In the previous exercise, we explored how to create a context manager using the contextlib module. However, we did not go over how to deal with errors just as we did with the class-based approach. Like any other pattern, you may run into errors when invoking your context manager using the @contextmanager decorator.

For the class-based context manager, the __exit__ method dealt with exceptions. For the decorator method, errors are most commonly dealt with within an except block. We will build on top of our try/finally block by incorporating an except. There are two main ways to deal with errors:

• To throw an error and stop the execution of our entire program, we can:

  • Simply do nothing by excluding an except block

• To catch errors and continue the execution of our program, we can:

  • Handle the exception via an except block.

Let’s look at an example of what a decorator based context manager that catches errors can look like:

from contextlib import contextmanager

@contextmanager
def open_file_contextlib(file, mode):
  open_file = open(file, mode)

try:
   yield open_file

 # Exception Handling
 except Exception as exception:
   print('We hit an error: ' + str(exception))

 finally:
   open_file.close()

with open_file_contextlib('file.txt', 'w') as opened_file:
 opened_file.sign('We just made a context manager using contexlib')

Notice:

• The inclusion of the except clause
• The except attempts to catch a generic Exception and, if it is hit, saves it to a variable exception.
  • Note: we can use any exception object, not just a generic one, if we know the specific exception we are trying to catch.
• The handler then prints out the error

When this context manager is called in the with statement above, it will hit the exception block because .sign() is not a file method. The output would look like this:

We hit an error: '_io.TextIOWrapper' object has no attribute 'sign'

This tells us what our error is, so we know what to fix. Now, let’s practice upgrading our poem_files context manager to catch exceptions.