撰寫情境管理器

Python 函式寫作

Shayne Miel

Software Architect @ Duo Security

定義情境管理器的兩種方式

以類別為基礎
以函式為基礎

定義情境管理器的兩種方式

以類別為基礎
以函式為基礎 *

如何建立情境管理器


def my_context():

  # Add any set up code you need

  yield

  # Add any teardown code you need

定義一個函式。
（選用）加入情境所需的前置設定程式碼。
使用「yield」關鍵字。
（選用）加入情境所需的收尾程式碼。

如何建立情境管理器

@contextlib.contextmanager
def my_context():

  # Add any set up code you need

  yield

  # Add any teardown code you need

定義一個函式。
（選用）加入情境所需的前置設定程式碼。
使用「yield」關鍵字。
（選用）加入情境所需的收尾程式碼。
加上 `@contextlib.contextmanager` 裝飾器。

「yield」關鍵字

@contextlib.contextmanager
def my_context():
  print('hello')
  yield 42
  print('goodbye')

with my_context() as foo:
  print('foo is {}'.format(foo))

hello
foo is 42
goodbye

設定與收尾

@contextlib.contextmanager
def database(url):
  # set up database connection
  db = postgres.connect(url)

  yield db

  # tear down database connection
  db.disconnect()

url = 'http://datacamp.com/data'
with database(url) as my_db:
  course_list = my_db.execute(
    'SELECT * FROM courses'
  )

空白區塊

設定與收尾

@contextlib.contextmanager
def database(url):
  # set up database connection
  db = postgres.connect(url)

  yield db

  # tear down database connection
  db.disconnect()

url = 'http://datacamp.com/data'
with database(url) as my_db:
  course_list = my_db.execute(
    'SELECT * FROM courses'
  )

空白區塊

設定與收尾

@contextlib.contextmanager
def database(url):
  # set up database connection
  db = postgres.connect(url)

  yield db

  # tear down database connection
  db.disconnect()

url = 'http://datacamp.com/data'
with database(url) as my_db:
  course_list = my_db.execute(
    'SELECT * FROM courses'
  )

空白區塊

傳回值或 None

@contextlib.contextmanager
def database(url):
  # set up database connection
  db = postgres.connect(url)

  yield db

  # tear down database connection
  db.disconnect()

url = 'http://datacamp.com/data'
with database(url) as my_db:
  course_list = my_db.execute(
    'SELECT * FROM courses'
  )

@contextlib.contextmanager
def in_dir(path):
  # save current working directory
  old_dir = os.getcwd()

  # switch to new working directory
  os.chdir(path)

  yield

  # change back to previous
  # working directory
  os.chdir(old_dir)

with in_dir('/data/project_1/'):
  project_files = os.listdir()

一起來練習吧！

Python 函式寫作