argparse: document that add_argument returns an Action object

[Sworddragon]

BPO	25035
Nosy	@bitdancer

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2015-09-08.23:01:55.636>
labels = ['type-feature', '3.7', 'docs']
title = 'Getter/setter for argparse keys'
updated_at = <Date 2016-12-12.06:06:04.814>
user = 'https://bugs.python.org/Sworddragon'

bugs.python.org fields:

activity = <Date 2016-12-12.06:06:04.814>
actor = 'rhettinger'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2015-09-08.23:01:55.636>
creator = 'Sworddragon'
dependencies = []
files = []
hgrepos = []
issue_num = 25035
keywords = []
message_count = 8.0
messages = ['250263', '250305', '250330', '250350', '250352', '250353', '250423', '282878']
nosy_count = 5.0
nosy_names = ['bethard', 'r.david.murray', 'docs@python', 'Sworddragon', 'paul.j3']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue25035'
versions = ['Python 3.7']

Linked PRs

• gh-69223: Document that add_argument returns an Action object #144452

[Sworddragon]

On making a look at the argparse documentation to figure out if I can get the value of the choices key of a specific argument after argument parsing or if I have to implement it myself I noticed that there is a getter/setter for the default key (ArgumentParser.get_default and ArgumentParser.set_defaults) but not for the other keys. Maybe this could also be implemented for the other keys, probably as a generic function that takes as an extra argument the requested key.

[bitdancer]

Can you provide some specific use cases, please?

[Sworddragon]

I was myself in the case where I needed the values of the choices key of 2 specific arguments. Currently I'm solving this by storing them in variables but probably it could be cleaner by using a getter.

[paulj3]

parser.set_defaults lets you set a default for any dest. It does not have to be connected with an argument. See what the docs say about using it to set an action linked to a subparser.

arg1 = parser.add_argument(...) returns an Action object. The parser collects these objects in its lists, but you can also save a reference to it yourself.

I'd suggest doing this in an interactive shell, and see for yourself the attributes of this object. Simply doing a 'print' on the object shows a number of the attributes (but not all). It is possible to view, and in some cases, even modify these attributes directly.

args.choices gives you access to the choices attribute.

I don't think there's a need for getter/setter methods for Action attributes. The attributes are accessible by normal Python object approaches.

p.s. Another forum for asking argparse how-to questions is Stackoverflow. Just tag it 'argparse'.

[Sworddragon]

I'm actually not fully sure why you are telling me this all, especially in this specific way.

But I would also go the other way, by removing ArgumentParser.get_default and ArgumentParser.set_defaults if we think the current ways of getting/setting are enough. Mainly I think consistence is the important here.

[paulj3]

get_default and set_defaults are parser methods, and do more than set or get a particular Action attribute. 'Set' for example changes both the 'parser._defaults' attribute, and a 'action.default' attribute. Defaults are complex and can be defined in at least 3 different ways.

'choices' (and other things like 'required', 'nargs', even 'default') is an attribute of a specific Action (argument object). You normally only set these in one way, as parameters of the add_argument method.

'action.default' and 'action.choices' are 'public' attributes. 'parser._defaults' is a 'private' attribute. A setter method is the right way to change a 'private' attribute (if needed). It usually isn't need for a 'public' one.

[Sworddragon]

In this case probably all is fine then. But there is a minor thing I noticed from one of your previous posts: You said parser.add_argument returns an Action object but I noticed that this is not mentioned on "16.4.3. The add_argument() method". Is it maybe mentioned somewhere else or probably just missing?

[paulj3]

Yes, the information that add_argument returns a Action object is missing from the documentation. It might be useful addition.

The documentation has many faults. It isn't basic enough to be a tutorial, but many first time users use it as such, and get confused by its complexity. But it also is not a formal reference document. And the automatic 'help(argparse)' isn't much better.

A regular user does not need to use the Action object directly. But as a developer I often test argparse in an interactive shell (IPython), and the 'out' line shows a 'str' of the object (as a summary of its attributes). So assigning that object to a variable is just as natural to me as saving the parser object returned by argparse.ArgumentParser, or the group object returned by parser.add_argument_group.