Answer

When a Go application starts, all non-test packages are initialized in a strict order: first, all imported dependencies are initialized (in chain), then the package itself is initialized. The following are used for initialization:

• package-level variables (initialized top to bottom in the file)
• init() functions (there can be multiple per package)

The init functions are called after the package variables are initialized. If a package is imported, its init is executed once before the variables/functions of that package are used.

Example:

var a = getA()
func getA() int {
    fmt.Println("getA called")
    return 42
}

func init() {
    fmt.Println("init called")
}

// Output:
// getA called
// init called

Features:

• The order of initialization can become critical if variables from one package depend on variables from another imported package.
• Multiple init() functions in one file or package are executed in the order they appear in the source code.
• The main package is initialized last.

Trick Question

Can you explicitly call the init function of any package?

Answer: No, the init function is never called directly from the program code. It is called only by the runtime during the package initialization process at a predetermined moment, explicit calls to init() are not allowed — this will result in a compilation error.

Examples of real errors due to ignorance of nuances

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Story

In the project, two packages imported each other to initialize global variables via init(). This led to cyclic dependencies and build errors — Go could not determine the correct order of initialization.

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Story

A complex bug was caused by init functions using package global variables that had not yet been initialized. The result: ambiguous behavior at runtime with unpredictable states.

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Story

In the project, a package was imported for side effects without explicit use (via import _ "some/lib"). After code reorganization, the init function of the required package was called too early (before the dependent variables in another package were initialized), leading to a hard-to-debug bug.