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Except, Else, and Finally

You've already learned that try/except catches errors and prevents crashes. But what if your code could encounter different types of errors? What if you want to run code only when NO error occurs? What if you need to clean up resources no matter what happens?

In this lesson, you'll master the complete four-block exception handling structure through prediction and discovery. You'll start by predicting flow control in file operations, learn from AI about else vs finally semantics, challenge AI with return statement edge cases, and build a file operation template for production use.

Part 1: Predict Flow Control in File Operations​

Your Role: Flow control analyst predicting execution paths

Before learning the syntax, develop intuition about control flow. Professional developers reason about execution paths before writing code.

Prediction Exercise: Which Block Runs When?​

Study this code structure (don't run it yet):

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Your Prediction Task​

Before running code, predict which print statements execute for each scenario:

Scenario 1: File exists and is readable

  • Which prints execute?
  • In what order?
  • Your prediction: ________________

Scenario 2: File does not exist

  • Which prints execute?
  • In what order?
  • Your prediction: ________________

Scenario 3: File exists but has permission issues (PermissionError, not caught)

  • Which prints execute?
  • What happens to the program?
  • Your prediction: ________________

Test Your Predictions​

Now create test files and run the code:

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Compare your predictions to actual output.

Discovery Questions​

After running tests, answer these:

  1. When does the else block run? (Only when try succeeds? Even if except runs?)
  2. When does the finally block run? (Always? Only on success? Only on error?)
  3. What's the execution order? (try → except → else → finally, or different?)
  4. What happens if you have multiple except blocks? (Which one runs? All of them? First match?)

Flow Control Rules You Discovered​

Based on your experiments, document these rules:

Scenariotryexceptelsefinally
No errorRuns completelySkippedRunsRuns
Error caughtRuns until errorMatching block runsSkippedRuns
Error NOT caughtRuns until errorNone matchSkippedRuns, then propagates

Deliverable: Create flow_control_predictions.txt documenting:

  • Your initial predictions for all 3 scenarios
  • Actual results from running the code
  • The rules you discovered about when each block executes
  • One new scenario you designed to test an edge case

Part 2: Learn Else (Success Path) vs Finally (Cleanup Guarantee)​

Your Role: Student learning semantic distinctions from AI Teacher

Now that you've observed the behavior, understand the WHY behind it. Ask AI to explain the semantic purpose of each block.

AI Teaching Prompt​

Ask your AI companion:

"I've experimented with try/except/else/finally blocks. I observed that:

  • else runs only when try succeeds (no exception)
  • finally runs always, even after exceptions

Explain:

  1. What is the SEMANTIC purpose of the else block? Why not just put success code at the end of try?
  2. What is the SEMANTIC purpose of the finally block? Why is 'always runs' important?
  3. Show me a real-world file operation example where else and finally have different purposes."

What You'll Learn from AI​

Expected AI Response (summary):

Else block purpose:

  • Separates "success-only" code from "might fail" code
  • Makes exception source clear (if exception happens in else, it's not from try block)
  • Communicates intent: "This code should only run if NO errors occurred"

Finally block purpose:

  • Guarantees cleanup regardless of success, failure, or even return statements
  • Essential for resource management (files, network connections, locks)
  • Runs even if exception is re-raised or not caught

Key distinction:

  • else = "success path logic"
  • finally = "cleanup that must happen no matter what"

Convergence Activity​

After AI explains, test your understanding:

Ask AI: "Show me a file operation where I open a file, process it, log success in else block, and close it in finally. Then explain: what happens if processing raises an exception not caught by any except block?"

Example AI might show:

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Your turn: Explain back to AI:

  • Why is file.close() in finally, not in else?
  • What happens if an IOError occurs during line processing (not caught)?
  • Does finally run before or after the function returns?

Deliverable: Write a 1-paragraph summary explaining the semantic difference between else (success-specific logic) and finally (guaranteed cleanup), with one real-world example.

Part 3: Challenge AI with Return Statement Edge Cases​

Your Role: Student teaching AI by exploring complex control flow

Now reverse the roles. You'll design challenges that test AI's understanding of try/except/finally with return statements—one of the trickiest edge cases.

