File Handling

with open("notes.txt", "w") as f:          # "w" creates or overwrites
    f.write("Line one\n")
    f.write("Line two\n")
    print("Line three", file=f)            # print can write to files too

with open("notes.txt") as f:               # default mode is "r" (read text)
    print(f.read())

with open("notes.txt", "a") as f:          # "a" appends
    f.write("Line four\n")

with open("notes.txt") as f:
    for number, line in enumerate(f, start=1):   # iterate line by line: memory-friendly
        print(number, line.rstrip("\n"))

Line one
Line two
Line three

1 Line one
2 Line two
3 Line three
4 Line four

with open("poem.txt", "w") as f:
    f.write("Roses are red\nViolets are blue\nPython is neat\nAnd so are you\n")

with open("poem.txt") as f:
    first = f.readline()                 # one line
    rest = f.readlines()                 # list of the remaining lines
print(repr(first))
print(rest)

with open("poem.txt") as f:
    text = f.read()
print(len(text.split()), "words,", text.count("\n"), "lines")

'Roses are red\n'
['Violets are blue\n', 'Python is neat\n', 'And so are you\n']
13 words, 4 lines

import csv

rows = [["roll", "name", "marks"], [101, "Asha", 88], [102, "Ravi", 93], [103, "Meera, K", 79]]
with open("marks.csv", "w", newline="") as f:
    csv.writer(f).writerows(rows)

with open("marks.csv", newline="") as f:
    reader = csv.DictReader(f)                    # each row becomes a dict
    records = list(reader)

for r in records:
    print(f"{r['roll']} {r['name']:10} {r['marks']}")
average = sum(int(r["marks"]) for r in records) / len(records)
print(f"Class average: {average:.2f}")

101 Asha       88
102 Ravi       93
103 Meera, K   79
Class average: 86.67

import json

profile = {"name": "Asha", "skills": ["C", "Python"], "cgpa": 9.1, "placed": False}
with open("profile.json", "w") as f:
    json.dump(profile, f, indent=2)

with open("profile.json") as f:
    loaded = json.load(f)
print(loaded["skills"], loaded == profile)
print(json.dumps({"ok": True, "count": 3}))      # to a string

['C', 'Python'] True
{"ok": true, "count": 3}

from pathlib import Path

folder = Path("reports")
folder.mkdir(exist_ok=True)
for term in ("sem1", "sem2"):
    (folder / f"{term}.txt").write_text(f"Results for {term}\n")

for p in sorted(folder.glob("*.txt")):
    print(p.name, p.suffix, p.stat().st_size, "bytes:", p.read_text().strip())
print(Path("reports/sem1.txt").exists(), Path("missing.txt").exists())

sem1.txt .txt 17 bytes: Results for sem1
sem2.txt .txt 17 bytes: Results for sem2
True False