Python files and context managers

Write and read a UTF-8 text file with automatic cleanup.

Python

from pathlib import Path
from tempfile import TemporaryDirectory

with TemporaryDirectory() as folder:
    path = Path(folder) / "notes.txt"
    path.write_text("Learn\nPractice\n", encoding="utf-8")
    with path.open(encoding="utf-8") as handle:
        lines = [line.strip() for line in handle]
    print(len(lines))
    print(lines[1])

Output

2
Practice

Explanation

A context manager gives a resource a well-defined lifetime. A with block closes a file even if an exception occurs while using it.

TemporaryDirectory creates an isolated location that is removed when its context exits. Explicit UTF-8 encoding keeps this text example independent of the system's default encoding.

Explore the dry run

Guided walkthroughReady

1from pathlib import Path

2from tempfile import TemporaryDirectory

4with TemporaryDirectory() as folder:

5 path = Path(folder) / "notes.txt"

6 path.write_text("Learn\nPractice\n", encoding="utf-8")

7 with path.open(encoding="utf-8") as handle:

8 lines = [line.strip() for line in handle]

9 print(len(lines))

10 print(lines[1])

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Create a temporary directory; its exact path is system-dependent.	`temporary directory open`	—
Write two lines and close the write handle.	`temporary directory open`	—
Read both lines and strip surrounding whitespace.	`lines = ['Learn', 'Practice']`	—
The read handle has closed; print the line count.	`lines = ['Learn', 'Practice']`	2
Print the second line. Leaving the outer block cleans up the directory.	`lines = ['Learn', 'Practice']`	2 Practice

Cost and common mistakes

O(n) time and O(n) memory for n characters read into the list.

write_text replaces existing file contents.
strip removes all leading and trailing whitespace, not only the newline.

Try it yourself

Append a third line 'Review' to the text and predict the count.

Reveal answer and reasoning

The count becomes 3; lines[1] remains Practice.

Python files and context managers

Explanation

Follow the example

Cost and common mistakes

Try it yourself

Further reading