paul j3 added the comment: The handling of defaults is a bit complicated.
Note that `set_defaults` both sets a `_defaults` attribute, and any actions with a matching `dest`. So it could change the `default` of 0, 1 or more actions. def set_defaults(self, **kwargs): self._defaults.update(kwargs) # if these defaults match any existing arguments, replace # the previous default on the object with the new one for action in self._actions: if action.dest in kwargs: action.default = kwargs[action.dest] `add_argument` gets `default` from the default parameter (kwarg), the container's `_defaults` or the parser `.argument_default`. The earliest in that list has priority. Finally, at the start of `parse_known_args`, each action's `default` is added to the namespace (IF it isn't already there), and values from `_defaults` are also add (also IF not already present). So here the priority is user supplied `namespace`, action `default`, parser `_defaults`. And finally, at the end of `_parse_known_args`, any string values in the namespace that match their action default are passed through `get_values`, which may convert them via the 'type'. I've skimmed over a couple of things: - defaults defined by the Action class (e.g. 'store_true' sets a False default) - how parent's `_defaults` are copied - when `_defaults` affects that final `get_values` action. - the relative priority of parser and subparsers _defaults (a current bug issue). So as you observed, `set_defaults` can change a previously defined argument's `default`, but it does not have priority over parameters supplied to new arguments. In my opinion, `set_default` is most useful as a way of setting Namespace values that do not have their own argument. The documentation has an example where each subparser sets its own 'func' attribute. parser_foo.set_defaults(func=foo) Maybe the documentation could be refined, though it might be tricky to do so without adding further confusion. Changing the priorities is probably not a good idea. The recent bug issues about parser and subparser `set_defaults` a cautionary tale. Even the earlier change in how 'get_values' is applied to defaults has potential pitfalls. ---------- nosy: +paul.j3 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue23814> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com