Handling a File Error

As already pointed out in the exceptions lesson, Python contains a vast set of built-in exceptions. One important class has to do with file handling. The example presented in the lesson will help to review file handling methods and put them in the context of handling exceptions.


Exceptions are something nobody wants to see but they are virtually unavoidable. Maybe you try to read a file that doesn't exist, the user of your code has chosen invalid values, the matrix you are analyzing has different dimensions than expected, etc. Handling exceptions is a sensitive topic because it can lead to even more problems downstream. An Exception is a clear message that there is something wrong going on and if you don't fix it properly, it is going to become even worse.

Handling exceptions can help you to avoid having inconsistent data, not closing resources such as devices, connections or files, etc. However, not handling exceptions correctly can lead to even more problems later on. The  try/except block is very handy when you know what kind of exceptions can appear and you know how to handle them. Imagine you are performing several steps of a complex operation, like writing to a database. If an error happens, you can revert all the changes and avoid inconsistencies.

As with almost any other Python topic, the best way to learn is to look closely at other's code and judge by yourself. Not all packages define their own exceptions, nor handle them in the same way. If you are looking for inspiration, you can see the errors of Pint, a relatively small package, or the exceptions of Django, a much more complex package.

Remember, that both the code and the text of the article is available, in case you have any comments or suggestions to improve it.