Command line with Python

Question

I'm developing a simple project with the purpose or learning Python, actually I have version 3.6 and I wanted to build a command line tool to generate password with specific criteria. So fare here is what I got:

import argparse
parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument("-a", "-auto", help="auto mode", action="store_true")
group.add_argument("-m", "-manual", help="manual mode", action="store_true")
parser.parse_args()

the dead end is I have no idea how to limit a command, example -l -lenght to the reqisite of having -m another stop is, how do I define -l so that it can accept a value, for example -l:8 to specify a password of 8 characters, I also want to put a range limit on -l values, example -l:8-256 and finally, I dont understand the difference between - and -- when defining the parameters.

I have already done all the part of generating passwords, just need a way to control its flow from outside so implementing parameters looked like a good way of doing this.

Keegan Ferrett · Accepted Answer · 2018-05-07 20:19:14Z

2

What you are looking for is the choices option.

add_argument('--length', type=int, choices=range(8,257)

This will only accept integer values between 8 and 256 (inclusive).

As for your second question, - indicates the shorthand for an option, which is common in most CLI tools, well -- indicates the long hand. It is common practice is CLI tools to provide both a long and a short hand for options.

answered May 7, 2018 at 20:19

Keegan Ferrett

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7 Comments

Jason 'Slingshot' Miller Over a year ago

how do I bing a parameter to only be used with another paramenter, say for example I can specify -l only when it is run -m while in auto modo I cannot do that and will not accept the lenght value.

chepner Over a year ago

That is best done in post-processing, after you have parsed all the options.

Keegan Ferrett Over a year ago

To may knowledge you cannot do that with argparse, but that is a very easy if statement that can be done after your arguments have been parsed

chepner Over a year ago

Note that your usage message will show all 240+ values if you use choices like this.

Keegan Ferrett Over a year ago

@chepner an easy way around this is to change the usage with the metavar option. Checkout stackoverflow.com/questions/25295487/… for an explanation

|

chepner · Accepted Answer · 2018-05-07 20:50:08Z

1

You can define a custom type to check for valid values:

from argparse import ArgumentTypeError

def passwd_len(s):
    try:
        s = int(s)
    except ValueError:
        raise ArgumentTypeError("'%s' is not an integer" % (s,))

    if not (8 <= s <= 256):
        raise ArgumentTypeError("%d is not between 8 and 256, inclusive" % (s,))

    return s

parser.add_argument("--length", "-l", type=passwd_len)

The difference between -- and - is one of conventions. Historically, options were single-letter arguments prefixed with a -, and groups of options could be combined: -ab was the same as -a -b. In order to support that convention and allow longer option names, a new convention of using -- to prefix multi-character names was devised. --length is a single option, not a group of six options -l, -e, -n, -g, -t, and -h. Long options cannot be grouped.

I would define the -a and -m options like this:

group = parser.add_mutually_exclusive_group()
group.add_argument("-a", "--auto", help="auto mode", action="store_const", const="auto", dest="mode")
group.add_argument("-m", "--manual", help="manual mode", action="store_const", const="manual", dest="mode")
group.add_argument("--mode", choices=["auto", "manual"])

Now instead of having two Boolean attributes that can never have the same value, you have just one attribute whose value you can check directly. Think of --mode as being the canonical way of choosing a mode, with -a and -m as shortcuts for selecting a specific mode.

edited May 7, 2018 at 20:50

answered May 7, 2018 at 20:34

chepner

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2 Comments

Jason 'Slingshot' Miller Over a year ago

what is the advantage of havind a third attribute named --mode respect using alternatively only -a or -m?

chepner Over a year ago

You could drop that option completely, and still have -a and -m as the only way to set the mode destination. I threw it in as sort of an afterthought. (You could think of it as demonstrating just how flexible you can be in setting a single destination.)

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