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Here the loop is only broken (by the break statement in the else clause) when no exception is raised. In other words, as long as something wrong happens, the program keeps asking for new input. The following is an example run: Enter the first number: 1 Enter the second number: 0 Invalid input. Please try again. Enter the first number: 'foo' Enter the second number: 'bar' Invalid input. Please try again. Enter the first number: baz Invalid input. Please try again. Enter the first number: 10 Enter the second number: 2 x/y is 5 As mentioned previously, an alternative to using an empty except clause is to catch all exceptions of the Exception class (which will catch all exceptions of any subclass as well). You cannot be 100 percent certain that you ll catch everything then, because the code in your try/except statement may be naughty and use the old-fashioned string exceptions, or perhaps create a custom exception that doesn t subclass Exception. However, if you go with the except Exception version, you can use the technique from the section Catching the Object, earlier in this chapter, to print out a more instructive error message in your little division program: while 1: try: x = input('Enter the first number: ') y = input('Enter the second number: ') value = x/y print 'x/y is', value except Exception, e: print 'Invalid input:', e print 'Please try again' else: break The following is a sample run: Enter the first number: 1 Enter the second number: 0 Invalid input: integer division or modulo by zero Please try again Enter the first number: 'x' Enter the second number: 'y' Invalid input: unsupported operand type(s) for /: 'str' and 'str' Please try again Enter the first number: foo

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render :layout => 'some_other_layout'

10

Note You can learn more about layouts at http://api.rubyonrails.org/classes/

In 8 you looked at Ruby s unit testing abilities, and in 12 you used them to test a library you developed. Tests allow you to specify expected outcomes and then to have various elements of functionality (mostly results from methods) tested to see if the actual outcomes match the expectations. If they match, you assume the code is perfect (or as close as it can get!), but if you get errors, you can use the results to debug your code. Rails is particularly well known for its testing features, and a lot of this is due to Ruby s fine unit testing library that you looked at in s 8 and 12. Test suites are run using Rake commands from the root directory of the application, and tests take place

Delivering Your Presentation Using Three Ground Rules Rule 1: Step into the Screen Rule 2: Remove the Distractions Rule 3: Manage the Flow Removing Distractions Preparing the Environment Checking the Technology Planning for Problems Rehearsing Away Distractions Using Notes Developing a Dialog Being Authentic Working Con dently with Your Slides Handling Q & A Improvising Within Constraints Keeping Control of Your Story Preparing for Different Contexts Handing Out Handouts

under the test environment, meaning that testing operations can take place upon a different database from live data At present, Rails supports three main types of testing as follows: Unit testing: You use unit testing within Rails to test your models You supply the Rails testing system with fixtures (that is, dummy data with which to populate the test database) and a test suite for each model where you perform operations against the data and then perform assertions against the results of these operations These test suites are located in test/unit, and the fixtures are in YAML format in test/fixtures Functional testing: You use this to test your controllers You have a test suite for each controller, whereupon you make certain requests to controller actions (in code) and then assert against the responses You define functional tests in test/functional.

Invalid input: name 'foo' is not defined Please try again Enter the first number: 10 Enter the second number: 2 x/y is 5

Integration testing: The highest level of testing, integration tests let you test your entire application You can make requests to controller actions, follow redirects, assert against the responses, and then continue making more requests of different types Integration tests are often considered story level tests, as you can test an entire branch of functionality from start to finish, in the same way a real user would while interacting with the application with a Web browser Integration tests are defined in test/integration The use of the preceding testing techniques varies from developer to developer It s common to download open source Rails applications to see no testing used at all It can be hard to develop the habit to test consistently, and harder still to learn to write the tests before you write the code to produce the results.

However, it s still encouraged, because once you perfect the technique, your code will shine, and you ll be able to rest secure in the knowledge that your code is easily testable and guaranteed..

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Note You can learn about the special types of assertions Rails adds to the standard Test::Unit assertions at http://api.rubyonrails.org/classes/Test/Unit/Assertions.html.