File handling

Programs need to read and write files: configuration, logs, data exports, results. Python makes this straightforward with the built in open() function and the with statement.

Writing a file

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

• "w" opens for writing and replaces any existing content.
• with guarantees the file is closed when the block ends, even if an error occurs. Always use it.
• write() does not add a newline; include \n yourself.

Reading a file

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

"r" (read) is the default mode. read() returns the entire file as one string.

Reading line by line

For anything large, iterate over the file object. It reads one line at a time.

with open("notes.txt") as f:
    for line in f:
        print(line.rstrip())

Each line keeps its trailing \n, hence the rstrip().

readlines() returns a list of all lines; readline() returns the next one.

Appending

"a" adds to the end without erasing.

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

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

File modes

Always pass encoding="utf-8" when working with text that might contain non ASCII characters, so behaviour is the same on every operating system.

Handling missing files

try:
    with open("does_not_exist.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("No such file")

Working with paths

The pathlib module gives you an object for file paths that works on every operating system.

from pathlib import Path

p = Path("notes.txt")
print(p.exists())
print(p.suffix, p.stem)
print(p.read_text())

data_dir = Path("data")
data_dir.mkdir(exist_ok=True)
(data_dir / "out.txt").write_text("hello")
print(sorted(x.name for x in data_dir.iterdir()))

Path.read_text() and write_text() are convenient shortcuts for small files.

Deleting

from pathlib import Path
Path("notes.txt").unlink()
print(Path("notes.txt").exists())

CSV files

The csv module handles quoting and commas inside values correctly. Do not split on commas by hand.

import csv

rows = [["name", "marks"], ["Asha", 88], ["Bilal", 92]]
with open("marks.csv", "w", newline="") as f:
    csv.writer(f).writerows(rows)

with open("marks.csv", newline="") as f:
    for row in csv.DictReader(f):
        print(row["name"], int(row["marks"]) + 5)

Practice

1. Write three lines of text to a file, then read it back and print the number of lines.
2. Append a timestamped line to a log file each time a script runs (use datetime.now()).
3. Write a CSV of five students and their marks, then read it and print the top scorer.