Catch Exception While Using Python with Statement

In Python, the with statement is used when working with files or network connections. It makes sure that resources (files) are opened, used, and then closed properly, even if an error occurs during the process.

If you want to catch any exceptions that happen inside a with block, you can wrap it in a try-except statement. Alternatively, you can use a custom context manager that handles exceptions on its own.

Catching Exceptions Inside with Block Using try-except

To catch errors that occur inside a with block, we need to place it within a try-except. This helps you to handle any exceptions that occur when working with resources.

Example

In this example, we open a file using a with statement and try to read an integer from the file. If the file contains invalid data, a ValueError may be raised, which we catch and handle -

try:
   # Create and write a number to the file
   with open("numbers.txt", "w") as file:
      file.write("ab3")  # You can change this to any integer or invalid value to test

   # Now open and read the file
   with open("numbers.txt", "r") as file:
      data = file.read()
      number = int(data)  # This may raise ValueError if data is not a valid integer
   print("Number read:", number)
except ValueError:
   print("Caught ValueError: invalid integer in file")
except FileNotFoundError:
   print("Caught FileNotFoundError: file not found")

Following is the output obtained -

Caught ValueError: invalid integer in file

Handling Exceptions Using Context Managers

A context manager is a Python object that defines what happens when you enter and exit a with block using its __enter__() and __exit__() methods. You can use it to automatically handle exceptions and even suppress them if needed.

In this case, the with statement is not only managing entry and exit but also helping with error handling by calling the __exit__() method when an exception occurs.

Example

Following is a simple custom context manager that catches exceptions internally and suppresses them -

class SuppressErrors:
   def __enter__(self):
      print("Entering context")
      return self
   def __exit__(self, exc_type, exc_val, exc_tb):
      if exc_type:
         print(f"Caught exception inside context manager: {exc_val}")
         return True  # Suppress the exception

with SuppressErrors():
   x = 1 / 0  # Raises ZeroDivisionError but is suppressed
print("Program continues after with block")

Here, the with statement calls the __enter__() method at the start and __exit__() method when an exception occurs. The context manager catches the error and returns True to suppress it, preventing the exception from going outside the block.

Entering context
Caught exception inside context manager: division by zero
Program continues after with block