CONTEXT MANAGER

With statement

Apa?

Fitur python yang mengatur proses setup dan teardown secara otomatis

Contoh

with open("file.txt") as f:
  data = f.read()

Alasan

• file akan ditutup otomatis
• aman dari error
• tidak perlu try/finally manual
• lebih rapi dan pythonic

Tanpa context manager

f = open("file.txt")
data = f.read()
f.close()

Jika error terjadi sebelum close(), file tidak akan ditutup menyebabkan

• memory leak
• lock file
• corrupted resource

Dengan context manager

with open("file.txt") as f:
  data = f.read()

saat keluar dari blok with:

• file ditutup otomatis
• resource dibersihkan
• tetap aman meskipun terjadi exception

Bagaimana with bekerja

• Python memanggil 2 method khusus (dunder)
  • enter()
  • exit()

class Example:
  def __enter__(self):
    print("Entered")
    return self
  
  def __exit__(self, exc_type, exc_value, traceback):
    print("Exited")

Pemakaian

with Example() as e:
  print("Inside")

Membuat context manager sendiri

Contoh timer

import time

class Timer:
  def __enter__(self):
    self.start = time.time()
    return self
  
  def __exit__(self, exc_type, exc, tb):
    self.end = time.time()
    print(f"Time: {self.end - self.start:.4f}s")

Pemakaian

with Timer():
  for _ in range(1_000_000):
    pass

Sangat berguna untuk bencmarking

Context manager dengan contextlib

Membuat context manager dengan generator

from contextlib import contextmanager

@contextmanager
def open_file(name):
  f = open(name)
  try:
    yield f
  finally:
    f.close()

Pemakaian

with open_file("file.txt") as f:
  print(f.read())

Lebih simple lebih bersih

Use case di dunia nyata

• File handling
• Database connection Contoh tanpa context

  conn = db.connect()
  try:
    result = conn.execute(...)
  finally:
    conn.close()
  

  Dengan context

  with db.connect() as conn:
    conn.execute(...)
  

• Locking (threading)

  from threading import Lock
  
  lock = Lock()
  
  with lock:
    # critical section
    counter += 1
  

• Resouce cleanup di FastAPI / Flask FastAPI dependency

  from contextlib import asynccontextmanager
  
  @asynccontextmanager
  async def lifespan(app):
    print("Starting app...")
    yield
    print("Shutting down...")
  

• Temporary directory / files

  from tempfile import TemporaryDirectory
  
  with TemporaryDirectory() as tmpdir:
    print("Using temp folder:", tmpdir)
  

• Redirect stdout (testing)

  from contextlib import redirect_stdout
  import io
  
  f = io.StringIO()
  with redirect_stdout(f):
    print("Hello")
  
  print(f.getvalue())  # "Hello\n"
  

• Timing, profiling, logging Timer

  with Timer():
    heavy_function()
  

  Logger

  with log_section("loading data"):
    load_data()
  

Handling exceptions inside context manager

Contoh context manager yang men-suppress exception

class IgnoreErrors:
  def __enter__(self):
    pass
  def __exit__(self, exc_type, exc, tb):
    return True  # ignore error

Pemakaian

with IgnoreErrors():
  1 / 0

print("Still running")

Useful untuk test/debug

exit parameters explained

exit(self, exc_type, exc_value, traceback) Jika:

• tidak ada error - semuanya None
• ada error - param berisi info error

Jika exit return True, error disuppress Jika return False (default), error tetap dilempar