What is Import Side Effect in Go?

In the realm of Go programming, you may have encountered the concept of "import side effects." This term refers to situations where the mere act of importing a package triggers actions that affect the program's behavior.

Consider the following import statement:

import (
    _ "github.com/lib/pq"
    _ "image/png"
    ...
)

Despite using the underscore prefix (which typically denotes unused imports), these imports actually have side effects. Specifically, they invoke initialization functions that register handlers, modify configuration files, or alter resources on disk.

Import side effects can stem from any code executed during package initialization. The primary one is the init() function. When a package is imported, its init() method is called before the main() function is executed. As a result, any actions performed within the init() function will occur at application startup and impact the program's state.

Additionally, package-scope variables that trigger side effects can also contribute to import side effects. For example, if a package contains a variable with an initializer that involves disk writes, that operation will occur upon importing the package, potentially modifying the system's state.

understanding the concept of import side effects in Go is crucial for effective code organization and reliability, as it allows you to anticipate and manage the potential impact of importing packages on your program's behavior.