Reading and Writing Files in Python

Why File I/O Matters

Reading and writing files lets programs persist data between runs. In HKDSE ICT, file I/O appears in 70% of papers. Master this topic and you unlock significant marks.

Opening Files

# Basic form
f = open("data.txt", "r")   # read mode
# ... use f ...
f.close()                   # MUST close

# Better form (auto-close)
with open("data.txt", "r") as f:
    # ... use f ...
# automatically closed

Always use with. It closes the file even if errors occur.

File Modes

Mode	Meaning	Behaviour
"r"	Read	Default. File must exist.
"w"	Write	Creates file. Overwrites if exists.
"a"	Append	Adds to end. Creates if doesn't exist.
"x"	Exclusive create	Fails if file exists.

Reading Files: 3 Ways

1. Read everything as one string

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

2. Read line by line (best for large files)

with open("data.txt") as f:
    for line in f:
        print(line.strip())   # .strip() removes \n

3. Read into a list of lines

with open("data.txt") as f:
    lines = f.readlines()
for line in lines:
    print(line.strip())

Writing Files

# Write a single string
with open("output.txt", "w") as f:
    f.write("Hello, HK!\n")
    f.write("Second line\n")

# Write multiple lines
lines = ["Chan\n", "Lee\n", "Wong\n"]
with open("names.txt", "w") as f:
    f.writelines(lines)

Appending to Files

with open("log.txt", "a") as f:
    f.write("New entry\n")
# Existing content preserved

Real HKDSE Pattern: CSV Processing

# Input file (scores.txt):
# Chan,85
# Lee,72
# Wong,91

students = []

with open("scores.txt") as f:
    for line in f:
        name, score_str = line.strip().split(",")
        students.append((name, int(score_str)))

# Find highest scorer
top = max(students, key=lambda x: x[1])
print(f"Top: {top[0]} with {top[1]}")

# Write sorted list to new file
students.sort(key=lambda x: x[1], reverse=True)
with open("ranked.txt", "w") as f:
    for name, score in students:
        f.write(f"{name}: {score}\n")

Common Mistakes

• Forgetting .strip() — lines keep the trailing newline
• Using "w" when you meant "a" — destroys existing data
• Not converting strings to numbers (int(), float())
• Opening a file without with and forgetting to close it

Reading a File That Might Not Exist

import os

if os.path.exists("data.txt"):
    with open("data.txt") as f:
        content = f.read()
else:
    content = ""   # default