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 #

[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

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] Helpful Examples #

Read File with While #

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()

Sometimes you might want to skip some lines. If this is your file:

Hi, this is Anna. Would you send your notes from today? I was sick.
Date: 12/9/2025
Sent: 21:10
Sure, just give me a sec to find them.
Date: 12/9/2025
Sent: 21:55
Youre the best!!! 🙏🙏🙏
Date: 12/9/2025
Sent: 22:38

You might want to only print every third line:

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 date line
    line = file.readline() # read sent line   
    line = file.readline() # read message line

Boolean Operators #

Boolean Operators are AND, OR, and NOT. You can use these in any condition.

AND #

The statement returns True if both conditions are True

# the student will only study for hl tests
has_test = True
is_hl = True
if has_test and is_hl:
    print("study for test") 

>> True

OR #

The statement returns True if either condition is True

# the student will stay home if they have a fever or if they are sneezing
has_fever = True
sneezing = False

if has_fever or sneezing:
    print("stay home from school -- too sick")

>> True

NOT #

Reverses the value of the boolean. Works similarly to !.

while not line == "":
    line = file2.readline()

Strip() #

The string function strip() removes any extra spaces or newline characters \n from the end of a line of text.

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

cleaned_line is now "Hello World!"

Casting #

To convert a string to an int, use the int() function:

text = "392"
number = int(text) ## cast a string to an integer

[2] File I/O Exercises #

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:

1000
2000
3000

4000

5000
6000

7000
8000
9000

10000

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 who_has_food.py, write a function finds the student carrying the most calories and prints out how many total calories that student carrying.

• Parameter: filepath to the food log
• Add exception handling to your code to handle incorrect file paths and incorrectly formatted files.
• Test your function thoroughly to be sure the exception handling is working

Merge Poem #

A beautiful poem has sadly been broken into two incomplete files. The lines from the two incomplete files should be interleaved into the output file (e.g., first line from poem_pt_1, first line from poem_pt_2, second line from poem_pt_1, etc.)

💻 In merge_poem.py, write a function that merges the text fron poem_pt_1.txt and poem_pt_2.txt into a single poem, and writes the whole poem to a new file, wholepoem.txt.

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 show_inventory.py, write a function that takes in a file path and prints out only the in-stock items in the following format:

Laptop - $1200.00
Mouse - $25.00
...

• Parameter: filepath to the inventory
• Add exception handling to your code to handle incorrect file paths and incorrectly formatted files.
• Test your function thoroughly to be sure the exception handling is working

[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