Challenge Design Pattern​

Create scenarios where:

  1. Return statements appear in different blocks
  2. AI must predict which value is actually returned
  3. Finally block interacts with return statements

Challenge 1: Multiple Return Points​

Your prompt to AI:

"Predict what this function returns for each scenario. Explain your reasoning BEFORE running the code.

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What does this return when:

  1. mystery_return(False) — which return executes?
  2. mystery_return(True) — which return executes?
  3. Why doesn't the else block return ever execute?"

Expected AI Response:

  1. Returns "A: try block return" (try succeeds, returns immediately, else is skipped)
  2. Returns "B: except block return" (exception caught, except returns)
  3. Else only runs when try completes WITHOUT returning (rare case)

Key insight: Return in try or except skips else, but finally still runs.

Challenge 2: Finally Overriding Return​

Your prompt to AI:

"What does this function return? Explain what's happening and whether this is good practice.

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When called with finally_override(10), what's returned: 20 or 30? Why is this pattern dangerous?"

Expected AI Response: Returns 30 (finally return overrides try return). This is dangerous because finally is meant for cleanup, not altering control flow. It hides the intended return value.

Challenge 3: Finally Without Return​

Your prompt to AI:

"Contrast the previous example with this one. What's the difference?

Loading Python environment...

When called with finally_cleanup(10), what's returned? Why is this version better than the previous one?"

Expected AI Response: Returns 20 (try block return). Finally runs, prints the message, modifies local variable, but doesn't override return. This is proper use of finally—cleanup and logging without altering control flow.

Your Analysis​

After AI responds to all three challenges, write:

  • Which pattern is correct professional practice?
  • Why should finally avoid return statements?
  • When would you use return in except blocks?

Deliverable: Document three return statement challenges you posed to AI, AI's predictions, and your analysis of best practices for return placement in try/except/else/finally blocks.

Part 4: Build File Operation Template with Proper Cleanup​

Your Role: Template designer creating reusable production patterns

Now integrate everything into a production-ready file operation template that handles all scenarios correctly.

Your File Operation Template Library​

Create a Python file called file_operations_template.py with these patterns:

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Template Requirements​

Your file operation template library must include:

  1. Read text file template

    • FileNotFoundError handling
    • PermissionError handling
    • Finally block for cleanup
    • Else block for success logging

  2. Write text file template

    • IOError handling
    • Permission handling
    • Success/failure return value

  3. Load JSON template

    • Multiple except blocks for different errors
    • Fallback to default values
    • Proper JSON decoding error handling

  4. Process lines template

    • Nested try/except (file-level and line-level)
    • Graceful degradation (skip bad lines)
    • Error threshold enforcement

  5. Testing section

    • Test cases for each template
    • Both success and failure scenarios

Validation with AI​

Once your template is complete, validate it by asking AI:

"Review my file operation templates. For each function:

  1. Is the exception handling appropriate for the error types?
  2. Are else and finally blocks used correctly?
  3. What edge cases am I missing?
  4. Are there any resource leaks or cleanup issues?
  5. Suggest one improvement for each template."

Deliverable: Complete file_operations_template.py with all 4 templates, comprehensive error handling, and test cases demonstrating both success and failure paths.

Try With AI​

Ready to master control flow in exception handling and build bulletproof file operations?

🔍 Explore Flow Control:

"Show me code with all four blocks (try/except/else/finally) and trace execution for three scenarios: no error, caught error, uncaught error. For each scenario, list which blocks execute in what order. Explain why else is skipped when except runs."

🎯 Practice File Cleanup:

"Help me write a function that reads a file, processes each line, logs success in the else block, and closes the file in finally. Walk me through what happens if: file doesn't exist, file exists but is empty, processing raises unexpected error."

🧪 Test Return Edge Cases:

"Create a function with try/except/else/finally blocks where try has a return statement. Predict what gets returned when: no error occurs, error occurs and except has return, finally has return (bad practice). Explain why finally with return is dangerous."

🚀 Apply to Database Operations:

"I'm building a database query function that must close the connection no matter what. Show me proper exception handling with: try for query execution, except for SQL errors, else for commit, finally for connection cleanup. Include rollback on error."