Files, Paths, CSV & JSON

pathlib.Path is the modern way to work with files: join paths with /, check .exists(), list files with .glob("*.csv"), and read or write small files with .read_text() / .write_text().

The csv module handles quoting and commas inside values correctly — never split CSV lines by hand. csv.DictReader gives each row as a dictionary keyed by the header.

The json module converts between Python objects and JSON text: json.dumps/json.loads for strings, json.dump/json.load for files. JSON is the standard format for APIs and config files.

convert.pyPython

import csv
import json
from pathlib import Path

data_dir = Path("data")
data_dir.mkdir(exist_ok=True)

csv_path = data_dir / "results.csv"
csv_path.write_text(
    "name,subject,score\n"
    "Amina,Maths,88\n"
    "Amina,Biology,79\n"
    'Juma,"Book-Keeping, Paper 1",42\n',
    encoding="utf-8",
)

report: dict[str, dict[str, int]] = {}
with csv_path.open(encoding="utf-8", newline="") as f:
    for row in csv.DictReader(f):
        report.setdefault(row["name"], {})[row["subject"]] = int(row["score"])

json_path = data_dir / "results.json"
json_path.write_text(json.dumps(report, indent=2), encoding="utf-8")

loaded = json.loads(json_path.read_text(encoding="utf-8"))
print(loaded["Juma"])                         # {'Book-Keeping, Paper 1': 42}
print([p.name for p in sorted(data_dir.glob("*"))])

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Key points

Use pathlib.Path instead of string paths.
Use the csv module — values can contain commas and quotes.
json.dumps(obj, indent=2) for readable output; json.loads to parse.

Exercise

Write a script that reads every .csv file in a folder, combines them, and writes a summary.json with each student's average and best subject.

Show solution

Try the exercise yourself first — then compare your approach with this one.

Path.glob("*.csv") finds every CSV file in the folder. Each row is added to a per-student dictionary of subject → score; then the summary computes each student's average and best subject and is written as JSON.

summarise.pyPython

import csv
import json
from pathlib import Path

folder = Path("results")
folder.mkdir(exist_ok=True)
(folder / "term1.csv").write_text("name,subject,score\nAmina,Maths,88\nJuma,Maths,42\n", encoding="utf-8")
(folder / "term2.csv").write_text("name,subject,score\nAmina,Biology,79\nJuma,Biology,61\n", encoding="utf-8")

scores: dict[str, dict[str, int]] = {}
for path in sorted(folder.glob("*.csv")):
    with path.open(encoding="utf-8", newline="") as f:
        for row in csv.DictReader(f):
            scores.setdefault(row["name"], {})[row["subject"]] = int(row["score"])

summary = {
    name: {
        "average": round(sum(subjects.values()) / len(subjects), 1),
        "best_subject": max(subjects, key=subjects.get),
    }
    for name, subjects in scores.items()
}

Path("summary.json").write_text(json.dumps(summary, indent=2), encoding="utf-8")
print(Path("summary.json").read_text(encoding="utf-8"))

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Key points

Exercise

Check your understanding