Python Argparse: Issue with optional arguments which are negative numbers


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 -2e-5. However this does not work – argparse interprets this is a positional argument. If I do -0.00002 it works: argparse reads it as a negative number. Is it possible to have able to read in -2e-3?

The code is below, and an example of how I would run it is:

./ --xlim -2.e-3 1e4 

If I do the following it works:

./ --xlim -0.002 1e4 

The code:

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

Whilst I can get it to work this way I would really rather be able to use scientific notation. Anyone have any ideas?


Asked By: Ger



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:

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:

$ ./ @@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:

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

Example output:

$ ./ --xlim "-2e-3 1e4"
Namespace(xlim=[-0.002, 10000.0])
Answered By: jcollado

If you are up to modifying itself, you could change the negative number matcher to handle scientific notation:

In class _ActionsContainer.__init__()

self._negative_number_matcher = _re.compile(r'^-(d+.?|d*.d+)([eE][+-]?d+)?$')

Or after creating the parser, you could set parser._negative_number_matcher to this value. This approach might have problems if you are creating groups or subparsers, but should work with a simple parser.

Answered By: hpaulj

One workaround I’ve found is to quote the value, but adding a space. That is,

./ --xlim " -2.e-3" 1e4

This way argparse won’t think -2.e-3 is an option name because the first character is not a hyphen-dash, but it will still be converted properly to a float because float(string) ignores spaces on the left.

Answered By: itub

Here is the code that I use. (It is similar to jeremiahbuddha’s but it answers the question more directly since it deals with negative numbers.)

Put this before calling argparse.ArgumentParser()

for i, arg in enumerate(sys.argv):
  if (arg[0] == '-') and arg[1].isdigit(): sys.argv[i] = ' ' + arg
Answered By: andrewfn

Another workaround is to pass in the argument using ‘=‘ symbol in addition to quoting the argument – i.e., --xlim="-2.3e14"

Answered By: toes

Inspired by andrewfn’s approach, I created a separate helper function to do the sys.argv fiddling:

def _tweak_neg_scinot():
    import re
    import sys
    p = re.compile('-\d*\.?\d*e', re.I)
    sys.argv = [' ' + a if p.match(a) else a for a in sys.argv]

The regex looks for:

  • - : a negative sign
  • \d* : zero or more digits (for oddly formatted values like -.5e-2 or -4354.5e-6)
  • \.? : an optional period (e.g., -2e-5 is reasonable)
  • \d* : another set of zero or more digits (for things like -2e-5 and -7.e-3)
  • e : to match the exponent marker

re.I makes it match both -2e-5 and -2E-5. Using p.match means that it only searches from the start of each string.

Answered By: hBy2Py

If you specify the value for your option with an equals sign, argparse will not treat it as a separate option, even if it starts with -:

./ --xlim='-0.002 1e4'
# As opposed to --xlim '-0.002 1e4'

And if the value does not have spaces in it (or other special characters given your shell), you can drop the quotes:

./ --xlim=-0.002


With this, there is no need to write your own type= parser or redefine the prefix character from - to @ as the accepted answer suggests.

Answered By: mxxk
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