How to pass None keyword as command line argument

Question

I am trying to pass None keyword as a command line parameter to a script as follows, if I explicity mention Category=None it works but the moment I switch to sys.argv[1] it fails, any pointers on how to fix this?

category = None --> works
#category=sys.argv[1] --> doesn't work 

so I tried as below which still didn't work

  if sys.argv[1].strip()==None:
    category = None
else:
    category=sys.argv[1].strip()

Command line passage:

 script.py None

Answer 1

As Stephen mentioned, None is not a string, so when comparing a str type to a NoneType type, the result will always be False .

If I just put " around None on the right-hand-side of the comparison, we get:

import sys

if sys.argv[1].strip() == "None":
    category = None
else:
    category = sys.argv[1].strip()

print(category)
print(type(category))

Resulting in:

~/temp $ python script.py 123
123
<type 'str'>
~/temp $ python script.py None
None
<type 'NoneType'>

argparse instead?

However, I recommend using argparse instead, if you're in a position to do so. I use it all the time.

The above code could be replaced with:

import argparse

parser = argparse.ArgumentParser(description='Do some cool stuff.')

def none_or_str(value):
    if value == 'None':
        return None
    return value

parser.add_argument('category', type=none_or_str, nargs='?', default=None,
                    help='the category of the stuff')

args = parser.parse_args()

print(args.category)
print(type(args.category))

Which will also tolerate no parameters

~/temp $ python script.py 
None
<type 'NoneType'>
~/temp $ python script.py 123
123
<type 'str'>
~/temp $ python script.py None
None
<type 'NoneType'>

And it auto-formats some help text for you!

~/temp $ python script.py --help
usage: script.py [-h] [category]

Do some cool stuff.

positional arguments:
  category    the category of the stuff

optional arguments:
  -h, --help  show this help message and exit

Answer 2

just an FYI. You can use ast module to convert sting 'None' to None type

Ex:

import ast
if not ast.literal_eval(sys.argv[1]):
    print "Input Arg is None"

or

if sys.argv[1].strip() == 'None':
     print "Input Arg is None"

Faster and better solution.

Answer 3

I think more appropriate way to do this would be to check if there's any parameter passed or not. If not then set category to None , otherwise set the value that is passed.

Ex:

if len(sys.argv) == 2: // or >= 2 in case of more parameter
    category = sys.argv[1].strip()
else:
    category = None

So if you just call script.py , category will be set to None .
If you call like script.py Value , category will be set to Value .

Answer 4

Piggybacking on @FraggaMuffin's answer, you may use a lambda function for convenience. Thus, instead of

parser.add_argument('category', type=none_or_str, nargs='?', default=None,
                    help='the category of the stuff')