Files: Making Data Stick Around
Everything we have done so far disappears when the program stops. Variables? Gone. Calculated results? Vanished. If you want your Python program to save data permanently — like a game saving your high score — you need to write it to a file.
The open() function and modes
Everything starts with the built-in open() function. It takes two things: the filename, and a mode that tells Python what you intend to do with the file.
"r"
Open to read. Fails if file doesn't exist.
"w"
Creates file if needed. Overwrites existing content.
"a"
Adds to the end without deleting existing content.
Getting data out of a file
After opening a file in read mode, you can use several methods to get its contents.
.read()— returns the entire file as one big string..readline()— returns just one line at a time..readlines()— returns a list where each item is one line of the file.
file = open("notes.txt", "r") content = file.read() print(content) file.close() # Always close when done!
Saving data to a file
Open the file in write mode "w" to create a file and write to it.
Use "a" (append) if you want to add to a file without destroying what's already there.
# Write mode — creates or overwrites file = open("score.txt", "w") file.write("High Score: 9500\n") file.close() # Append mode — adds to the end file = open("score.txt", "a") file.write("Second Run: 8700\n") file.close()
\n yourself to move to the next line.The safer, smarter way to open files
Remembering to call file.close() every time is annoying and easy to forget.
Python provides a cleaner approach: the with statement.
It automatically closes the file for you the moment you're done with it, even if an error occurs.
# The Pythonic way — no manual close() needed! with open("notes.txt", "r") as file: content = file.read() print(content) # File is automatically closed here ✅
with open(...) as file:. It's cleaner, safer, and it's what professional Python developers use.
Working with files is like working with a physical notebook.
open("r"): Taking the notebook off the shelf to read it.
open("w"): Tearing out all the pages and starting fresh on a blank notebook.
open("a"): Opening to the last page and continuing to write from where you left off.
close() / with: Putting the notebook back on the shelf when you're done. If you don't, it could fall off the desk and get damaged!
Read vs Write vs Append
The Diary
open("diary.txt", "w")
"w" mode overwrites. He needed "a" mode!
Reading and Writing Files
Since we're running in the browser, we'll simulate file reading and writing using Python's built-in tools.
- Read the file back using
open("scores.txt", "r")and print the contents. - Change the write mode to
"a"and see how a second run appends instead of overwriting. - Loop over
readlines()to print each score on its own line.
Working with Files — Done! 📄
- Use open(filename, mode) to open a file.
- "r" = Read, "w" = Write (overwrites!), "a" = Append (adds to end).
- After opening, use .read(), .readline(), or .readlines() to get content.
- Use .write("text") to write. Remember
\nfor new lines. - Always close() a file when done, or better yet use the
with open(...) as file:block which closes automatically.
