Python 3: Accept Positional Command Line Arguments

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This article describes how to use the argparse module, which comes built into Python versions 3.2 and up.

Let's say we want to write a program called hello.py which takes a name as a positional command line argument and says hello to that name. We want the user to be able to run the program by typing something like the following in a command line terminal:

$ python3 hello.py Wolfgang
Hello, Wolfgang!

How would we write hello.py?

The argparse module allows us to accept command line arguments. Here's how we can write hello.py using argparse:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('name')
args = parser.parse_args()

name = args.name
print( f'Hello, { name }!' )

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import argparse

parser = argparse.ArgumentParser()
parser.add_argument('name')
args = parser.parse_args()

name = args.name
print( f'Hello, { name }!' )

If the user runs hello.py without any name argument, then argparse displays a help message automatically for us:

$ python3 hello.py
usage: hello.py [-h] name
hello.py: error: the following arguments are required: name

The user can also run with the -h flag to display a more detailed help message:

$ python3 hello.py -h
usage: hello.py [-h] name

positional arguments:
  name

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

We can add additional arguments using the add_argument() function. For example, to allow the user of our example hello.py program to specify a first, middle and last name, we can write hello.py like this:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('first_name')
parser.add_argument('middle_name')
parser.add_argument('last_name')
args = parser.parse_args()

first_name  = args.first_name
middle_name = args.middle_name
last_name   = args.last_name

print( f'Hello, { first_name } { middle_name } { last_name }!' )

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import argparse

parser = argparse.ArgumentParser()
parser.add_argument('first_name')
parser.add_argument('middle_name')
parser.add_argument('last_name')
args = parser.parse_args()

first_name  = args.first_name
middle_name = args.middle_name
last_name   = args.last_name

print( f'Hello, { first_name } { middle_name } { last_name }!' )

Now our program produces the following output:

$ python3 hello.py Wolfgang Amadeus Mozart
Hello, Wolfgang Amadeus Mozart!

If the user only supplies a first and middle name, then argparse automatically displays the following help message:

$ python3 hello.py Wolfgang Amadeus
usage: hello.py [-h] first_name middle_name last_name
hello.py: error: the following arguments are required: last_name

• argparse — Parser for command-line options, arguments and sub-commands — Python 3 documentation

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