Welcome, guest | Sign In | My Account | Store | Cart

Many times I find myself write a cli that takes two/more positional arguments. Something like mycp file1 file2 [options] I have to write extra code everytime to show correct usage/hints to user if he invokes command this way mycp file1 Positional arguments are required ones unlike optional arguments.

The solution below lets cli writer add a positional argument so parser can generate usage friendlier to positional args.

Some inspiration: http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/573441

Python, 56 lines
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
import sys
from optparse import OptionParser, Option, SUPPRESS_HELP

class PAOptionParser(OptionParser, object):
    def __init__(self, *args, **kw):
        self.posargs = []
        super(PAOptionParser, self).__init__(*args, **kw)

    def add_posarg(self, *args, **kw):
        pa_help = kw.get("help", "")
        kw["help"] = SUPPRESS_HELP
        o = self.add_option("--%s" % args[0], *args[1:], **kw)
        self.posargs.append((args[0], pa_help))

    def get_usage(self, *args, **kwargs):
        self.usage = "%%prog %s [options]\n\nPositional Arguments:\n %s" % \
        (' '.join(["<%s>" % arg[0] for arg in self.posargs]), '\n '.join(["%s: %s" % (arg) for arg in self.posargs]))
        return super(self.__class__, self).get_usage(*args, **kwargs)

    def parse_args(self, *args, **kwargs):
        args = sys.argv[1:]
        args0 = []
        for p, v in zip(self.posargs, args):
            args0.append("--%s" % p[0])
            args0.append(v)
        args = args0 + args
        options, args = super(self.__class__, self).parse_args(args, **kwargs)
        if len(args) < len(self.posargs):
            msg = 'Missing value(s) for "%s"\n' % ", ".join([arg[0] for arg in self.posargs][len(args):])
            self.error(msg)
        return options, args

if __name__ == '__main__':
    #parser = PAOptionParser("My usage str")
    parser = PAOptionParser()
    parser.add_posarg("Foo", help="Foo usage")
    parser.add_posarg("Bar", dest="bar_dest")
    parser.add_posarg("Language", dest='tr_type', type="choice", choices=("Python", "Other"))
    parser.add_option('--stocksym', dest='symbol')
    values, args = parser.parse_args()
    print values, args

# python mycp.py  -h
# python mycp.py
# python mycp.py  foo
# python mycp.py  foo bar
#
# python mycp.py foo bar lava
# Usage: pa.py <Foo> <Bar> <Language> [options]

# Positional Arguments:
# Foo: Foo usage
# Bar: 
# Language: 
#
# pa.py: error: option --Language: invalid choice: 'lava' (choose from 'Python', 'Other')

4 comments

Steven Bethard 13 years, 3 months ago  # | flag

If you want full support for positional arguments, take a look at argparse (http://argparse.python-hosting.com/), where your code would look something like:

parser = argparse.ArgumentParser()
parser.add_argument("foo", help="foo usage")
parser.add_argument("bar", dest="bar_dest")
parser.add_argument("language", dest='tr_type', choices=("Python", "Other"))
parser.add_argument('--stocksym', dest='symbol')
args = parser.parse_args()
print args.foo, args.bar, args.language, args.symbol
Vijayendra Bapte 9 years, 9 months ago  # | flag

You should avoid using super(self.__class__, self) for calling parent class method.

example:

class A(object):
    def greet(self):
        print "Hello world from A"

class B(A):
    def greet(self):
        super(B, self).greet()
        print "Hello world from B"

class C(A):
    def greet(self):
        super(C, self).greet()
        print "Hello world from C"

class D(C, B):
    def greet(self):
        super(D, self).greet()
        print "Hello world from D"

if __name__ == "__main__":
    D().greet()

output:

Hello world from A
Hello world from B
Hello world from C
Hello world from D

If you replace first parameter of super with self.__class__ then you will get

RuntimeError: maximum recursion depth exceeded while calling a Python object.
Shekhar Tiwatne (author) 9 years, 9 months ago  # | flag

Yes, as you point out using self this way isn't safe. Agree. Even though self.__class__ is effectively same as D (or B or A for that matter). Have any explanation to that?

On side note, if we take multiple inheritance out this works well. Like D inheriting 'C and A' and C inherits A, is brain exploding.

Shekhar Tiwatne (author) 9 years, 9 months ago  # | flag

While I have modified recipe as per Vijayendra's solution, as Steven mentions argparse is definitely a better solution. I have recently used it :)