In [2]:

    

class Context(object):
    def __init(self):
        print('__init__')
    def __enter__(self):
        print('__enter__')
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        print('__exit__')

with Context():
    print('Do working in the context')


    

    




__enter__
Do working in the context
__exit__

In [3]:

    

class WithinContext:

    def __init__(self, context):
        print('WithinContext.__init__({})'.format(context))

    def do_something(self):
        print('WithinContext.do_something()')

    def __del__(self):
        print('WithinContext.__del__')


class Context:

    def __init__(self):
        print('Context.__init__()')

    def __enter__(self):
        print('Context.__enter__()')
        return WithinContext(self)

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('Context.__exit__()')


with Context() as c:
    c.do_something()


    

    




Context.__init__()
Context.__enter__()
WithinContext.__init__(<__main__.Context object at 0x1099a66a0>)
WithinContext.do_something()
Context.__exit__()

In [4]:

    

class Context:

    def __init__(self, handle_error):
        print('__init__({})'.format(handle_error))
        self.handle_error = handle_error

    def __enter__(self):
        print('__enter__()')
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('__exit__()')
        print('  exc_type =', exc_type)
        print('  exc_val  =', exc_val)
        print('  exc_tb   =', exc_tb)
        return self.handle_error


with Context(True):
    raise RuntimeError('error message handled')

print()

with Context(False):
    raise RuntimeError('error message propagated')


    

    




__init__(True)
__enter__()
__exit__()
  exc_type = <class 'RuntimeError'>
  exc_val  = error message handled
  exc_tb   = <traceback object at 0x10999df48>

__init__(False)
__enter__()
__exit__()
  exc_type = <class 'RuntimeError'>
  exc_val  = error message propagated
  exc_tb   = <traceback object at 0x10999dec8>






    




---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-4-c2f64363f95f> in <module>()
     23 
     24 with Context(False):
---> 25     raise RuntimeError('error message propagated')

RuntimeError: error message propagated

In [5]:

    

import contextlib


class Context(contextlib.ContextDecorator):

    def __init__(self, how_used):
        self.how_used = how_used
        print('__init__({})'.format(how_used))

    def __enter__(self):
        print('__enter__({})'.format(self.how_used))
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('__exit__({})'.format(self.how_used))


@Context('as decorator')
def func(message):
    print(message)


print()
with Context('as context manager'):
    print('Doing work in the context')

print()
func('Doing work in the wrapped function')


    

    




__init__(as decorator)

__init__(as context manager)
__enter__(as context manager)
Doing work in the context
__exit__(as context manager)

__enter__(as decorator)
Doing work in the wrapped function
__exit__(as decorator)

In [7]:

    

import contextlib


@contextlib.contextmanager
def make_context():
    print('  entering')
    try:
        yield {}
    except RuntimeError as err:
        print('  ERROR:', err)
    except ValueError as err:
        print( 'ERROR:', err)
    finally:
        print('  exiting')


print('Normal:')
with make_context() as value:
    print('  inside with statement:', value)

print('\nHandled error:')
with make_context() as value:
    raise RuntimeError('showing example of handling an error')

print('\nUnhandled error:')
with make_context() as value:
    raise ValueError('this exception is not handled')


    

    




Normal:
  entering
  inside with statement: {}
  exiting

Handled error:
  entering
  ERROR: showing example of handling an error
  exiting

Unhandled error:
  entering
ERROR: this exception is not handled
  exiting

In [11]:

    

import contextlib


class NonFatalError(Exception):
    pass


def non_idempotent_operation():
    raise NonFatalError(
        'The operation failed because of existing state'
    )


with contextlib.suppress(NonFatalError):
    print('trying non-idempotent operation')
    non_idempotent_operation()
    print('succeeded!')

print('done')


    

    




trying non-idempotent operation
done

In [ ]: