Lesson 37 of 59

5 mins read

Python Records: Anonymous Composite Tuples in Dart 3

In Plain English

Introduced in Dart 3, **Records** (`(TypeA, {TypeB name})`) are anonymous, aggregate, immutable tuple values. Records allow functions to return multiple values type-safely without having to create throwaway DTO classes. Records support positional fields (`$1`, `$2`) and named fields (`rec.name`).

The Mental Model

"A Record is like a partitioned bento box: you can hand someone a single box containing rice in compartment $1, salad in compartment $2, and salmon in the labeled "mainDish" compartment."

Deep Dive: How It Works

Anonymous & Strongly Typed: `(String, int)` is a concrete type with value equality out of the box.

Field Access: Positional fields are accessed via `$1`, `$2`, etc.; named fields are accessed by name (`record.fieldName`).

Value Equality: Two records with identical field values evaluate as equal (`==`) automatically.

Syntax Blueprint

// Return multiple values
(double lat, double lng) getCoordinates() {
  return (37.7749, -122.4194);
}

// Usage with destructuring
final (lat, lng) = getCoordinates();

Record return type and pattern destructuring.

Core Rules to Remember

Multiple Return Values: Eliminates boilerplate throwaway wrapper classes.

Built-In Value Equality: Records automatically implement `==` and `hashCode` based on contents.

Positional and Named: Mix positional elements and labeled named fields seamlessly.

Common Beginner Traps & How to Fix Them

Mistake:Single-element positional record syntax `(42)` instead of `(42,)`.

Why it happens: `(42)` is parsed as parenthesized integer expression 42.

How to fix: Include a trailing comma `(42,)` for single-element positional records.

Pro Tip: Combine records with pattern destructuring `final (name, :age) = getUser();` for clean JSON parsing.

Live Interactive Example

Hit Run Code to see it live

Returning and Destructuring Records

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: Returned a record with positional and named fields, then destructured fields cleanly.

Your Turn: Micro Challenge

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

Micro Exercise

Return Min and Max as a Record

Define `(int min, int max) getBounds(List<int> list) => (list.reduce((a, b) => a < b ? a : b), list.reduce((a, b) => a > b ? a : b));`.
In `main()`, call `getBounds([10, 5, 20, 15])` and print `Min: ${b.$1}, Max: ${b.$2}`.

Sandbox Output

Click "Run & Check" to test your solution.

Finished reading and practicing?

Mark this lesson as completed to update your course progress.