Ruby Exception Handling

Ruby Exception Handling: EOF Error

Next on the docket in our Ruby Exception Handling series, today we’re going to examine the EOFError in Ruby.EOFErrors are a descendants of the IOError class, and consequently, occur only in the specific scenario that an IO method is called for a file stream, in which that stream has already reached the end of the file.

In this article we’ll explore just what might cause an EOFError, see where it sits within the overall Ruby Exceptionclass hierarchy, and take a peak at how you might avoid running into EOFErrors of your own, so let’s get to it!

The Technical Rundown

  • All Ruby exceptions are descendants of the Exception class, or a subclass therein.
  • StandardError is a direct descendant of the Exception class, and is also a superclass with many descendants of its own.
  • IOError is a direct descendant of the StandardError class, and is a superclass to one descendant of its own.
  • EOFError is a direct descendant of the IOError class.

When Should You Use It?

Most commonly, EOFErrors will occur when a further attempt to read from a file stream is made, after the end of that file stream has already been reached. Ruby’s built-in IO class features a slew of methods and related classes that allow for significant manipulation of file objects. Rather than cover them all here, we’ll just take a brief look at some example code and how it ends up producing an EOFError, along with how to solve it.

In this example, our goal is to create a comma-separated value (CSV) file with a list of names and their respective IDs. Nothing fancy, but this will be helpful when we read the file later. To create our csv file, we’ve included a simple append_data function that loops through the data in our provided array, then writes each element (and associated index) to a new line in our truncated names.csv file.

Within the begin-rescue block, our actual examination code takes place. Here we’re first creating our new csv file and populating it, then we reopen the file and call the .read() method, which (by default) grabs a string of the full file stream, which we’re outputting to the console.

Finally, before we’ve issued a .close() command on our file stream, we lastly attempt one more read attempt using the .readline() method.

Initially, the first call to file.read() works fine, and our output displays the contents of our names.csv file. However, this has forced the file stream to read the entire file and thus is sitting at the end of the stream. When our next call to file.readline() is made, sure enough, we throw (and must rescue) an EOFError:

The challenge is that this isn’t really a “fixable” error in the normal sense. That is to say, Ruby is kind enough to report that we’ve reached the end of the file, and throws an EOFError for our troubles, but the production of such an error typically means there’s a fundamental flaw in the structure of the code, since reaching the end of a file that’s being read isn’t inherently a bad thing.

In most cases (as with our example) the culprit is our non-idiomatic use of File.open, in which we assign the resulting file object to a variable, rather than using an inclusive block following the File.open method call. By assigning it to a variable, it leaves us open to forgetting to manage our file stream status manually. This typically requires calling the .close() method on our file, which closes the file and flushes any pending write operations before doing so.

The advantage to using a File.open code block to handle the file object we just created, is that once the block concludes, .close() is called for us automatically. Thus, in our example above, we could change our begin-rescue-end block to something like this:

Admittedly, this isn’t pretty code, particularly because we’d almost never want to actually perform the actions we’re taking on this file (reading it all, then reading the first line of it immediately afterward). However, for our example, it serves the purpose: We never manipulate our file object outside of a File.open() method code block, so our handling of .close() is done for us. Thus, our output no longer shows an EOFError, and instead displays the entirety of the file (file.read()), then the first line (file.readline()):

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