Using argparse to parse arguments of form "arg= val"

Question

I want to use argparse to parse command lines of form "arg=val" For example, the usage would be:

script.py conf_dir=/tmp/good_conf

To achieve it, I am doing this:

desc   = "details"
parser = argparse.ArgumentParser(description=desc, add_help=False)
args = parser.add_argument("conf_dir")
args = parser.parse_args("conf_dir=FOO".split())
args = parser.parse_args()
print args.conf_dir

But, the problem is that, on invocation of the script with:

python script.py conf_dir=/tmp/good_conf

I get:

conf_dir=/tmp/good_conf

Where as I expect

/tmp/good_conf

So, the question is: Can I use argparse to parse cmd line, which contains name value pairs? Any hints?

Edit: The reason I want to do this and not some thing like --conf_dir=/tmp/good_dir is because there are other tools (written in other language), which uses conf_dir=/tmp/good_dir style of arguments. To maintain consistency, I was to parse args in this way.

I don't have argparse available to check this, but shouldn't "conf_dir=FOO".split() actually be "conf_dir=FOO".split('=')? — lafras
– lafras, Commented Mar 1, 2011 at 13:11

chepner · Accepted Answer · 2020-12-23 01:03:05Z

29

You need a custom action

class StoreNameValuePair(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        n, v = values.split('=', 1)
        setattr(namespace, n, v)

args = parser.add_argument("conf_dir", action=StoreNameValuePair)

edited Dec 23, 2020 at 1:03

answered Aug 1, 2012 at 15:00

chepner

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3 Comments

Gringo Suave Over a year ago

str.partition() is less prone to ValueError than str.split() as it always returns three items.

chepner Over a year ago

I'd rather handle that with values.split('=', 1), which doesn't unnecessarily return the =. Arguably, you'd want a ValueError if the name value pair wasn't actually a pair. (Though whether you want to catch the error--or let it propagate upwards--or check if values.partition("=")[1] is empty is probably a matter of personal preference.)

Gringo Suave Over a year ago

Ok, although .partition is built primarily for this use case. Delimiter can be assigned to _, also error can be checked by checking if v is empty. All without try except. ;-)

ncoghlan · Accepted Answer · 2011-03-01 13:09:05Z

6

As per the documentation, argparse doesn't natively let you have unprefixed options like that. If you omit the leading -, it assumes you are describing a positional argument and expects it to be provided as:

python script.py /tmp/good_conf

If you want it to be optional, it needs to be correctly marked as a flag by calling it --conf_dir, and invoking the script like:

python script.py --conf_dir=/tmp/good_conf

However, to accept name-value pairs, you can implement a custom action. In combination with nargs, such an action could accept an arbitrary number of name-value pairs and store them on the argument parsing result object.

answered Mar 1, 2011 at 13:09

ncoghlan

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Comments

Michael Wyraz · Accepted Answer · 2018-11-13 15:26:16Z

2

@chepner This is great. I improved this to support multiple args as well and store the result as dict:

class StoreDict(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        kv={}
        if not isinstance(values, (list,)):
            values=(values,)
        for value in values:
            n, v = value.split('=')
            kv[n]=v
        setattr(namespace, self.dest, kv)

answered Nov 13, 2018 at 15:26

Michael Wyraz

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Comments

Thomas K · Accepted Answer · 2011-03-01 13:05:14Z

1

The usual way to put name value pairs on the command line is with options. I.e. you would use

python script.py --confdir=/tmp/good_conf

argparse can certainly handle that case. See the docs at:

http://docs.python.org/library/argparse.html#option-value-syntax

answered Mar 1, 2011 at 13:05

Thomas K

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