Is it ‘allowed’ to get parameters like this

[Peter Otten]

Cecil Westerhof wrote:

> It has been a while since I worked with Python. I wanted to get some
> stats about the idle time of my computer, so that was a good moment to
> pick up Python again. ;-)
> 
> 
> As I understood it getopt is the way to get the parameters for your
> script. But at the moment it looks a bit like overkill, my script does
> not have much parameters as parameter_error shows:
> def parameter_error():
>     print('Error: {0} [ --5 | --10 | --20 | --25 ] --all | --this-month |
>     --this-year | --today'
>           .format(basename(sys.argv[0])))
>     sys.exit(1)
> 
> So at the moment this is my init function:
> def init():
>     global conn
>     global cursor
>     global vmstat_params
> 
>     arg_length = len(sys.argv)
>     if   (arg_length == 1):
>         period       = '--today'
>         slice_length = 20
>     elif (arg_length == 2):
>         period       = sys.argv[1]
>         slice_length = 20
>     elif (arg_length == 3):
>         period = sys.argv[2]
>         if (sys.argv[1] in ['--5', '--10', '--20', '--25']):
>             slice_length = int(sys.argv[1][2:])
>         else:
>             parameter_error()
>     else:
>         parameter_error()

<snip>

Contrast that with the following cli based on argparse:

$ cat demo.py
#!/usr/bin/env python3

def init():
    import argparse

    parser = argparse.ArgumentParser()
    parser.add_argument(
        "period", nargs="?",
        choices="today this-month this-year all".split(),
        default="today",
        help="Period covered")
    parser.add_argument(
        "slice_length", type=int, nargs="?",
        choices=[5, 10, 20, 25],
        default=20,
        help="Whatever...")

    print(parser.parse_args())

init()
$ ./demo.py -h
usage: demo.py [-h] [{today,this-month,this-year,all}] [{5,10,20,25}]

positional arguments:
  {today,this-month,this-year,all}
                        Period covered
  {5,10,20,25}          Whatever...

optional arguments:
  -h, --help            show this help message and exit
$ ./demo.py all
Namespace(period='all', slice_length=20)
$ ./demo.py none
usage: demo.py [-h] [{today,this-month,this-year,all}] [{5,10,20,25}]
demo.py: error: argument period: invalid choice: 'none' (choose from 
'today', 'this-month', 'this-year', 'all')
$ ./demo.py all 5
Namespace(period='all', slice_length=5)
$ ./demo.py all 7
usage: demo.py [-h] [{today,this-month,this-year,all}] [{5,10,20,25}]
demo.py: error: argument slice_length: invalid choice: 7 (choose from 5, 10, 
20, 25)

The interface is not exactly the same, but the code that defines it is 
declarative rather than trying to cover all possible combinations of input 
and should be easier to maintain in the long run.

Thus the answer to your question is no, the Python Police will have to 
arrest you if you stick with your original approach ;)