Command-line arguments

Learning outcomes

• Can read the command-line arguments
• Can run a Python script with command-line arguments

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'.

flowchart TD
    python[[Python]]

    %% Give a white background to all nodes, instead of a transparent one
    classDef node fill:#fff,color:#000,stroke:#000

    subgraph sub_programming_language[Programming language]
      interpreted_language[Interpreted language]
      programming_language[Programming language]
      scripting_language[Scripting language]
      interpreter[Interpreter]
      scripts[Scripts]
      text_files[Text files]
    end
    style sub_programming_language fill:#ccf,color:#000,stroke:#fcc

    subgraph sub_programming_terms[Programming terms]
      user_input[User input]
      file_io[File I/O]
      command_line_arguments[Command-line arguments]
      variables[Variables]
      operators[Operators]
      %% functions[Functions]
      python_packages[Python packages]
      graphics[Graphics]
    end 
    style sub_programming_terms fill:#cfc,color:#000,stroke:#fcc

    subgraph sub_naiss[NAISS]
      your_hpc_center[Your HPC center]
      modules[software modules]
      your_hpc_cluster[Your HPC cluster]
      text_editors[Text editors]
      x_forwarding[X-forwarding]
      remote_desktop[Remote desktop]
    end
    style sub_naiss fill:#fcc,color:#000,stroke:#fcc

    python --> |is a| programming_language
    python --> |is a| interpreted_language
    python --> |is a| scripting_language

    programming_language --> |uses| text_files
    interpreted_language --> |has a| interpreter
    scripting_language --> |is|interpreted_language
    scripting_language --> |runs| scripts
    scripts --> |are| text_files
    scripts --> |can use| command_line_arguments

    python --> |has| python_packages
    python --> |has| variables
    %%python --> |has| functions
    python --> |has| operators
    %%functions --> |use| variables
    operators --> |work on| variables
    python_packages --> |allow| graphics
    python_packages --> |allow to use| user_input
    python_packages --> |allow to do| file_io
    python_packages --> |allow to use| command_line_arguments

    your_hpc_center --> |has| your_hpc_cluster
    your_hpc_cluster --> |have| modules
    your_hpc_cluster --> |have| text_editors
    your_hpc_cluster --> |allow| x_forwarding
    your_hpc_cluster --> |has| remote_desktop
    modules --> |allow the use of| python_packages
    modules --> |allow the use of| interpreter

    text_editors --> |work on|text_files
    x_forwarding --> |allows|graphics
    remote_desktop --> |allows|graphics


    variables ~~~ your_hpc_center

Exercises

See the exercise procedure here.

Exercise 1: Read the command-line arguments

Learning outcomes

• Rehearse creating a script
• Read the command-line arguments

Read the official Python documentation about sys.argv here.

Create a script with the following text:

read_argv.py

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

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

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

• python read_argv.py "hello world"

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

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

• python read_argv.py 'hello world'

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

Exercise 2: Read a command-line argument

Learning outcomes

• Experience sys.argv is a list
• First experience in working with a list

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.

Exercise 3: Work with a command-line argument

Learning outcomes

• Do a first string comparison

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.

Links

• How to Think Like a Computer Scientist: Learning with Python 3