PYTHON — Using The With Statement In Python
In the software world, the moment you start using someone else’s software, you are living in their world, under their philosophy. — Richard Stallman
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PYTHON — Python Basics Class Overview
The “with” statement in Python is a useful tool for properly managing external resources in your programs. It allows you to take advantage of existing context managers to automatically handle the setup and teardown phases whenever you’re dealing with external resources or with operations that require those phases.
What’s a context manager? It’s a block of code that has side effects upon entering and exiting. The context management protocol allows you to create your own context managers so you can customize the way you deal with system resources.
In this article, we’ll cover how context managers work, some common context managers in the Python standard library, and how to write a custom context manager.
How Context Managers Work
A common part of writing programs is managing resources, such as files, network connections, or database connections. The typical interaction is to open one of these resources, do some work, then close the resource. That last part is important. You have to remember to clean up after yourself. Your program can actually crash if you leave too many of these kinds of things open at a time. One way of making sure things get cleaned up, even if there’s an exception, is to use a finally section in a try block. Or, as you might have guessed from the course’s title, another option is to use a context manager.
A context manager is instantiated using the with keyword. Underneath the with is a block of code. When that block exits, a special method gets called on the context manager, mostly used to do resource cleanup.
Common Context Managers in the Python Standard Library
Context managers can be found throughout the standard library, and they are often one of two ways of doing things. You can use the open() built-in function inside of a try … finally block, or you can use the same function as a context manager. In the latter case, the file gets closed automatically when the block finishes.
Writing a Custom Context Manager
You can even write your own context managers, either through the creation of a custom class or by writing a function that uses the context manager decorator in the contextlib library.
Example:
from contextlib import contextmanager
@contextmanager
def open_file(path, mode):
the_file = open(path, mode)
yield the_file
the_file.close()
files = []
for x in range(100000):
with open_file('foo.txt', 'w') as infile:
infile.write('foo')
files.append(infile)The above example shows how to create a custom context manager using the contextlib library to open and write to files.
With the knowledge of context managers and the with statement, you can write more expressive code and avoid resource leaks in your programs.
