Pythonで` with `ステートメントを持つコンテキストマネージャ

with open('test_ch30.txt' , 'w+') as file:
    file.write('Blew the lid of ur life')

 # the open file has automaticlly been clossed 

Opening a file using with statement ensures that File descriptor are closed automatically.

class ContextManager(object):
    """docstring for ContextManager"""
    def __init__(self):
        pass

    def __enter__(self):
        print("__enter__()")
        # optionaly return an obj
        return "an instance"

    def __exit__(self , exc_type , exc_val , traceback):
        print("__exit__ with " , exc_val if exc_val else ' nope')


with ContextManager() as e:
    print("is a " , e)

with open('test_ch30.txt') as input_file, open('test_ch77.txt', 'w') as output_file:
    # do something with both files.
    # e.g. copy the contents of input_file into output_file
    for line in input_file:
        output_file.write(line + '\n')

# It has the same effect as nesting context managers:
with open('test_ch30.txt') as input_file:
    with open('test_ch77.txt', 'w') as output_file:
        for line in input_file:
            output_file.write(line + '\n')

class File():
    def __init__(self, filename, mode):
        self.filename = filename
        self.mode = mode
    def __enter__(self):
        self.open_file = open(self.filename, self.mode)
        return self.open_file
    def __exit__(self, *args):
        self.open_file.close()


for _ in range(10000):
    with File('test_ch30.txt', 'w') as f:
        f.write('foo')