Lesson 11 of 20

14 mins read

Python Explicit Error Handling, Wrapping & Inspection

In Plain English

Go treats errors as regular values rather than hidden exceptions. Master returning (result, error), wrapping errors with fmt.Errorf("%w"), and inspecting error chains with errors.Is and errors.As.

The Mental Model

"Instead of an unexpected building fire alarm (exceptions), every room inspection produces a signed report card. If a defect is found, the inspector attaches a note and hands it up."

Deep Dive: How It Works

Errors as Values: error is a standard built-in interface with Error() string.

Explicit Handling: Errors are returned as the final return value and handled immediately with if err != nil.

Error Wrapping with %w: fmt.Errorf("ctx: %w", err) wraps errors so errors.Is and errors.As can unwrap them.

Syntax Blueprint

var ErrNotFound = errors.New("not found")
wrapped := fmt.Errorf("query failed: %w", ErrNotFound)
if errors.Is(wrapped, ErrNotFound) { ... }

Create sentinel errors, wrap with %w context, and inspect using errors.Is.

Core Rules to Remember

Explicit Values: No hidden exceptions; control flow is completely transparent and explicit.

Error Wrapping: Use fmt.Errorf with %w to maintain the full chain of error provenance.

Sentinel Inspection: Use errors.Is to inspect sentinel errors across nested wrapper layers.

Common Beginner Traps & How to Fix Them

Mistake:Using %v instead of %w when formatting errors with fmt.Errorf.

Why it happens: %v converts error to string, discarding the underlying error type chain needed for errors.Is.

How to fix: Always use %w when wrapping underlying errors.

Pro Tip: Define package-level sentinel errors (var ErrNotFound = errors.New(...)) so callers can test against them with errors.Is.

Live Interactive Example

Hit Run Code to see it live

Error Wrapping and Sentinel Inspection

Go 1.22 / Runtime

Output Console

Click "Run Code" to view the rendered output.

How it works: errors.Is walked the wrapped %w chain and matched ErrDatabaseOffline.

Your Turn: Micro Challenge

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

Micro Exercise

Safe Division with Custom Error

Define a sentinel error var ErrDivideByZero = errors.New("cannot divide by zero").
Write a function divide(a, b float64) (float64, error) that returns ErrDivideByZero if b == 0.
In main(), call divide(10, 0), check if errors.Is(err, ErrDivideByZero), and print "Caught: divide by zero".

Sandbox Output

Click "Run & Check" to test your solution.

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