Apress Pro Python System Administration 2nd (2014) by unknow

Author:unknow
Format: epub
Tags: Computers, Linux & UNIX Administration, Programming Languages, Python, System Administration
ISBN: 9781484202180
Google: tWDRoAEACAAJ
Published: 0101-01-01T00:00:00+00:00

The short answer is “no” and the slightly longer answer is “it depends.” The reason for an exception to be raised is that something unexpected happened. Let’s say we have a web application that serves files from our server. All files are linked from external pages, and the general assumption is that whoever creates the listing would only list files that do exist. But, being human, we all make mistakes, and the operator of the listing may have made a typo, so the resulting link would point to a file that does not exist. Now, if a user clicks on the link, the application tries to do exactly what it is told to—retrieve the file. But the file does not exist, and so the code that is responsible for reading in the file would fail and throw an exception saying that the file does not exist.

Should the application check for the missing files and react appropriately? In this example, probably yes; but in more complicated situations, it is not always possible to predict and write code for every possible outcome. Even with simple applications like my file retriever service, it’s not always possible to think about every possible thing that can wrong.

As an example, let’s run the application as a Tomcat user and assume that all files being written to the file system have permissions set such that Tomcat users have read access to them. It’s been like that for a long time, and the application works flawlessly. One day, a new system administrator joins the crew and, without knowing, deploys a file with different user permissions. Suddenly there’s a file access error. The file is not missing, but the process that runs with Tomcat user permissions cannot read it. The developer has not thought of this situation, and so there’s no code to handle it. This is where exception handling is really helpful; the application would encounter a situation that is different from a normal program flow and cannot handle it, so the code raises an exception and either the system administrator or the developer can examine why things have gone wrong.

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