Determine actually given command line arguments

Question

I am writing a program that gets its parameters from a combination of config file (using configparser) and command line arguments (using argparse). Now I would also like the program to be able to _write_ a
configparser config file that contains only the parameters actually given on the commandline. Is there a simple way to determine which command line arguments were actually given on the commandline, i.e. does argparse.ArgumentParser() know which of its namespace members were actually hit during parse_args().

I have tried giving the arguments default values and then looking for those having a non-default value but this is really awkward, especially if it comes to non-string arguments. Also, parsing sys.argv looks clumsy because you have to keep track of short and long options with and without argument etc. i.e. all things that I got argparse for in the first place.

Answer 1

Have you looked into docopt? It's pretty awesome, and might really help in this case.

Answer 2

I don't know about that but I imagine that you could compare values with their defaults to see which have been changed.

Comments

Yes, I was trying that and it sort of works with strings if I use something sufficiently improbable like "__UNSELECTED__" as default. But it gets difficult with boolean or even number arguments where you just may not have valid "improbable" defaults. You could now say, so what, it's the default anyway. But in my program I would like to distinguish between given and not given arguments rather than between default and
non-default.

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Initialize all your arg variables to None, then after command line processing, any which remain as None weren't set on the command line. At that point, set them to the actual defaults. I think that's a
pretty common idiom.

Note: I am an old cranky dude and still use getopt. This idiom ispretty easy there. YMMV with argparse or optparse.

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Unfortunately, argparse wants to know the type of the argument and the boolean arguments (those with action=store_true) can't be initialized with None.

However, maybe I could convert boolean arguments to something like
parser.add_argument('--foo', type=str, nargs='?', const='True', default=None)

I'd then have to check for string 'True' rather than for boolean True, though.

Answer 3

I think what you're looking for is sys.argv:

$ cat argv.py
import sys
print sys.argv

$ python argv.py foo bar
['argv.py', 'foo', 'bar']

Answer 4

That's what the program does at the moment. However, I'm not quite happy with it. Generally, the user doesn't know what's the default and it would be confusing if 'prog -i 3' doesn't make 'iopt = 3' turn up in
the file at the end. There may also be the case when the user (for whatever reason) _wants_ the default in the file.

I think I will try the opposite instead: the program writes the whole set of options to the file. This would produce a complete and consistent configuration which automatically reflects the hierarchy in which the options were set. And the user can sort it out by hand if he wants.