead of doing it before
+                    # parsing arguments to avoid calling convert functions
+                    # twice (which may fail) if the argument was given, but
+                    # only if it was defined already in the namespace
+                    if (action.default is not None and
+                            hasattr(namespace, action.dest) and
+                            action.default is getattr(namespace, action.dest)):
+                        setattr(namespace, action.dest,
+                                self._get_value(action, action.default))
 
         # make sure all required groups had one option present
         for group in self._mutually_exclusive_groups: