python get specific arg

Question

I got a programme with initial values:

SIZE = 100
NB = 100
WIDTH_CANVAS = 150
[...]

But I want to allow the user to update these values from terminal:

python3 main.py NB=10

python3 main.py NB=10 WIDTH_CANVAS=200 SIZE=50

or nothing to change :

python3 main.py

so I tried something like that:

import sys
SIZE = sys.argv[1]
NB = sys.argv[2]
WIDTH_CANVAS = sys.argv[3]

but the problem is: what if they don't want to specify the NB for example?

• Tom Ron: Check argparse - doughellmann.com/PyMOTW/argparse

Answer 1

argparse is exactly what you need. Here is a short example (https://docs.python.org/3/library/argparse.html):

import argparse
SIZE = 100
NB = 100
parser = argparse.ArgumentParser()
parser.add_argument('--NB', dest='NB', type=int, default=NB)
parser.add_argument('--SIZE', dest='SIZE', type=int, default=SIZE)
args = parser.parse_args()
print(args.SIZE)
print(args.NB)

So for running python prog.py --SIZE 3 you will get:

3
100

Answer 2

If you want to use exactly that syntax, probably set up a dict with your defaults.

value = {'NB': 100, 'SIZE': 100, 'WIDTH_CANVAS': 150}
for arg in sys.argv[1:]:
 if '=' in arg:
 k, v = arg.split('=', 1)
 if k not in value:
 raise KeyError('Invalid command-line argument %s' % k)
 value[k] = v
 else:
 raise MaybeGripe('Some other error here?')

Now go ahead and use value['SIZE'] as you would have used the bare SIZE variable previously, etc.

As suggested in comments, a more common arrangement is to have command-line options and use the standard ArgParse module to parse them. Maybe then your syntax would look more like

python main.py --size 25 --canvas-width 100

which will be more familiar and less distracting to most users.

• tripleee: I believe the Unix dd command was implemented with a syntax like you propose partly as a joke. Unfortunately, it stuck, and we all swear over it.