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CIT 383: Administrative Scripting
Exceptions
CIT 383: Administrative Scripting
Slide #1
Topics
1.
2.
3.
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5.
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Subclasses and Inheritance
What is an Exception?
Raising Exceptions
Catching Exceptions
Retry
Exception Classes
CIT 383: Administrative Scripting
Subclassing
A subclass is a class that is based on but
modified from an existing class.
 A subclass extends another class.
 Extends means the same as inherits from.
 The original class is known as the superclass.
A subclass is defined using < superclass:
class Point3D < Point
end
CIT 383: Administrative Scripting
Inheritance
Subclasses inherit methods of superclass.
Superclass methods can be called on subclass.
p3d = Point3D.new(0,0,0)
p3d.x
p3d.y
Class variables shared with superclass.
If subclass modifies value of a class variable,
the superclass sees the modified value too.
CIT 383: Administrative Scripting
Overiding and Adding Methods
To make Point3D useful, redefine initialize:
class Point3D < Point
def initialize(x,y,z)
@x,@y,@z = x,y,z
end
We also need to add accessors for the z coordinate:
def z
@z
end
def z=(value)
@z = value
end
end
CIT 383: Administrative Scripting
Calling Superclass Methods
How can you call an overidden method?
Use super(args) in method of subclass.
New definition of initialize:
class Point3D < Point
def initialize(x,y,z)
# pass first two args to
# superclass initialize method
super(x,y)
# initialize the third argument
@z = z
end
end
CIT 383: Administrative Scripting
What is an Exception?
An exception is an object that represents an
exceptional condition, such as an error of
some type.
An exception is raised when an error occurs.
A ruby program terminates on an exception.
An exception can be caught by an exception
handler, preventing program termination.
CIT 383: Administrative Scripting
Exception Classes
Exceptions belong to Exception class or its subclasses.
Exception
NoMemoryError
SystemExit
StandardError
ArgumentError
IOError
IndexError
TypeError
ZeroDivisionError
CIT 383: Administrative Scripting
Raising Exceptions
The raise method raises an exception
raise: creates a new RuntimeError and raises it.
raise(string): creates new RuntimeError
with the specified string as its message and
raises it.
raise(class): creates a new exception object
of the specified class and raises it.
raise(class, string): creates a new
exception object of the specified class with the
specified string as its message and raises it.
CIT 383: Administrative Scripting
Raise Example
Raise an exception on a bad username argument:
class User
def initialize(username)
if username =~ /^[a-z][a-z0-9]+$/
@username = username
else
raise 'Invalid username'
end
end
end
CIT 383: Administrative Scripting
Handling Exceptions
Use a rescue clause with a begin statement.
begin
# Ruby code that may raise
# an exception.
rescue
# Exception-handling code.
end
CIT 383: Administrative Scripting
Handling Exceptions by Type
Specify the exception class after rescue
By default, only StandardError is caught.
To catch any exception
rescue Exception
To catch specific exception and assign to e.
rescue ArgumentError => e
Accessing the exception object string
puts e.message
CIT 383: Administrative Scripting
Catching Multiple Exception Types
Handling multiple types identically:
rescue ArgumentError, IOError => e
Handling each type differently:
begin
#code that may raise exception
rescue ArgumentError => e
# code to handle ArgumentErrors
rescue IOError => e
# code to handle IOErrors
end
CIT 383: Administrative Scripting
Retry
Retry restarts at top of begin block.
# Try to get URL up to 10 times in case svr slow
tries = 0
begin
tries += 1
geturl(‘http://example.com/’)
rescue Exception => e
puts e.message # print error
if tries < 10
sleep(10)
# wait 10 seconds
retry
# try get_url again
end
end
CIT 383: Administrative Scripting
Writing Exception Handlers
Keep begin blocks short.
begin
# no more than 3 lines
rescue
end
Move longer blocks into a new method.
begin
try_to_geturl()
rescue
end
CIT 383: Administrative Scripting
Defining Exceptions
What is the purpose of exception classes?
Each class can be handled differently by its own
rescue block.
Creating a new exception class
class MyError < StandardError
end
Exception subclasses define no new methods
Only characteristics that matter are their name
and parent class.
CIT 383: Administrative Scripting
Defining Exceptions
Create a new exception class only when
The caller has to do something different to
recover from the error.
Do not create a class for each error message
Use the message string to contain the message
instead of creating a new exception class.
CIT 383: Administrative Scripting
References
1. Michael Fitzgerald, Learning Ruby, O’Reilly,
2008.
2. David Flanagan and Yukihiro Matsumoto, The
Ruby Programming Language, O’Reilly, 2008.
3. Hal Fulton, The Ruby Way, 2nd edition, AddisonWesley, 2007.
4. Robert C. Martin, Clean Code, Prentice Hall,
2008.
5. Dave Thomas with Chad Fowler and Andy Hunt,
Programming Ruby, 2nd edition, Pragmatic
Programmers, 2005.
CIT 383: Administrative Scripting
Slide #18