09: Fișiere

Read and write data from text, CSV, and JSON files.

python files reading writing csv json

Lesson 09 · Files

What you'll learn

  • Opening and closing files with with open()
  • Reading: read(), readline(), readlines()
  • Writing: write(), writelines()
  • Working with CSV and JSON files

Why files?

Until now, the data in our programs disappeared when the program closed. Files allow data persistence — you save data and read it back later.

Opening a file

with open("data.txt", "r") as f:
    content = f.read()
print(content)
  • "data.txt" — the file name
  • "r" — the open mode (read)
  • as f — the variable through which you access the file
  • with — automatically closes the file when leaving the block

Open modes

Mode Meaning
"r" read (file must exist)
"w" write (creates or overwrites the file)
"a" append at end
"r+" read and write

Reading

read() — the whole file as a single string

with open("hello.txt", "r", encoding="utf-8") as f:
    text = f.read()
print(text)

readlines() — a list of lines

with open("hello.txt", "r", encoding="utf-8") as f:
    lines = f.readlines()

for line in lines:
    print(line.strip())   # .strip() removes the trailing \n

readline() — one line at a time

with open("hello.txt", "r", encoding="utf-8") as f:
    first = f.readline()
    second = f.readline()

encoding='utf-8'

Always add encoding="utf-8" to support Romanian diacritics (ă, î, ș, ț, â).

Writing

with open("results.txt", "w", encoding="utf-8") as f:
    f.write("Andrew: 95\n")
    f.write("Mary: 87\n")
    f.write("Robert: 91\n")

'w' mode overwrites!

If the file already exists, "w" deletes it and starts over. Use "a" to add to the end.

Append to an existing file

with open("results.txt", "a", encoding="utf-8") as f:
    f.write("Helen: 99\n")

CSV files

CSV (Comma-Separated Values) = values separated by commas. Ideal for simple tables.

import csv

# Write CSV
with open("students.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerow(["Name", "Grade", "Class"])   # header
    writer.writerow(["Anna", 9.5, "8A"])
    writer.writerow(["Michael", 8.0, "8B"])

# Read CSV
with open("students.csv", "r", encoding="utf-8") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)
# ['Name', 'Grade', 'Class']
# ['Anna', '9.5', '8A']
# ['Michael', '8.0', '8B']

JSON files

JSON (JavaScript Object Notation) = a structured data format. Ideal for dictionaries and lists.

import json

# Write JSON
data = {
    "student": "Andrew",
    "grades": [8, 9, 7, 10],
    "average": 8.5
}

with open("student.json", "w", encoding="utf-8") as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

# Read JSON
with open("student.json", "r", encoding="utf-8") as f:
    loaded_data = json.load(f)

print(loaded_data["student"])   # Andrew
print(loaded_data["grades"])    # [8, 9, 7, 10]

Exercises

Exercise 1 — Write and read

Write a program that saves 3 sentences to a file, then reads them back and displays them numbered.

Solution 
sentences = [
    "Python is fun.",
    "Files keep your data.",
    "Programming opens doors."
]

with open("sentences.txt", "w", encoding="utf-8") as f:
    for s in sentences:
        f.write(s + "\n")

with open("sentences.txt", "r", encoding="utf-8") as f:
    for i, line in enumerate(f, 1):
        print(f"{i}. {line.strip()}")

Exercise 2 — Word count

Read a text file and display the number of lines and words.

Solution 
with open("sentences.txt", "r", encoding="utf-8") as f:
    lines = f.readlines()

line_count = len(lines)
word_count = sum(len(line.split()) for line in lines)
print(f"Lines: {line_count}, Words: {word_count}")

Mini-project: Persistent scoreboard

Build a program that saves and displays a leaderboard of scores in JSON format.

Functionality: - Asks for a name and a score - Adds it to the leaderboard - Displays the sorted top - Persists between runs

Solution 
import json
import os

FILE = "scores.json"

def load_scores():
    if os.path.exists(FILE):
        with open(FILE, "r", encoding="utf-8") as f:
            return json.load(f)
    return []

def save_scores(scores):
    with open(FILE, "w", encoding="utf-8") as f:
        json.dump(scores, f, ensure_ascii=False, indent=2)

scores = load_scores()

name = input("Your name: ")
score = int(input("Your score: "))
scores.append({"name": name, "score": score})

scores.sort(key=lambda x: x["score"], reverse=True)
save_scores(scores)

print("\n=== LEADERBOARD ===")
for i, entry in enumerate(scores[:5], 1):
    print(f"#{i}: {entry['name']} — {entry['score']}")

Summary

  • with open("file", "mode", encoding="utf-8") as f: — opens the file safely
  • Modes: "r" read, "w" write, "a" append
  • f.read() — everything as a string; f.readlines() — list of lines
  • The csv module for tables; the json module for structured data
  • encoding="utf-8" — required for Romanian characters

Next step: → Project: Text adventure game

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