Parsing command-line arguments in Python

Let's talk about parsing command line arguments in Python.

Parsing command-line arguments

We have a program here called add.py that uses Python's argparse module to parse two arguments, x and y:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('x', type=float)
parser.add_argument('y', type=float)
args = parser.parse_args()

print(args.x + args.y)

When called, it converts the arguments to floating point numbers and then adds them up:

$ python3 add.py 3 4
7.0

Now, we have a lot of code here just to parse two command-line arguments. Why did we use argparse for this?

Why not use sys.argv to get command-line arguments?

Instead of all that code, we could just read sys.argv (which is a list of all command-line arguments as strings) and then convert our two expected arguments to floating point numbers:

import sys

[program, x, y] = sys.argv

print(float(x) + float(y))

As you can see, this does work:

$ python3 add_old.py 3 4
7.0

But the downside of doing things this way is, if we make a mistake when calling this command-line program, we won't see very helpful error messages.

For example, if we pass too few arguments Python doesn't really tell us much about what's going on:

$ python3 add_old.py 3
Traceback (most recent call last):
  File "/home/trey/_/add_old.py", line 3, in <module>
    [program, x, y] = sys.argv
ValueError: not enough values to unpack (expected 3, got 2)

Similarly, if we pass in something that isn't a number, we get only a slightly helpful error message:

$ python3 add_old.py 3 a
Traceback (most recent call last):
  File "/home/trey/_/add_old.py", line 5, in <module>
    print(float(x) + float(y))
ValueError: could not convert string to float: 'a'

And if we ask for help, we won't get very helpful information at all:

$ python3 add_old.py --help
Traceback (most recent call last):
  File "/home/trey/_/add_old.py", line 3, in <module>
    [program, x, y] = sys.argv
ValueError: not enough values to unpack (expected 3, got 2)

In all of these cases we're seeing an unhelpful traceback printed out instead of a helpful error message. Python's argparse module will print out helpful error message in all of these cases instead.

Comparing argparse to manually reading sys.argv

Here's our command-line program from before that uses argparse:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('x', type=float)
parser.add_argument('y', type=float)
args = parser.parse_args()

print(args.x + args.y)

When we ask this program for help (by passing --help argument) it shows us usage information:

$ python3 add.py --help
usage: add.py [-h] x y

positional arguments:
  x
  y

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

If we parse in too few arguments, it tells us there's another argument that's required:

$ python3 add.py 3
usage: add.py [-h] x y
add.py: error: the following arguments are required: y

Similarly, if we pass in the wrong value for an argument (a non-number), it tells us that our argument can't be converted to a floating point number:

$ python3 add.py 3 a
usage: add.py [-h] x y
add.py: error: argument y: invalid float value: 'a'

The argparse module is really handy for making command-line interfaces that are friendly.

But argparse is not just for simple command-line interfaces like this one, we can also accept optional arguments using argparse.

Accepting optional command-line arguments

This version of our add.py program accepts an optional --expected or -e argument:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('x', type=float)
parser.add_argument('y', type=float)
parser.add_argument('--expected', '-e', type=float)
args = parser.parse_args()

total = args.x + args.y
print(total)
if args.expected is not None and args.expected != total:
    print(f"Expected {args.expected} but got {total}")

This line of code adds an option, that is an optional argument, to our command-line parser:

parser.add_argument('--expected', '-e', type=float)

When running this program, this optional argument can be specified with either --expected=SOMETHING or -e SOMETHING.

If this expected argument isn't provided then it will default to the value None.

When this expected argument is not None we check to see it was equal to total (which is the total sum of our x and y arguments):

if args.expected is not None and args.expected != total:
    print(f"Expected {args.expected} but got {total}")

For example if we pass in 3 and 4 and an expected value of 7 we won't see anything printed out (since 3 + 4 is 7):

$ python3 add.py 3 4 -e 7
7.0

We could have also specified that expected argument with --expected= instead of -e:

$ python3 add.py 3 4 --expected=7
7.0

This argument might is a little bit silly, but we could use it for confirming issues with floating point addition.

For instance, we're adding 0.1 and 0.02 to check whether it's exactly equal to 0.12 (it's not):

$ python3 add.py 0.1 0.02 --expected=0.12
0.12000000000000001
Expected 0.12 but got 0.12000000000000001

This addition gave us a number very close to 0.12 (but not exactly 0.12) so we see an error message printed out.

Optional command-line arguments without a value

We can also make optional command-line arguments that don't accept a value.

In this version of add.py we're accepting an optional --verbose argument:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('x', type=float)
parser.add_argument('y', type=float)
parser.add_argument('--verbose', action='store_true')
args = parser.parse_args()

if args.verbose:
    print(f"Adding {args.x} and {args.y}")
print(args.x + args.y)

The mere presence of a --verbose argument will store the value True:

parser.add_argument('--verbose', action='store_true')

If that argument is not given, the verbose attribute will be something falsey (see truthiness) and if it is given, we'll get True.

if args.verbose:
    print(f"Adding {args.x} and {args.y}")

So when the argument --verbose is given, our program print out the numbers we're adding together:

$ python3 add.py --verbose 3 4
Adding 3.0 and 4.0
7.0

When --verbose is not given our program does exactly what it did before:

$ python3 add.py 3 4
7.0

Adding documentation for command-line interface using argparse

The argparse module also allows you to add documentation for your command-line interfaces.

Here's one more version of our add.py program with lots of help text added:

import argparse

parser = argparse.ArgumentParser("Add two numbers together")
parser.add_argument('x', type=float, help="First number to add")
parser.add_argument('y', type=float, help="Second number to add")
parser.add_argument('--verbose', action='store_true', help="Show more context")
args = parser.parse_args()

if args.verbose:
    print(f"Adding {args.x} and {args.y}")
print(args.x + args.y)

If the end user asks for help when running this program they'll see some very helpful usage information:

$ python3 add.py --help
usage: Add two numbers together [-h] [--verbose] x y

positional arguments:
  x           First number to add
  y           Second number to add

optional arguments:
  -h, --help  show this help message and exit
  --verbose   Show more context

This usage information let's end users know that this program:

1. Will "add two numbers together"
2. Accepts two positional arguments, the "first number to add" and the "second number to add"
3. Accepts a --verbose argument to "show more context"

Summary

If you're trying to make a friendly command-line interface in Python you can use the argparse (included in Python's standard library).

The argparse module makes pretty standard command-line interfaces which feel typical or normal on Linux, Mac, and Windows.