File Handling

Why this matters

Programs forget everything when they stop running. Files let your code remember things: scores, notes, settings, stories—anything made of text or bytes.

Core ideas

Files and paths: A file lives in a folder (directory). A path is "where" it lives (like data/notes.txt).
Opening files: Use open(path, mode, encoding). Common modes:
- 'r' = read (file must exist)
- 'w' = write (creates or overwrites)
- 'a' = append (adds to the end)
Always close files: The with statement closes files for you, even if errors happen.
Reading text: read() gets the whole file; for line in file reads line by line; readline() gets one line.
Writing text: write("text") writes a string. Remember to add "\n" for new lines.
Encoding: Use encoding="utf-8" for text files.
Safer paths: Use pathlib.Path to build paths that work across systems.
Errors: FileNotFoundError happens if you try to read a file that isn't there.

Code examples

1) Write to a new file (overwrites if it exists)

# Creates (or overwrites) greetings.txt
with open("greetings.txt", "w", encoding="utf-8") as f:
    f.write("Hello, PyVerse!\n")
    f.write("This is a saved line.\n")

2) Append to a file (keeps old content and adds more)

with open("greetings.txt", "a", encoding="utf-8") as f:
    f.write("Another line at the end.\n")

3) Read the whole file at once

with open("greetings.txt", "r", encoding="utf-8") as f:
    text = f.read()
print(text)

4) Read line by line (better for big files)

with open("greetings.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())  # strip() removes the trailing newline

5) Write many lines from a list (remember the newline!)

names = ["Ada", "Linus", "Guido"]
with open("names.txt", "w", encoding="utf-8") as f:
    for name in names:
        f.write(name + "\n")

6) Build paths safely with pathlib

from pathlib import Path

data_folder = Path("data")
data_folder.mkdir(exist_ok=True)  # make folder if it doesn't exist

file_path = data_folder / "notes.txt"
with file_path.open("w", encoding="utf-8") as f:
    f.write("Saved in the data folder!\n")

7) Friendly error handling

try:
    with open("missing.txt", "r", encoding="utf-8") as f:
        print(f.read())
except FileNotFoundError:
    print("Oops! missing.txt was not found.")

Mini Project: Personal Journal (text-based)

Build a tiny journal app that saves entries to a file. You'll practice reading, writing, appending, and searching.

Requirements

Create or open journal.txt.
Menu:
1. Add entry: ask for text, save with a timestamp.
2. Read entries: show all saved entries.
3. Search entries: ask for a keyword and show matching lines.
4. Quit.
Handle the case where the file doesn't exist yet.

Starter code

from pathlib import Path
from datetime import datetime

JOURNAL = Path("journal.txt")

def add_entry():
    text = input("Write your entry: ").strip()
    timestamp = datetime.now().strftime("%Y-%m-%d %H:%M")
    # TODO: open JOURNAL in append mode and write "[timestamp] text\n"
    with JOURNAL.open("a", encoding="utf-8") as f:
        f.write(f"[{timestamp}] {text}\n")
    print("Entry saved.")

def read_entries():
    if not JOURNAL.exists():
        print("No entries yet.")
        return
    with JOURNAL.open("r", encoding="utf-8") as f:
        print("---- Journal ----")
        print(f.read())
        print("-----------------")

def search_entries():
    if not JOURNAL.exists():
        print("No entries to search.")
        return
    keyword = input("Keyword: ").strip().lower()
    print(f"Results for '{keyword}':")
    found = False
    with JOURNAL.open("r", encoding="utf-8") as f:
        for line in f:
            if keyword in line.lower():
                print(line.strip())
                found = True
    if not found:
        print("No matches found.")

def main():
    while True:
        print("\n1) Add entry  2) Read entries  3) Search  4) Quit")
        choice = input("Choose: ").strip()
        if choice == "1":
            add_entry()
        elif choice == "2":
            read_entries()
        elif choice == "3":
            search_entries()
        elif choice == "4":
            print("Goodbye!")
            break
        else:
            print("Please choose 1-4.")

if __name__ == "__main__":
    main()

Challenge extensions

Add an option to export only today's entries to today.txt.
Count how many entries are in the journal and show the count.
Prevent empty entries.

Summary

Use open(path, mode, encoding) and prefer with to auto-close files.
'r' reads, 'w' overwrites, 'a' appends.
read() gets all text; loop line by line for large files.
write() saves strings; add "\n" for new lines.
Use pathlib.Path for safe, clean paths.
Handle FileNotFoundError to keep programs friendly.

With these skills, your programs can remember, share, and analyze information across runs—welcome to the world of persistent data!

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