Learn

Now that we know the structure of implementing our own class-based context manager. Let’s walk through a context manager that manages actual files as well as explore each of the methods we saw earlier. Here is what our context manager will look like:

class WorkWithFile: def __init__(self, file, mode): self.file = file self.mode = mode def __enter__(self): self.opened_file = open(self.file, self.mode) return self.opened_file def __exit__(self, *exc): self.opened_file.close()

We have written a class-based context manager called WorkWithFile! Let’s break down each method and what happens inside of it.

  • The __init__ method:

    This method is standard across most classes, even ones that are not context managers themselves. In this case, we have three parameters:

    • self: This is standard for any class we work with and allows us to work with methods and properties we assign to an instance of a class.
    • file: Since we are working with files, we need to be able to take in a file argument when we call the class with a with statement.
    • mode: Lastly, we need to provide the file a mode. This allows us to manage what our context manager will actually be doing, such as reading, writing, or both!

    Both file and mode arguments allow us to accomplish the following syntax:

    with WorkWithFile('file.txt', 'r')
  • The __enter__ method:

    This is where we deal with opening the file we want to work on. Since any new instance of our context manager will have a file and mode property, we can pass them into the open() function to open a specific file with a specific mode. Then, we save it as a variable called self.opened_file, and return it.

    By returning self.opened_file, the file will be passed into the variable we define when we call it with the with statement. So for example:

    with WorkWithFile('file.txt', 'r') as file

    Will assign the open file 'file.txt' to the variable called file that follows the as clause and thus allowing us to use it in the with statement code block (which we will look at shortly).

  • The __exit__ method:

    Lastly, but one of the most important steps, we have to close the file we work on. Here we are still taking in a *exc argument, but we won’t touch on that until the next exercise. For now, this method is solely responsible for closing the resource we opened in __enter__.

Now that we created our context manager, we can now use it in a with statement like so:

with WorkWithFile("file.txt", "r") as file: print(file.read())
Open this for a step by step breakdown of the above code!
Okay, here we go:
  1. The with statement executes and a context manager object is created from the class provided, with the values of the arguments required in the __init__ method.
  2. The __enter__ method is run.
  3. The __enter__ method opens the file and returns a handle (reference) to the target variable file in the with statement. This is why in the method we return self.opened_file.
  4. Then the code block in the with statement executes; printing the contents of the file to the console.
  5. Once the code in the with statement is completed, The __exit__ method is called.
  6. The __exit__ method closes the file.

Phew, these context managers are a handful! Take some time to let it all sink in, then let’s create our own context manager that works on poem files!

Instructions

1.

Let’s build our poem context manager from earlier again! This time we will allow it to work on files. By the end of these exercises, we will have a custom context manager that has written to a file!

Create a class called PoemFiles and give it a __init__ method that defines a self, poem_file, and mode parameter.

Inside the method, print 'Starting up a poem context manager'

2.

Next, let’s built the properties of the class via __init__. Remember this is so we can pass a file name and a mode when we call the context manager with the with statement.

Inside of the __init_ method and under the print statement, assign two properties to the class:

  • file that is equal to the poem_file parameter
  • mode that is equal to the mode parameter
3.

Next, let’s work on the __enter__ method to set up what happens when we want to start working on a file.

Create an __enter__ method. Have the method print 'Opening poem file'.

4.

In the __enter__ method, we will need to open the file we want to work on and return it! This way, it will be assigned to the variable we declare when we work with the with statement.

Inside __enter__ method give the class a new property called opened_poem_file and assign it to a call of the open() function that takes two arguments:

  • self.file: our classes file property
  • self.mode: our classes mode property

Lastly, return the opened_poem_file property!

5.

Lastly, we need to create an __exit__ method.

Write a __exit__ method that defines a self parameter and a *exc parameter. Make the method print 'Closing poem file'.

6.

We need to make sure we close our file in the __exit__ method so we properly manage our resources.

In the __exit__ method, under the print statement, call the .close() built-in function on the opened_poem_file property of the class.

7.

Uncomment the with statement and run the code. Check out poem.txt to see if the poem has been added!

Note: in reality, we wouldn’t have to create a context manager that opens a file because there’s already an open() built-in function that you can run with a with statement that will open and close a file. However, open() has its limitations, and knowing this base structure will allow us to create our own custom and more advanced context managers that can do much more than open()!

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