Day 22 to 24 - Context managers and the “with” statement

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Topics: with statement and context managers

Context managers and the with statement

Similar to decorators, context managers are a concept many people use but only few understand. If you haven’t heard of the term ‘context manager’ before: you probably encountered them already while reading or writing from/to a file using the with statement.

The most common use of context managers is the proper management of resources. In simple terms this means that we want to make sure that we open, read, write and close files correctly. Before creating our own context manager, let’s take a look at the most common use of the with statement and why the with statement is so useful.

Using with to open files

In the world of Cleon Bery a lot of communication is done via good old letters. So let’s imagine that Cleon wants to write a letter to Bromley asking if he, Flynn and Cassidy can stop by for tea in the afternoon. To do so, we open a file called ‘letter.txt’ and write a few lines of text to the file:

with open('letter.txt', 'w') as letter:
    letter.write("Hi Bromley! \n"
                 "Can Flynn, Cassidy and I stop by for a tea this afternoon? \n"
                 "Cleon")

Internally, this is translated to something like this (details of the full translation can be found in PEP 343):

letter= open('letter.txt', 'w')
try:
    letter.write("Hi Bromley! \n"
                 "Can Flynn, Cassidy and I stop by for a tea this afternoon? \n"
                 "Cleon")
finally:
    letter.close()

The try ... finally statement is the key part: it guarantees that the code in the finally block is always executed no matter what happens in the try block.

Why the with statement is useful

In the with statement above, the open keyword opens a file descriptor. As you probably know it’s very important that every time you open a file, you also need to close it. Otherwise, when opening too many files, your operating system will throw an error at some point.

By using the with statement we ensure that the open file descriptor is closed automatically after the control flow leaves the context of the with statement. So even when an exception occurs before the end of the with block, the opened file will be closed. Also when using a return, continue or break statement in the with block, the file will be closed automatically. We don’t have to write try ... except ... finally blocks ourselves - the with statement takes care of properly closing the file. This makes sure that we are not leaking any resources, i.e. forget to close opened files.

So in short: the with statement
a) Makes code that deals with resources more readable
b) Ensures that resources are not leaked

If you want to see an example of how context managers are used within Python, take a look at the threading.Lock class. It can be found here.

Creating our own context manager

Let’s say we want to create a Letter class in our Magical Universe that functions as a context manager. This would allows us to create a function called write_letter() that opens a letter object and writes text to it. So in the end we want to be able to use:

with Letter('lettername.txt', 'w') as letter:
    letter.write(...)

To achieve this functionality, our Letter class needs to contain two methods: __enter__() and __exit__(). When these methods are implemented, our class follows the context manager protocol and supports the with statement. More about context managers can be found in the Python docs.

Letter class

Creating a context manager is not difficult. Our Letter class will look as follows:

class Letter:
    def __init__(self, letter_name):
        self.letter_name = letter_name

    def __enter__(self):
        self.letter = open(self.letter_name, 'w')
        return self.letter

    def __exit__(self, exc_type, exc_value, traceback):
        if self.letter:
            self.letter.close()

That’s it! With a proper implemetation of __enter__() and __exit__() our Letter class supports the with statement:

with Letter('dear_bromley.txt') as letter:
    letter.write("Hi Bromley! \n"
                 "Can Flynn, Cassidy and I stop by for a tea this afternoon? \n"
                 "Cleon")

Steps of the with statement

You might wonder when exactly __enter__() and __exit__() are called and what happens behind the scences when using the with statement. Therefore, we will take a closer look at the individual steps.

To be more precise: when the with statement is executed with a single file like ‘letter.txt’:

  1. The with statement invokes a context manager.
  2. The context manager’s __exit__() method is loaded for later use.
  3. The context manager’s __enter__() method is invoked.
  4. The value returned by the __enter__() method is bound to the identifier in the as clause of the with statement (i.e. the return value of __enter__() is bound to the variable ‘letter’).
  5. The code in the body of the with statement is executed (i.e. the letter.write(...) part).
  6. The context manager’s __exit__() method is invoked no matter what happened in the code body.

If an exception occured in the code body, its type, value and traceback are passed as arguments to __exit__(). So we can use __exit__() to handle those exceptions, for example, we could suppress them.

For further details on these steps, take a look at the Python docs.

Writing letters

Currently, our Letter class is very simple and not doing more than the open() statement. However, Letter is a regular class, so we could extend it with all kinds of methods. As long as __enter__() and __exit__() are implemented properly, the class will support the with statement. For example, we could keep track of how many letters have been created so far:

class Letter:
    total_number_of_letters = 0

    def __init__(self, letter_name):
        self.letter_name = letter_name
        self.__class__.total_number_of_letters += 1

    def __enter__(self):
        self.letter = open(self.letter_name, 'w')
        return self.letter

    def __exit__(self, exc_type, exc_value, traceback):
        if self.letter:
            self.letter.close()

As a last step, we will add a write_letter() method to our CastleKilmereMember class. This will allow all Castle Kilmere members to write actual letters (even if we can’t send them by owl).

class CastleKilmereMember:
    """
    Creates a member of the Castle Kilmere School of Magic
    """

    ...

    def write_letter(self, recipient, content):
        letter_name = f"dear_{recipient}.txt"
        with Letter(letter_name) as l:
            l.write(content)

    ...
if __name__ == "__main__":
    cleon = Pupil.cleon()

    letter_content = "Hi Bromley! \nCan Flynn, Cassidy and I stop by for a tea this afternoon? \nCleon"
    cleon.write_letter('Bromley', letter_content)

    print(f"Total number of letter creates so far: {Letter.total_number_of_letters}")

Contextlib.contextmanager

Context managers don’t have to be class-based. We could also use the contextlib module to support the with statement.