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Files

Learning outcomes

  • Have read a file
  • Have created a file
For teachers

Teaching goals are:

  • Learners have read a file
  • Learners have created a file

Lesson plan:

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

Prior:

  • What is file I/O?
  • What are some problems you can have when reading a file?
  • What are some problems you can have when creating a file?

Overview

Most programmers need to work on data and produce some result. In Python, we -of course- can read from files and write to files. Here we do just that.

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: create a simple text file

Learning outcomes

  • Create a simple text file

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

  • 13.1. About files
  • 13.2. Writing our first file

Then do:

  • Put the code at the top of 13.2 in a Python script
  • Run that code
  • Verify that it works as expected

Exercise 2: read and create a simple text file

Learning outcomes

  • Read a simple text file
  • Repeat creating a file
  • Practice to search the internet how to reverse the order of text lines

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

  • 13.4. Turning a file into a list of lines

Consider searching for 'Python reverse order', as you will need to do this in the exercise.

Then do:

  • Exercise 13.11.1

13.11.1

Write a program that reads a file and writes out a new file with the lines in reversed order (i.e. the first line in the old file becomes the last one in the new file.)

Answer

The new thing is to use reversed:

f = open("friends.txt", "r")
xs = f.readlines()
f.close()

xs = reversed(xs)

g = open("sortedfriends.txt", "w")
for v in xs:
    g.write(v)
g.close()