README.md

Built-ins and libraries

contextlib

The contextlib module allows easy creation of context managers to be used in with clauses.

• @contextmanager

  Doc:

  This function is a decorator that can be used to define a factory function for with statement context managers, without needing to create a class or separate __enter__() and __exit__() methods.

  It needs to be used with a specific syntax:

  try:
      yield ...  # value
  finally:
      ...  # exit

  (Script available)

  import os
  
  from contextlib import contextmanager
  
  def getcwd_abs():
      return os.path.abspath(os.getcwd())
  
  @contextmanager
  def cd(path):
      cwd = getcwd_abs()
      try:
          os.chdir(path)
          yield
      finally:
          os.chdir(cwd)
  
  if __name__ == '__main__':
      print(getcwd_abs())
      with cd('..'):
          print(getcwd_abs())
      print(getcwd_abs())

  There is also @asynccontextmanager for async with.

• ContextDecorator

  Doc:

  A base class that enables a context manager to also be used as a decorator.

  Context managers inheriting from ContextDecorator have to implement __enter__ and __exit__ as normal. __exit__ retains its optional exception handling even when used as a decorator.

  (Script available)

  import os
  
  from contextlib import ContextDecorator
  
  def getcwd_abs():
      return os.path.abspath(os.getcwd())
  
  class CD(ContextDecorator):
      def __init__(self, path):
          self.original = getcwd_abs()
          self.path = os.path.abspath(path)
  
      def __enter__(self):
          os.chdir(self.path)
  
      def __exit__(self, exc_type, exc_val, exc_tb):
          os.chdir(self.original)
  
  @CD('..')
  def list_dir(path='.'):
      print(getcwd_abs())
      print(os.listdir(path))
  
  if __name__ == '__main__':
      with CD('..'):
          print(getcwd_abs())
          print(os.listdir('.'))
  
      list_dir()

Prev / Up / Next