Python Argparse: Issue with optional arguments which are negative numbers

Question

I\'m having a small issue with argparse. I have an option xlim which is the xrange of a plot. I want to be able to pass numbers like <

Answer 1

As already pointed out by the comments, the problem is that a - prefix is parsed as an option instead of as an argument. One way to workaround this is change the prefix used for options with prefix_chars argument:

#!/usr/bin/python
import argparse

parser = argparse.ArgumentParser(prefix_chars='@')
parser.add_argument('@@xlim', nargs = 2,
                  help = 'X axis limits',
                  action = 'store', type = float,
                  default = [-1.e-3, 1.e-3])
print parser.parse_args()

Example output:

$ ./blaa.py @@xlim -2.e-3 1e4
Namespace(xlim=[-0.002, 10000.0])

Edit: Alternatively, you can keep using - as separator, pass xlim as a single value and use a function in type to implement your own parsing:

#!/usr/bin/python
import argparse

def two_floats(value):
    values = value.split()
    if len(values) != 2:
        raise argparse.ArgumentError
    values = map(float, values)
    return values

parser = argparse.ArgumentParser()
parser.add_argument('--xlim', 
                  help = 'X axis limits',
                  action = 'store', type=two_floats,
                  default = [-1.e-3, 1.e-3])
print parser.parse_args()

Example output:

$ ./blaa.py --xlim "-2e-3 1e4"
Namespace(xlim=[-0.002, 10000.0])