Ensure for cleanup in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the ensure block in Ruby?

The ensure block runs code that must execute no matter what happens in the begin block, such as cleaning up resources or closing files.

beginner

How does ensure differ from rescue in Ruby?

rescue handles exceptions when they occur, while ensure always runs after begin, whether an exception happened or not.

intermediate

What happens if an exception is raised inside an ensure block?

If an exception occurs inside ensure, it will override any previous exception and propagate up, potentially hiding the original error.

beginner

Why is ensure useful for resource cleanup?

ensure guarantees that cleanup code like closing files or releasing locks runs even if an error interrupts normal flow.

beginner

Example: What will this Ruby code print?

begin
  puts 'Start'
  raise 'Oops'
rescue
  puts 'Rescued'
ensure
  puts 'Always runs'
end

It prints:
Start
Rescued
Always runs
The ensure block runs last no matter what.

What does the ensure block guarantee in Ruby?

A Code inside it runs before the <code>begin</code> block

B Code inside it runs only if no exception occurs

C Code inside it runs only if an exception occurs

D Code inside it always runs, even if an exception occurs

The ensure block always runs after begin, regardless of exceptions.

Which block handles exceptions in Ruby?

A <code>begin</code>

B <code>ensure</code>

C <code>rescue</code>

D <code>finally</code>

rescue handles exceptions; ensure runs cleanup code.

If an exception is raised in ensure, what happens?

A It replaces any previous exception

B The program exits silently

C It is caught by <code>rescue</code>

D It is ignored

Exceptions in ensure override previous exceptions and propagate.

Where should you put code to close a file to ensure it always runs?

A Inside <code>begin</code>

B Inside <code>ensure</code>

C Inside <code>rescue</code>

D Outside all blocks

Closing files should be in ensure to guarantee execution.

What will this code print?

begin
  puts 'Hello'
ensure
  puts 'Goodbye'
end

A Hello Goodbye

B Goodbye

C Hello

D Goodbye Hello

Both puts run; ensure runs after begin.

Explain how the ensure block works in Ruby and why it is important for cleanup.

Think about what happens when errors occur and you still want to clean up.

Describe the difference between rescue and ensure blocks in Ruby exception handling.

One catches errors, the other guarantees code runs.

Practice

1. What is the main purpose of the ensure block in Ruby?

easy

A. To run code that must execute regardless of errors

B. To catch exceptions and handle them

C. To define a method inside a class

D. To start a loop that repeats code

Solution

1. Step 1: Understand the role of ensure

   The ensure block runs code no matter what happens in the begin block, even if an error occurs.

2. Step 2: Compare with other blocks

   Unlike rescue which handles errors, ensure always runs for cleanup tasks.

3. Final Answer:

   To run code that must execute regardless of errors -> Option A

4. Quick Check:

   ensure = always runs cleanup [OK]

Hint: Remember: ensure always runs, even after errors [OK]

Common Mistakes:

• Confusing ensure with rescue
• Thinking ensure only runs on errors
• Using ensure to catch exceptions

2. Which of the following is the correct syntax to use ensure in Ruby?

easy

A. if
# code
else
# cleanup code
end

B. try
# code
finally
# cleanup code
end

C. begin
# code
rescue
# cleanup code
end

D. begin
# code
ensure
# cleanup code
end

Solution

1. Step 1: Identify Ruby's syntax for ensure

   Ruby uses begin ... ensure ... end to guarantee cleanup code runs.

2. Step 2: Eliminate other language or incorrect syntax

   Options B and D use non-Ruby keywords; C uses rescue which is for error handling, not guaranteed cleanup.

3. Final Answer:

   begin # code ensure # cleanup code end -> Option D

4. Quick Check:

   Ruby ensure syntax = begin ... ensure ... end [OK]

Hint: Use begin ... ensure ... end for guaranteed cleanup [OK]

Common Mistakes:

• Using try/finally (not Ruby syntax)
• Confusing rescue with ensure
• Using if/else for cleanup

3. What will be the output of this Ruby code?

def test
  begin
    puts "Start"
    raise "Error!"
  ensure
    puts "Cleanup"
  end
end

test

medium

A. Start\nCleanup\nRuntimeError: Error!

B. Start\nRuntimeError: Error!\nCleanup

C. Cleanup\nStart\nRuntimeError: Error!

D. Start\nCleanup

Solution

1. Step 1: Trace the code execution

   The method prints "Start", then raises an error. The ensure block runs next, printing "Cleanup".

2. Step 2: Understand error propagation

   After ensure, the error is not rescued, so it propagates and shows the RuntimeError message.

3. Final Answer:

   Start\nCleanup\nRuntimeError: Error! -> Option A

4. Quick Check:

   ensure runs before error propagates [OK]

Hint: Ensure runs before error stops program [OK]

Common Mistakes:

• Thinking ensure stops error
• Assuming cleanup prints after error message
• Ignoring error propagation

4. Identify the error in this Ruby code using ensure:

begin
  puts "Processing"
  # missing end for begin
ensure
  puts "Cleaning up"
end

medium

A. Cannot use ensure without rescue

B. Missing end for begin block

C. The puts statements are invalid inside ensure

D. The ensure block must come before begin

Solution

1. Step 1: Check block structure

   The begin block is missing its closing end before ensure.

2. Step 2: Understand ensure placement

   ensure must be inside the begin ... end block, so the missing end causes a syntax error.

3. Final Answer:

   Missing end for begin block -> Option B

4. Quick Check:

   Every begin needs matching end [OK]

Hint: Count your begin and end keywords carefully [OK]

Common Mistakes:

• Thinking ensure needs rescue
• Placing ensure outside begin-end
• Assuming puts invalid in ensure

5. You want to open a file, write data, and always close it even if an error occurs. Which Ruby code correctly uses ensure for this cleanup?

hard

A. begin
file = File.open('data.txt', 'w')
file.write('Hello')
rescue
file.close
end

B. file = File.open('data.txt', 'w')
file.write('Hello')
file.close

C. file = File.open('data.txt', 'w')
begin
file.write('Hello')
ensure
file.close
end

D. begin
file = File.open('data.txt', 'w')
file.write('Hello')
ensure
file = nil
end

Solution

1. Step 1: Identify correct use of ensure for cleanup

   file = File.open('data.txt', 'w') begin file.write('Hello') ensure file.close end opens the file before begin, writes inside, and closes in ensure, guaranteeing closure.

2. Step 2: Check other options for errors

   begin file = File.open('data.txt', 'w') file.write('Hello') rescue file.close end closes in rescue, which only runs on errors, not always. file = File.open('data.txt', 'w') file.write('Hello') file.close doesn't handle errors. begin file = File.open('data.txt', 'w') file.write('Hello') ensure file = nil end sets file to nil but doesn't close it.

3. Final Answer:

   file = File.open('data.txt', 'w') begin file.write('Hello') ensure file.close end -> Option C

4. Quick Check:

   Ensure always closes file [OK]

Hint: Put cleanup like file.close inside ensure [OK]

Common Mistakes:

• Closing file only in rescue block
• Not closing file on error
• Setting variable to nil instead of closing