python-decorators

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()
  

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