With Keyword

Introduction

The with keyword can be used for dealing with unmanaged resources. Let’s consider the following scenario.

• Open file
• Do something
• Close file

We are most likely going to wrap this code in a try - finally block to ensure that the call to Close the file is actually done, even if something happens (exception) during the Do something phase.

We might have code that looks like this:

try:
    f = open('data/dummyData1.txt')
    lines = f.read()
    print(lines)
finally:
    f.close()

Using with

Using the with keyword we can now use code like this instead:

with open("data/dummyData1.txt") as f:
    lines = f.read()
    print(lines) 

We don’t have to worry about closing the file because that has already been done as part of the with keyword.

Internals of with

Python’s with statement supports the concept of a runtime context defined by a context manager.

An explanation of the history of with can be found here, and below is a basic summary using code.

class controlled_execution:
    def __enter__(self):
        set things up
        return thing
    def __exit__(self, type, value, traceback):
        tear things down

with controlled_execution() as thing:
    some code

When the with statement is executed, Python evaluates the expression, calls the enter method on the resulting value (which is called a “context guard”), and assigns whatever enter returns to the variable given by as. Python will then execute the code body, and no matter what happens in that code, call the guard object’s exit method.

Building your class with a context manager

Naturally you can build your own classes to behave the same.