Reading and Writing Files

Variables disappear when a program ends. To keep data, write it to a file. The built-in open() function gives you a file object, and the with statement makes sure the file is closed properly when you are done, even if an error happens.

Writing a file

Mode "w" creates the file, or empties it if it already exists.

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

print("Saved!")

Saved!

Reading a file

Mode "r" (the default) opens a file for reading. read() gives you the whole content as one string. Looping over the file gives you one line at a time.

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

with open("notes.txt") as f:
    content = f.read()
print(repr(content))

with open("notes.txt") as f:
    for number, line in enumerate(f, start=1):
        print(number, line.strip())

'apple\nbanana\ncherry\n'
1 apple
2 banana
3 cherry

line.strip() removes the newline character at the end of each line.

Appending

Mode "a" adds to the end of a file without erasing what is already there.

with open("log.txt", "w") as f:
    f.write("started\n")

for event in ["loaded data", "finished"]:
    with open("log.txt", "a") as f:
        f.write(event + "\n")

with open("log.txt") as f:
    print(f.read(), end="")

started
loaded data
finished

Missing files

Opening a file that does not exist for reading raises FileNotFoundError. Combine files with what you learned about exceptions:

try:
    with open("does_not_exist.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("File not found. Check the name and folder.")

File not found. Check the name and folder.