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
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