Default values for optional positional arguments

[Paddy3118]

In the following:

Usage:
  block_lines (rep|exp) [options]  [INPUT-FILE [OUTPUT-FILE]]
  block_lines gen [options]  [OUTPUT-FILE]
  block_lines (-h|--help|--version)

Modes:
  rep               Find the repeated blocks of lines.
  exp               Expand the output from rep.
  gen               (Random gen of output suitable for example rep mode processing).

Options:
  -h --help         Show this screen.
  -v --version      Show version.
  -w N, --width N   Digits in field for repetition count [default: 4].
  -d --debug        Debug:
                      * In rep mode remove anything after '#' on all input lines.
                      * In gen mode annotate with repeat counts after '#'.

I/O Defaults:
  INPUT-FILE        Defaults to stdin.
  OUTPUT-FILE       Defaults to stdout.

I would like to specify default values for INPUT-FILE and OUTPUT-FILE but instead have to look for None values and replace them with defaults manually,

Is there a better way?

[jhrmnn]

I personally try to stick to the design rule that if an optional positional argument has a default textual value, it should be an option. In your case, there is actually none, the behaviour is different, say

if args['INPUT-FILE']:
    s = open(args['INPUT-FILE'])
else:
    s = sys.stdin

[poleguy]

I agree with Paddy3118. It should be added, as it's not that strange a request. Docopt shouldn't force you to work around this deficiency.

[dradetsky]

This seems to have been around for some time, but I'd like it as well. I came here after trying the same default syntax as for options, and being surprised it didn't work. Surely it can be made to work.

[yunake]

wow, really surprised this doesn't work with docopt. this is such a common thing amongst standard unix tools. this particular stdin/stdout example is a corner-case, and @Azag0 argument against it is valid, but the default values for positional parameters do not end with files do they

[keleshev]

This is how to accomplish this:

from docopt import docopt
args = docopt("usage: prog <input-file>")
file = open(args["<input-file>"] or "default-file.txt")

I would be interested to hear in what ways the above would not be adequate.

[yunake]

this is perfectly valid option @keleshev, thank you. this is what i ended up doing too. do you think it's worth documenting this somewhere outside of this bugreport?

[Paddy3118]

@keleshev, unfortunately your fix does not work in the docopt manner. It jars.
A docopt solution would involve reading the default from the help text, ideally

[GhostofGoes]

So the solution is to encode the defaults wherever in your code you're parsing the arguments. Isn't that somewhat contrary to the purpose of docopt?

[scharch]

Here's another vote for docopt handling this natively