File handling

What you'll learn

How to write text to a file, read it back, append to it, and why to use with open(...).

Why it matters

Programs need to save things: notes, logs, settings, results. Files are the simplest way to keep data after the program ends.

Explanation

You open a file with open(path, mode). The mode says what you want:

• "r": read (the default). The file must exist.
• "w": write. Creates the file, or erases an existing one.
• "a": append. Adds to the end.

Always open files with with. It closes the file for you, even if an error happens:

Ways to read: f.read() gives the whole text, f.readlines() gives a list of lines, and looping for line in f: goes one line at a time.

line.strip() removes the newline at the end. Add encoding="utf-8" to avoid surprises with non-English text.

Example

Step-by-step walkthrough

1. Mode "w" creates todo.txt and writes two lines.
2. Mode "a" adds a third line without erasing the first two.
3. Mode "r" reads the file. The comprehension strips the newline from each line.
4. The list is ['Buy milk', 'Call Ravi', 'Study Python'], with 3 tasks.
5. enumerate(lines, start=1) numbers them: 1 Buy milk, 2 Call Ravi, 3 Study Python.

Try it yourself

Append a fourth task and print the number of lines again.

Read and write a filePython

Editable codewith open("todo.txt", "w", encoding="utf-8") as f: f.write("Buy milk\n") f.write("Call Ravi\n") with open("todo.txt", "a", encoding="utf-8") as f: f.write("Study Python\n") with open("todo.txt", "r", encoding="utf-8") as f: lines = [line.strip() for line in f] print(lines) print("Tasks:", len(lines))

Run ResetCopyCtrl+Enter to run

Common mistakes

• Using "w" when you meant "a", erasing the file.
• Forgetting the newline \n, so everything ends up on one line.
• Not closing the file. Use with.
• Reading a file that does not exist, which raises FileNotFoundError.

Quick quiz

Summary

• open(path, mode) with modes r, w, a.
• Always use with so files are closed.
• Loop over a file or use read() and readlines().

Next: organising code with classes.

with open("notes.txt", "w") as f:
    f.write("Hello\n")

with open("todo.txt", "w", encoding="utf-8") as f:
    f.write("Buy milk\n")
    f.write("Call Ravi\n")

with open("todo.txt", "a", encoding="utf-8") as f:
    f.write("Study Python\n")

with open("todo.txt", "r", encoding="utf-8") as f:
    lines = [line.strip() for line in f]

print(lines)
print("Tasks:", len(lines))
for i, task in enumerate(lines, start=1):
    print(i, task)