Python Tutorial (6): File I/O dan Exception Handling

# Cara basic
f = open("data.txt", "r")
content = f.read()
f.close()
 
# Context manager (SELALU gunakan ini)
with open("data.txt", "r") as f:
    content = f.read()
# File otomatis ditutup setelah blok with

# Baca seluruh isi
with open("data.txt") as f:
    content = f.read()          # string
 
# Baca per baris
with open("data.txt") as f:
    lines = f.readlines()       # list of strings (termasuk \n)
 
# Iterasi per baris (memory-efficient untuk file besar)
with open("data.txt") as f:
    for line in f:
        print(line.strip())
 
# Baca N karakter
with open("data.txt") as f:
    chunk = f.read(1024)

# Write (overwrite)
with open("output.txt", "w") as f:
    f.write("Baris pertama\n")
    f.write("Baris kedua\n")
 
# Append (tambah di akhir)
with open("log.txt", "a") as f:
    f.write(f"[{datetime.now()}] Event logged\n")
 
# Write multiple lines
lines = ["satu\n", "dua\n", "tiga\n"]
with open("output.txt", "w") as f:
    f.writelines(lines)
 
# Print ke file
with open("output.txt", "w") as f:
    print("Hello, file!", file=f)

# Binary file
with open("image.png", "rb") as f:
    data = f.read()
 
with open("copy.png", "wb") as f:
    f.write(data)

from pathlib import Path
 
# Path manipulation
p = Path("data") / "output" / "result.csv"
p.parent          # data/output
p.name            # result.csv
p.stem            # result
p.suffix          # .csv
 
# File operations
p = Path("myfile.txt")
p.write_text("Hello, pathlib!")
content = p.read_text()
p.exists()
p.is_file()
p.is_dir()
p.unlink()        # delete
 
# Directory operations
Path("output").mkdir(parents=True, exist_ok=True)
 
# Iterate files
for f in Path(".").glob("**/*.py"):
    print(f)
 
# Home directory
home = Path.home()
config = home / ".config" / "myapp" / "settings.json"

import json
 
# Write JSON
data = {"name": "Alice", "scores": [90, 85, 92]}
with open("data.json", "w") as f:
    json.dump(data, f, indent=2)
 
# Read JSON
with open("data.json") as f:
    loaded = json.load(f)
 
# String conversion
json_str = json.dumps(data, indent=2)
parsed = json.loads(json_str)

import csv
 
# Write CSV
with open("users.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "age", "city"])
    writer.writerow(["Alice", 30, "Jakarta"])
    writer.writerow(["Bob", 25, "Bandung"])
 
# Read CSV
with open("users.csv") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(f"{row['name']} ({row['age']})")

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Tidak bisa bagi dengan nol!")
 
# Multiple exceptions
try:
    value = int(input("Angka: "))
    result = 100 / value
except ValueError:
    print("Input bukan angka")
except ZeroDivisionError:
    print("Tidak bisa bagi nol")
except (TypeError, AttributeError) as e:
    print(f"Error: {e}")
 
# else & finally
try:
    f = open("data.txt")
    data = f.read()
except FileNotFoundError:
    print("File tidak ditemukan")
else:
    print(f"Berhasil baca {len(data)} karakter")
finally:
    print("Blok ini SELALU dieksekusi")

BaseException
 └── Exception
      ├── ValueError
      ├── TypeError
      ├── KeyError
      ├── IndexError
      ├── FileNotFoundError
      ├── IOError
      ├── RuntimeError
      └── ...

class AppError(Exception):
    """Base exception untuk aplikasi."""
    pass
 
class ValidationError(AppError):
    """Input tidak valid."""
    def __init__(self, field, message):
        self.field = field
        self.message = message
        super().__init__(f"{field}: {message}")
 
class NotFoundError(AppError):
    """Resource tidak ditemukan."""
    pass
 
 
def create_user(name, age):
    if not name:
        raise ValidationError("name", "Nama tidak boleh kosong")
    if age < 0:
        raise ValidationError("age", "Umur tidak valid")
    return {"name": name, "age": age}
 
 
try:
    user = create_user("", 25)
except ValidationError as e:
    print(f"Validation failed: {e.field} - {e.message}")
except AppError as e:
    print(f"Application error: {e}")

from contextlib import contextmanager
 
@contextmanager
def timer(label):
    """Ukur waktu eksekusi blok kode."""
    import time
    start = time.perf_counter()
    try:
        yield
    finally:
        elapsed = time.perf_counter() - start
        print(f"{label}: {elapsed:.3f}s")
 
 
with timer("Data processing"):
    data = [x**2 for x in range(1_000_000)]

# Class-based context manager
class DatabaseConnection:
    def __init__(self, url):
        self.url = url
        self.conn = None
 
    def __enter__(self):
        self.conn = connect(self.url)
        return self.conn
 
    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.conn:
            self.conn.close()
        return False    # False = don't suppress exceptions
 
 
with DatabaseConnection("postgres://localhost/db") as conn:
    conn.execute("SELECT 1")

# EAFP (Easier to Ask Forgiveness than Permission): Pythonic
try:
    value = data["key"]
except KeyError:
    value = default
 
# vs LBYL (Look Before You Leap): less Pythonic
if "key" in data:
    value = data["key"]
else:
    value = default