Cameron Simpson wrote:
> Mats has mentioned the modules getopt and argparse etc. These are
> primarily aimed at option parsing ("-v", "-o foo"). Your situation
> occurs _after_ the option parsing (in your case, there are no options).
Not argparse. The main advantage over optparse is its handling of positional
arguments. Your custom logic
> def main(argv):
> cmd = argv.pop(0) # collect the command word
> badopts = False
> # mandatory first argument
> if not argv:
> print("%s: missing first argument" % cmd, file=sys.stderr)
> badopts = True
> else:
> first = argv.pop(0)
> # optional second argument
> if argv:
> second = argv.pop(0) # explicit argument 2, use it
> else:
> second = None # or some otherdefault
> if argv:
> print("%s: extra arguments: %r" % (cmd, argv), file=sys.stderr)
> badopts = true
> if badopts:
> print("%s: invalid invocation, aborting" % cmd, file=sys.stderr)
> return 2
> ... work with first and second ...
can roughly be replicated with the two lines
parser.add_argument("first")
parser.add_argument("second", nargs="?")
A working script:
$ cat test.py
#!/usr/bin/python3
import argparse
def main():
parser = argparse.ArgumentParser()
parser.add_argument("first")
parser.add_argument("second", nargs="?")
args = parser.parse_args()
print("first:", args.first)
print("second:", args.second)
if __name__ == "__main__":
main()
$ ./test.py
usage: test.py [-h] first [second]
test.py: error: the following arguments are required: first
$ ./test.py -h
usage: test.py [-h] first [second]
positional arguments:
first
second
optional arguments:
-h, --help show this help message and exit
$ ./test.py ONE
first: ONE
second: None
$ ./test.py ONE TWO
first: ONE
second: TWO
$ ./test.py ONE TWO THREE
usage: test.py [-h] first [second]
test.py: error: unrecognized arguments: THREE
Argparse makes a usable standard command line interface easy to set up (if
you need non-standard behaviour it gets a bit harder).
There is also a companion module argcomplete (not in the stdlib) that
enables autocompletion.
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