File Handling

Everything you've written so far disappears the moment your program ends. Reading and writing files is how a Python program remembers something, or picks up data that lives outside the program itself.

Writing to a file

open() gives you a file object to work with. Pass it a filename and a mode — "w" for write — and use with so the file is automatically closed for you when the block ends:

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

print("Done writing.")

Done writing.

Nothing prints from inside the with block itself — this creates notes.txt in the current folder (or overwrites it completely if it already existed, since "w" mode starts from empty) containing the two lines just written.

Reading a file back

"r" mode opens a file for reading. .read() grabs the whole thing as one string, while looping over the file object directly gives you it one line at a time:

with open("notes.txt", "r") as f:
    contents = f.read()

print(contents)

First line
Second line

with open("notes.txt", "r") as f:
    for line in f:
        print("Line:", line.strip())

Line: First line
Line: Second line

.strip() removes the trailing newline character that comes along with each line — without it, you'd get a blank-looking extra line after every value printed.

Appending instead of overwriting

"a" mode adds to the end of an existing file instead of replacing it:

with open("notes.txt", "a") as f:
    f.write("Third line\n")

with open("notes.txt", "r") as f:
    print(f.read())

First line
Second line
Third line