The with statement

You saw .close() in the last lesson. Forgetting it is a real bug — data can sit unwritten in a buffer, and the file stays locked.

with closes the file for you, automatically, even if an error is raised inside the block:

with open("notes.txt", "w") as f:
    f.write("Hello, file!")

print("File is closed now.")

Read that as "open this file as f, and close it when the indented block ends." Always use with — the manual open()/.close() pair is only worth knowing so you recognise it in older code.

with is a context manager. Module 9 shows how to write your own.

Writing

"w" mode completely overwrites the file. This is the most common way beginners lose data:

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

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

with open("log.txt") as f:
    print(f.read().strip())

Only "Second run" survives. If the file exists, "w" empties it the moment it opens.

Appending

"a" adds to the end instead:

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

with open("log.txt", "a") as f:
    f.write("Second run\n")

with open("log.txt") as f:
    print(f.read().strip())

Use "w" to replace a file, "a" to add to a log.

.write() doesn't add newlines

Unlike print(), .write() writes exactly what you give it:

with open("out.txt", "w") as f:
    f.write("one")
    f.write("two")

with open("out.txt") as f:
    print(f.read())

That produces onetwo on a single line. Add \n yourself.

Writing a list

.writelines() writes every string in a list — again, adding no newlines of its own:

lines = ["apple\n", "banana\n", "cherry\n"]

with open("fruits.txt", "w") as f:
    f.writelines(lines)

with open("fruits.txt") as f:
    print(f.read().strip())

A loop is often clearer, and lets you add the newline as you go:

fruits = ["apple", "banana", "cherry"]

with open("fruits.txt", "w") as f:
    for fruit in fruits:
        f.write(f"{fruit}\n")

with open("fruits.txt") as f:
    print(f.read().strip())

Opening several files

One with can manage more than one file — handy for reading from one and writing to another:

with open("source.txt", "w") as f:
    f.write("hello\nworld\n")

with open("source.txt") as src, open("upper.txt", "w") as dst:
    for line in src:
        dst.write(line.upper())

with open("upper.txt") as f:
    print(f.read().strip())

Try it yourself

Write a list of fruits, append one more, then read the file back.

Checkpoint

• with open(...) as f: closes the file automatically — always prefer it.
• "w" overwrites from scratch; "a" appends.
• .write() adds no newline; include \n yourself.
• One with can open several files, separated by commas.