Context is Key: Context Managers

Python objects that follow the so-called "context manager protocol" can be used with Python's with statement. The with statement starts a code block that is processed "within a context", e.g. a resource that has been acquired:

>>> with open("myfile.txt", "w") as myfile:
...     myfile.write("Hello!")
... 
6
>>> with open("myfile.txt", "r") as myfile:
...     myfile.readlines()
... 
['Hello!']
>>> 
>> myfile
<_io.TextIOWrapper name='myfile.txt' mode='r' encoding='UTF-8'>
>>> myfile.closed
True
>>> 

After the with statement code block ends the context is "terminated", e.g. an acquired resource gets closed or destroyed.

This is a useful mechanism for (but not limited to) closing resources that shouldn't be left open. Prominent examples for such resources are

• files
• network connections
• database connections

It's possible to use multiple context managers for a code block:

>>> with open('in.txt', 'r') as infile, open('out.txt', 'w') as outfile:
>>>     for line in infile:
>>>         # better do s.th. sensible with line first...
>>>         outfile.write(line)
>>>

Context managers can be implemented by providing an object with the context manager protocol methods:

class MyContextManager:
    def __enter__(self):
        # context initialization goes here (e.g. resource acquisition)
        # ...

    def __exit__(self, exc_type, exc_value, traceback):
        # context finalization goes here (e.g. resource closing/cleanup)
        # ...

A context manager object gets entered through the with statement execution and exits when the with code block ends.

Useful tools for easily creating context managers can be found in the stdlib contextlib.