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.

Exercises¶

Exercise 1: 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¶

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 2.1: question 13.11.1¶

Create a script 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()

In modern Python, there are better solutions.

A better solution

This is an answer that would be superior in modern Python:

with open("friends.txt", "r") as f:
    xs = f.readlines()

xs = reversed(xs)

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

This is considered better, because on cannot forget to close the files anymore: this is now done automatically.