1.4 · File Handling

Goal: read, write, append, modify text files. Required operations: delete, insert, append, amend records.

Opening modes

Mode	Meaning
"r"	Read (default; file must exist)
"w"	Write — truncates / creates
"a"	Append — creates if missing
"r+"	Read + write
"x"	Exclusive create — fails if exists
"b" suffix	Binary mode ("rb", "wb")

Reading

with open("students.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())

with ensures the file closes automatically — always prefer it.

Bulk read:

with open("students.txt", encoding="utf-8") as f:
    all_text = f.read()
    print(all_text)

Writing

with open("output.txt", "w", encoding="utf-8") as f:
    f.write("Hello\n")
    f.write("World\n")

Appending

with open("log.txt", "a", encoding="utf-8") as f:
    f.write(f"{datetime.now()}: event\n")

CSV processing

import csv

with open("students.csv", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["score"])

Record updates — the four required operations

Assume a CSV students.csv with header id,name,score.

Read all rows into memory

import csv

def read_all(path):
    with open(path, encoding="utf-8") as f:
        return list(csv.DictReader(f))

def write_all(path, rows):
    if not rows: return
    with open(path, "w", encoding="utf-8", newline="") as f:
        w = csv.DictWriter(f, fieldnames=rows[0].keys())
        w.writeheader()
        w.writerows(rows)

Append

rows = read_all("students.csv")
rows.append({"id": "1006", "name": "Frank", "score": "75"})
write_all("students.csv", rows)

Insert at a specific position

rows = read_all("students.csv")
rows.insert(2, {"id": "1007", "name": "Grace", "score": "80"})
write_all("students.csv", rows)

Amend (update)

rows = read_all("students.csv")
for r in rows:
    if r["id"] == "1001":
        r["score"] = "90"
write_all("students.csv", rows)

Delete

rows = read_all("students.csv")
rows = [r for r in rows if r["id"] != "1003"]
write_all("students.csv", rows)

Error handling

try:
    with open("missing.txt") as f:
        ...
except FileNotFoundError:
    print("File not found")

Common student mistakes

• Forgetting with → file stays open.
• Wrong mode ("r" to write, "w" to read).
• Mixing newlines on Windows — use newline="" for csv writer.
• Not specifying encoding="utf-8" → Chinese turns into mojibake.

Exam-style question

Q (5 marks): Write Python that reads marks.csv (columns id, name, score), adds 5 bonus points to every score below 50 (capped at 100), and writes the result back to the same file.

Sample answer:

import csv

with open("marks.csv", encoding="utf-8") as f:
    rows = list(csv.DictReader(f))

for r in rows:
    s = int(r["score"])
    if s < 50:
        r["score"] = str(min(100, s + 5))

with open("marks.csv", "w", encoding="utf-8", newline="") as f:
    w = csv.DictWriter(f, fieldnames=["id","name","score"])
    w.writeheader()
    w.writerows(rows)

Key takeaways

• with open(... , encoding="utf-8") always.
• 4 required operations: insert, append, amend, delete.
• Use csv module for tabular data.

➡️ Next: 1.5 Linear & Binary Search