Lab: Files

The problems in this lab will help you understand reading and writing files.

Part 1: Reading and Writing a Text File

This part will help you understand reading and writing.

1. Open and Read a File

Create a file called ``test.txt`` with a few lines in it. Read and print the first line of the file. Don’t forget to close the file.

2. Read Two Lines

Wirte a program that reads the first two lines of test.txt and prints them. Don’t forget to close the file.

3. Write a File

Use Python to write the following into a file called fish.txt:

One fish,
Two fish,
Red fish,
Blue fish.

The program should create the file if it doesn’t exist.

4. Write Data

Write a program that uses the widgets in the next cell to:

• Get the name of a file (widget_filename)

• Get a word (widget_word)

• Get a number (widget_times)

import ipywidgets
widget_filename = ipywidgets.Text(description="Filename:")
widget_word = ipywidgets.Text(description="Word:")
widget_times = ipywidgets.IntSlider(description="Times:")
display(widget_filename, widget_word, widget_times)

Open the file widget_filename.value then write widget_word.value widget_times.value number of times. Hint: Remember what happens when you multiply a string by an integer (e.g. ``”hello” * 100``)

5. Read Lines and Reorder

Write a program that reads the four lines in fish.txt and prints them in reverse order.

6. Read Into a List

Write a program that reads fish.txt into a list.

7. Print the Last Line

Use the list you created in the previous question to print the last line of the file.

Part 2: Functions with Files

In this part you’ll write functions that manipulate files.

1. Read a Whole File

Write a function called read_file that has an argument filename that contains the name of a file. The function should read the file and return it’s entire contents. The function must close the file before returning.

• Name: read_file

• Arguments:

  • filename (string) The name of a file to read.

• Returns: (string) The contents of the file.

Test your function below. You can use the file example.txt for testing purposes.

2. Write a Whole File

Write a function called write_file that takes two arguments filename and contents. The function opens the file named in filename removing any previous contents and replacing them with contents. Return the number of characters written to the file. The function must close the file before returning.

• Name: write_file

• Arguments:

  • filename (string) The name of a file to write.

  • contents (string) The stuff to write into the file.

• Returns: (integer) The number of characters written.

Test your function in the cell below. Write to a file named output.txt

3. Summing Values

Write a function called file_sum that takes one argument filename, the name of a file. The function should read the first four lines of the file and convert the lines to float, then return the sum of the numbers in the file.

• Name: file_sum

• Arguments:

  • filename (string) - The name of a file

• Returns: (float) The sum of the first four lines in the file.

Test your function in the code below. The file numbers.txt in the current directory can be used for testing.

4. Top to Bottom

Write a function called top_to_bottom that takes one argument filename. The function reads the file and re-rewrites it with the first line of the file moved to the bottom. For example if the file looked like this before top_to_bottom:

line 1
line 2
Mary had a little
Lamb.
And stuff

After top_to_bottom the file should look like this:

line 2
Mary had a little
Lamb.
And stuff
line 1

• Name: top_to_bottom

• Arguments:

  • filename (string) - The name of a file

• Returns: None

Test your function in the cell below: