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Ruby Language Creating a custom exception type


A custom exception is any class that extends Exception or a subclass of Exception.

In general, you should always extend StandardError or a descendant. The Exception family are usually for virtual-machine or system errors, rescuing them can prevent a forced interruption from working as expected.

# Defines a new custom exception called FileNotFound
class FileNotFound < StandardError

def read_file(path)
  File.exist?(path) || raise(FileNotFound, "File #{path} not found")

read_file("missing.txt")  #=> raises FileNotFound.new("File `missing.txt` not found")
read_file("valid.txt")    #=> reads and returns the content of the file

It's common to name exceptions by adding the Error suffix at the end:

  • ConnectionError
  • DontPanicError

However, when the error is self-explanatory, you don't need to add the Error suffix because would be redundant:

  • FileNotFound vs FileNotFoundError
  • DatabaseExploded vs DatabaseExplodedError