What is a Context Manager?
It's an object, that is notified when a context or a block of code starts and ends, of a class that implements the 2 magic methods __enter__ and __exit__ and any other methods if needed.
For example: when a context manager ends, the object file is closed.
with open('test_ch30.txt' , 'w+') as file:
file.write('Blew the lid of ur life')
# the open file has automatically been closed
Context manager is commonly used on reading and writing files to assist in conserving system memory and improve resource management by ensuring that each process holds its resources while executing.
Opening a file using
withstatement ensures that File descriptor are closed automatically.
How with statment implements a context manager?
1- When an object is called with a with statement, the
interpreter invokes the __enter__ method ,which is used to
call and perform the setup code required and needed for the
process to be executed, before the with statement.
2- When the with statements ends, the interpreter invokes the __exit__ method .
Illustration :
What's required to implement a contect manager class?
class ContextManager(object):
"""docstring for ContextManager"""
def __init__(self):
pass
def __enter__(self):
print("__enter__()")
# optional 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)
Multiple context managers
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')
Manage Resources
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')
ManageFile class follow.
ManageFile class follow/adhere the context manager protocol and support the with statement Code link
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