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Command-line arguments

Learning outcomes

For teachers

Teaching goals are:

  • Learners have used sys.args
  • Learners have run a Python script with command-line arguments

Lesson plan:

  • 5 mins: prior knowledge
  • 5 mins: presentation
  • 15 mins: challenge
  • 5 mins: feedback

Prior:

  • What are command-line arguments?
  • What is meant with 'a command-line interface'?
  • Why use command-line arguments?

Overview

Sometime, also programs need input, for example the name of the input file and the name of a results file:

python my_input.csv my_results.csv

Sometime you need to get 'stuff to work', for example, code written by someone else. In this session, we practice this, going through the problems and errors that occur when you want to 'just want to run the stupid code'.

Exercises

Exercise 1: Read the command-line arguments

Read the official Python documentation about sys.argv.

Create a script called read_argv.py with the following content:

import sys

print(sys.argv)

Run the Python script as shown below. What does it print?

python read_argv.py
Answer 
['read_argv.py']

Run the Python script as shown below. What does it print?

python read_argv.py hello world
Answer 
['read_argv.py', 'hello', 'world']

Run the Python script as shown below. What does it print?

python read_argv.py "hello world"
Answer 
['read_argv.py', 'hello world']

Run the Python script as shown below. What does it print?

python read_argv.py 'hello world'
Answer 
['read_argv.py', 'hello world']

Exercise 2: Read a command-line argument

sys.argv is a list. Here we use this list

Read the following sections of How to Think Like a Computer Scientist: Learning with Python 3:

  • 11. Lists
  • 11.1. List values
  • 11.2. Accessing elements

Modify the script in such a way that the script will only show the first command-line argument.

For example, running the script like this:

python read_argv.py hello world

it should show hello.

Answer

A possible implementation is this:

import sys

args = sys.argv
print(args[1])

Some comments:

  • print(args[0]) prints the name of the script
  • the script does not check if the user supplies command-line arguments

Exercise 3: Work with a command-line argument

Read the following sections of How to Think Like a Computer Scientist: Learning with Python 3:

  • 8.7. String comparison

Now, we make the script produce different output.

If the script is run like this:

python read_argv.py hello

it should show Hello human!.

If the script is run like this:

python read_argv.py bye

it should show Sad to see you go human!.

Write the script to do that.

Answer

A possible implementation is this:

import sys

args = sys.argv
word = args[1]

if word == 'hello':
    print('Hello human!')

if word == 'bye':
    print('Sad to see you go human!')

Some comments:

  • print(args[0]) prints the name of the script
  • the script does not check if the user supplies command-line arguments
  • instead of using two if statements (which is good enough for this exercise), an elif could be used in the second conditional: it would be considered better, because if the word is hello, then there is no need anymore to check if it is bye