With Statement in Python
Creating a program takes up memory and disk space and if we don’t manage the resources effectively, problems can arise, such as our program running sluggishly and consuming excessive memory/resources.
One common scenario where this issue can occur is when working with files. When a file is opened, any operation like writing text is temporarily stored in a buffer. A buffer is a temporary storage space used when the input and output speeds are not the same. When we open a file, the computer stores it in a buffer to allow for smooth operation. If we forget to close the file after performing operations on it, any changes made to the data will be lost as the computer will discard the changes as useless data. In simple terms, the changes made will be lost.
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The occurrence of exceptions and errors in a program can lead to similar issues. Python offers two solutions to handle such problems:
- Try…finally
- With
In this article, we will discuss the above solutions individually with special focus on the latter. We will be covering the following sections:
Try…Finally Statement in Python
In Python, the Try…Finally statement is used to ensure that a certain block of code is always executed, even if an exception is raised. It is a helpful technique to manage resources like files or network connections, where we want to make sure that they are always properly closed or released, regardless of whether an error occurs or not.
The code inside the “try” block is executed, and if an exception is raised, the code inside the “finally” block is executed before the exception is propagated further. The Try…Finally statement ensures that the code inside the “finally” block is executed even if an exception is not caught, making it a useful tool for managing resources and handling exceptions in Python.
The code snippet below demonstrates a method for managing file resources using the Try…Finally approach in Python. The file is opened using the open() method, the contents are written to the file, and then the file is closed using the close() method.
file = open('file-path', 'w') try: file.write('Lorem ipsum') finally: file.close()
This method can be prone to errors if the programmer forgets to close the file, leading to data loss or other issues.
A cleaner approach to file resource management is offered by the with statement, as shown below:
with open('file-path', 'w') as file: file.write('Lorem ipsum')
Instead of explicitly opening and closing the file, the “with” statement automatically handles resource management. When the code block within the “with” statement is executed, the file is opened and assigned to the variable file. Once the block is completed, the file is automatically closed by the “with” statement. This ensures that the file is properly closed and any errors are handled gracefully.
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With Statement in Python
The with statement in Python is used to simplify the management of resources that need to be acquired and released in a specific order. It provides a way to wrap the execution of a block of code with methods defined by a context manager. A context manager is an object that defines two special methods __enter__() and __exit__() which are used to acquire and release resources, respectively.
To put it simply, when we use the “with” statement in Python, the __enter__() method is executed when the program enters the “with” block, and the __exit__() method is executed when the program leaves the block. It’s important to note that “with” only works with objects that support the context management protocol and have these two methods.
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Using With Statement in Custom Methods
While “with” is commonly used with built-in methods like open() for file handling, it can also be used in custom methods. This can be helpful for tasks that require specific actions to be performed at the beginning or end of a procedure, such as opening and closing a file or establishing and terminating a connection to a database.
To use “with” in custom methods, the object must have the __enter__() and __exit__() methods. By defining these methods in a user-defined class, we can enable the use of “with” in our own code.
Below is an example of a simple class:
class ContextManager: def __enter__(self): print("Entering context") return "Entered" def __exit__(self, *args, **kwargs): print("Exiting context") with ContextManager() as myCM: print(myCM)
Output:
The code above defines a class called ContextManager with two methods, __enter__() and __exit__(). When the class is called using the “with” statement, the __enter__() method runs first, followed by the __exit__() method.
The __exit__() method takes three arguments, namely *args and **kwargs. These arguments are used to handle exceptions that may occur during the execution of the code.
- The first argument, execution_type, is the class of the exception.
- The second argument, execution_value, is an instance of the exception that indicates the type of exception (e.g., Zero Division Error or Floating Point Error).
- The third argument, traceback, is a traceback object.
When no exceptions are raised, these arguments return None, None, and None, respectively.
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Endnotes
Hope this article was helpful for you to understand how to use the with statement in Python to ensure that a file/database connection is properly closed and any errors are handled gracefully. If you wish to learn more about Python and practice Python programming, you can explore related articles here.
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