[Originally submitted by risto.kankkunen@iki.fi to python-hosting.org tracker]

The default behavior for options with append action is not very convenient:

>>> import argparse >>> parser = argparse.ArgumentParser() >>> parser.add_argument("--foo", action='append') >>> parser.parse_args(('--foo=1', '--foo=2')) Namespace(foo=['1', '2']) >>> parser.parse_args(()) Namespace(foo=None)

You would expect to get zero or more items in a list for options having an append action. Now you get either None or one or more items in a list.

Giving a default value to fix this doesn't work:

>>> import argparse >>> parser = argparse.ArgumentParser() >>> parser.add_argument("--foo", action='append', default=[]) >>> parser.parse_args(()) Namespace(foo=[]) >>> parser.parse_args(('--foo=1', '--foo=2')) Namespace(foo=['1', '2']) >>> parser.parse_args(()) Namespace(foo=['1', '2']) >>> parser.parse_args(('--foo=1', '--foo=2')) Namespace(foo=['1', '2', '1', '2'])

This also applies if you want to have any other default value: if the default is mutable, the default gets mutated. If the default is immutable, you get an exception.

I'm curious what your use case is that requires calling parse_args more than once. Could you elaborate?

The use case is:

"I needed to parse the command line twice, because I wanted to combine options from the command line and from a configuration file: I parse the command line and check if there is a '--config=FILE' option. If there is, I parse the config file and call set_defaults() for each item there. This way I get the same error checking etc. for free. After this I need of course to re-parse the command line so that its options override the ones from the config file."

The fix, I believe should be to rewrite _AppendAction and _AppendConstAction to do something like:

items = _ensure_value(namespace, self.dest, []) + [values]
setattr(namespace, self.dest, items)

instead of the current call to append.

All append actions copy their destination list as of r21.

Not sure why this was marked as fixed. argparse 1.2.1 still has this issue.

The issue that was fixed was that if a default is supplied, it is copied so that multiple calls to parse_args work correctly.

I assume you're asking for the default for the "append" action to be an empty list? If so, please open a new issue for that, at the argparse bug tracker, which is the Python bug tracker: http://bugs.python.org/ Bugs reported to the Google Code tracker are likely to be lost.