Basic File Handling

Friendly introduction

Have you ever written something in a notebook to save it for later? Files are like Python's notebooks. They let your programs remember things even after the code stops running. Today you'll learn how to make a file, write to it, read from it, and add more to it. It's easier than you think!

What you'll learn

How to open a file the safe way
How to write text to a file
How to read text from a file
How to add (append) new text to an existing file
Where your file gets saved

Important idea: the with block

We'll use with open(...) as file: because it automatically closes the file for us. This keeps things neat and prevents mistakes.

Step 1: Write (create) a file

Mode 'w' means "write." It creates a new file or overwrites an existing one.

Python example:

# Create a new text file and write some lines
with open('greeting.txt', 'w') as f:
    f.write('Hello, PyVerse!\n')
    f.write('Files are just text.\n')
    f.write('This is line 3.\n')

print('Done writing greeting.txt')

Notes:

'\n' makes a new line.
If greeting.txt already exists, 'w' will replace its contents.

Step 2: Read the whole file

Mode 'r' means "read." Use read() to get everything as one string.

with open('greeting.txt', 'r') as f:
    content = f.read()

print('--- File contents ---')
print(content)

Step 3: Append (add) more text

Mode 'a' means "append." It adds new text to the end of the file without erasing what's already there.

with open('greeting.txt', 'a') as f:
    f.write('Adding a new line at the end.\n')

print('Appended a line!')

Step 4: Read a file line by line

This is handy when you want to process each line separately.

with open('greeting.txt', 'r') as f:
    for line in f:
        print(line.strip())  # .strip() removes the ending newline

Step 5: Where is my file saved?

If you use just a name like 'greeting.txt', it saves in the same folder as your Python file (or where you run the program).
You can also give a path, like 'C:/Users/You/Documents/greeting.txt' on Windows or '/Users/you/Documents/greeting.txt' on macOS.

Quick tips and common mistakes

Forgetting '\n' means your lines might stick together.
If you try to read a file that doesn't exist, Python raises FileNotFoundError. First create it (write) or check the file name.
Always prefer with open(...) so the file closes automatically.

Mini exercise: Favorites list

Goal: Ask the user for three favorite snacks, save them to a file, then read and print them.

Instructions:

Ask the user three times for a snack.
Write each snack on its own line in a file called favorites.txt.
Open the file again and print the lines back to the user.

Try it yourself:

# 1) Collect favorites
snacks = []
for i in range(3):
    snack = input(f'Enter favorite snack #{i+1}: ')
    snacks.append(snack)

# 2) Write them to a file (one per line)
with open('favorites.txt', 'w') as f:
    for s in snacks:
        f.write(s + '\n')

# 3) Read them back and print
print('\nYou entered:')
with open('favorites.txt', 'r') as f:
    for line in f:
        print('- ' + line.strip())

Short summary

Use with open('name.txt', 'w') to create or overwrite a file and write text.
Use with open('name.txt', 'r') to read a file (read() or loop line by line).
Use with open('name.txt', 'a') to add new text at the end of a file.
'\n' makes a new line, and .strip() removes it when printing.
Files are saved in your current folder unless you give a full path.

You now know how to make your programs "remember" things with files. Great job!

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