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Command Line Arguments in Python: sys.argv, argparse

Modified: | Tags: Python

In Python, you can use sys.argv or the argparse module to handle command line arguments. The sys and argparse modules are both included in the standard library, so no additional installation is required.

Contents

To take input from the keyboard in your program, use the input() function.

For information on handling arguments in functions instead of command line arguments, refer to the following article.

sys.argv vs the argparse module

sys.argv is very simple and easy to use; however, you need to manage the number of arguments and perform type conversion yourself. On the other hand, while using the argparse module requires some configuration code, it allows you to utilize options and manage any number of arguments.

If the number of arguments is fixed and simple, sys.argv is usually sufficient. However, for programs used as command line tools, argparse is more convenient due to its flexible argument handling capabilities.

How to use sys.argv

Here is an example script:

import sys

print('sys.argv         : ', sys.argv)
print('type(sys.argv)   : ', type(sys.argv))
print('len(sys.argv)    : ', len(sys.argv))

print()

print('sys.argv[0]      : ', sys.argv[0])
print('sys.argv[1]      : ', sys.argv[1])
print('sys.argv[2]      : ', sys.argv[2])
print('type(sys.argv[0]): ', type(sys.argv[0]))
print('type(sys.argv[1]): ', type(sys.argv[1]))
print('type(sys.argv[2]): ', type(sys.argv[2]))
source: sys_argv_test.py

The results of running the python (or python3) command with arguments are as follows. Arguments are specified after the script file path, separated by spaces.

$ python3 sys_argv_test.py a 100
sys.argv         :  ['sys_argv_test.py', 'a', '100']
type(sys.argv)   :  <class 'list'>
len(sys.argv)    :  3

sys.argv[0]      :  sys_argv_test.py
sys.argv[1]      :  a
sys.argv[2]      :  100
type(sys.argv[0]):  <class 'str'>
type(sys.argv[1]):  <class 'str'>
type(sys.argv[2]):  <class 'str'>

As you can see from this result, sys.argv handles command line arguments as follows:

  • sys.argv is a list
    • You can get the number of items (= the number of arguments) with len()
  • The first element stores the script file path
    • Whether this is a full path or not depends on the operating system
  • All items are strings (str)
    • Even if specified as a number, it is stored as a string
    • Use int() or float() to convert to a number

See the following articles for len() to get the number of items in a list, and int() and float() to convert strings to numbers.

In the sample code above, specifying fewer arguments than expected causes an error.

$ python3 sys_argv_test.py a
sys.argv         :  ['sys_argv_test.py', 'a']
type(sys.argv)   :  <class 'list'>
len(sys.argv)    :  2

sys.argv[0]      :  sys_argv_test.py
sys.argv[1]      :  a
Traceback (most recent call last):
  File "sys_argv_test.py", line 11, in <module>
    print('sys.argv[2]      : ', sys.argv[2])
IndexError: list index out of range

To deal with such cases, for example, you can either use len() to get the number of arguments and then use an if statement for conditional branching, or handle exceptions.

The argparse module

For more flexible command handling, the argparse module allows you to define argument types, accept multiple arguments as a list, and use options like -i and -o.

For a detailed explanation of the argparse module's usage, refer to the comprehensive official tutorial. While argparse is versatile with extensive features, the quickest way to learn is by going through the tutorial from start to finish.

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