Lesson 12 of 59

5 mins read

Python Error Handling: try, catch, on, finally & assert

In Plain English

Robust Dart applications handle runtime failures using `try`, `on ExceptionType`, `catch (e, stackTrace)`, and `finally`. Dart distinguishes between **Exceptions** (intended to be caught and handled, like `FormatException` or `SocketException`) and **Errors** (program bugs like `RangeError`). `assert()` statements enforce invariants in development mode with zero production overhead.

The Mental Model

"`try/catch` is like a trapeze safety net: you attempt a dangerous maneuver, and if you slip, the safety net catches you cleanly without halting the entire circus."

Deep Dive: How It Works

`on SpecificType`: Catches only designated exception subtypes without catching unrelated errors.

`catch (e, s)`: Binds the exception object `e` and the detailed execution `StackTrace s`.

`finally`: Block guaranteed to execute regardless of whether an exception occurred (ideal for closing streams/files).

`assert(expr, "message")`: Active only in debug mode; stripped out completely in AOT release builds.

Syntax Blueprint

try {
  var num = int.parse("invalid");
} on FormatException catch (e) {
  print("Invalid number: $e");
} catch (e, stackTrace) {
  print("Unknown error: $e");
} finally {
  print("Cleanup completed");
}

Typed exception handling with fallback and guaranteed finalizer.

Core Rules to Remember

Targeted on Catching: Catch specific error types with `on` rather than catching generic `Object`.

StackTrace Inspection: Pass second argument to `catch(e, s)` to capture stack traces for logging.

Zero Release Cost Asserts: `assert()` runs during development and is stripped in release compilation.

Common Beginner Traps & How to Fix Them

Mistake:Catching `dynamic` and swallowing all errors silently without logging.

Why it happens: Hides critical programming bugs (like `NoSuchMethodError` or `RangeError`).

How to fix: Catch specific exceptions with `on` and rethrow unexpected errors with `rethrow;`.

Pro Tip: Use `rethrow;` inside a catch block to preserve the original stack trace when logging and propagating exceptions.

Live Interactive Example

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Typed Exception Handling with on, catch, and finally

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: The FormatException was caught specifically by the on clause, followed by the guaranteed finally block.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Catch FormatException Safely

Wrap `int.parse("not_a_number")` in a `try` block.
Catch `on FormatException` and print `Caught FormatException`.
Add a `finally` block that prints `Done`.

Sandbox Output

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