File Handling #

Syllabus Topics [SL] #

B2.5.1 Construct code to perform file-processing operations.
B2.1.3 Describe how programs use common exception handling techniques.

Key Vocabulary #

Word	Definition
Path	Location of a file
Exception	Less severe problem that occurs at runtime and can be managed using exception handling (e.g., invalid input, missing files)
ValueError	An exception that occurs when a function receives an argument of the correct type but with an invalid value (e.g., converting “abc” to an integer).
ZeroDivisionError	An exception that occurs when you attempt to divide a number by zero.
NameError	An exception that occurs when you use a variable or function name that has not been defined
FileNotFoundError	An exception that occurs when a file or directory is requested but cannot be found
Boolean Operators	Symbols used for performing arithmetic on the values `True` and `False`. Includes `AND` `OR` `NOT`
Relational Operators	Symbols used for comparing values. Includes `<`, `<=`, `>`, `>=`, `==`, `!=`.

[0] Set up #

💻 Go to your dpcs/unit01_cryptography folder.

cd ~/desktop/dpcs/unit01_cryptography/

💻 Clone your repo. This will copy it onto your computer. Be sure to replace yourgithubusername with your actual username.

git clone https://github.com/isf-dp-cs/lab_file_handling_yourgithubusername

💻 In the Terminal, type the following command to open the lab folder.

cd lab_file_handling_yourgithubusername

💻 Enter the Poetry Shell to start the lab. As a reminder, we will run this command at the start of each lab, but only when we are inside a lab folder.

poetry shell

👾 💬 Exiting the poetry shell
When you want to exit the shell, you can type exit or ^D

[1] File Handling #

In this lab you will learn how to process files.

📖 To read a whole file

file = open('example.txt', 'r')
file.read()
file.close()

open() - opens a file in a specific mode, if the file does not exisit it creates a new file
'example.txt' is the name of the file you want to open or create
'r' represents the mode. important modes to remember are:
- 'r' - read the text
- 'w' - write over existing text
- 'a' - append text to the end of the file
read() - returns all text in the file
close() - closes the file

📖 To read a single line

file = open('example.txt', 'r')
file.readline()
file.close()

📖 To read a file line-by-line

file = open('example.txt', 'r')
for line in file:
    print(line)
file.close()

📖 To append to the end of an existing file

file = open('log.txt', 'a')
file.write('A new entry. \n')
file.close()

📖 To write to a new file. If the file exists, it will overwrite it.

file = open('new_document.txt', 'w')
file.write('Hello world')
file.close()

📖 To write read a file with a while loop. You can use file.readline() to skip lines.

file = open("file_path.txt", "r")

line = file.readline() # read the first line
while line != "": # check that we're not at the end of the file
    print(line)
    line = file.readline() # read the next line

file.close()

📖 Use strip() remove extra spaces or new line characters \n

line = "Hello World!   \n"
cleaned_line = line.strip()     # cleaned_line = "Hello  World!"

[2] Exercises #

MTR Lines #

💻 Open mtr.py and construct code to perform the following actions.

Add the "Airport Express Line" to the end of mtr_lines.txt. It should appear on the next line.
Create a new file mtr_lines_upper.txt with each line in all uppercase letters.
- you can use .upper()

Check the inventory #

The file inventory.txt stores which items are for sale at a technology store. However, there is extra detail included, and some items are out of stock, and shouldn’t be shown to customers.

💻 In inventory.py, construct code to find the in-stock items and writes the item and price to a new file inventory_instock.txt

inventory_instock.txt should look something like this

Laptop - $1200.00
Mouse - $25.00
...

Book Count #

💻 In book.py construct functions to analyze any given book. The provided book in book.txt is Pride and Prejudice by Jane Austen and is provided by Project Gutenberg.

find_word_count(file, phrase)
num_characters(file)
num_words(file)

✅ Check each function! Try the names “Elizabeth” or “Darcy”. The names of the main character.

Words #

In words.py let’s learn a bit about the words in the English dictionary. The words_alpha.txt file has been sourced from this repo.

💻 Construct the function num_words_of_length(word_length). Then, print out the number of words for word length 1-20.

It should look something like

[3] Deliverables #

⚡✨ Once you complete the lab, be sure to complete these two steps:
✏️ Go to your Syllabus Content Checklist in your Google Drive and update it accordingly.
💻 Push your work to Github
git status
git add -A
git status
git commit -m "describe your code here"
git push
git remote

[4] Extensions #

In book.py write the following functions:

get_top_used_words(file, num) - returns a list of the top used words
get_word_length_distribution(file) - returns a dictionary where the keys are the word length and the values are the occurrences

Camping Trip #

On an ELP camping trip, each student has brought snacks with them. The students take turns writing down the number of calories contained by the various snacks that they’ve brought with them, one item per line. Each student separates their own inventory from the previous student’s inventory (if any) by a blank line.

For example, suppose the students finish writing their items’ calories and end up with the following list:

In case the students get hungry and need extra snacks, they need to know which student to ask: they’d like to know how many calories are being carried by the student carrying the most calories. In the example above, this is 24000 (carried by the fourth student).

The food log is stored in food.txt.

💻 In food.py, write a function finds the student carrying the most calories and returns how many total calories that student carrying.