Accidental removal of side-effect imports
Go provides the ability to import a package purely for the package’s
side-effects (i.e. you don’t actually use a symbol from the package in your
code, but just want any side-effects introduced by the package’s
As an example, look at
package expvar in the standard library.
Importing the package registers a HTTP handler at “/debug/vars” that prints
the state of published vars, as a side-effect of importing the package.
The importing source file may not even necessarily reference any symbols in
package expvar. However, this means that it’s easy to lose the import during
refactoring or when running
goimports(1) on the file.
Import with an underscore
To avoid losing the import, when you import a package for its side-effects,
import it with an underscore, so that it isn’t removed by
goimports. And add
a comment that makes the side-effect import stand out during code review if
Import separately for each kind of use
If you’re also importing the same package for normal use, import it twice—once
with the underscore and once without. This way, the
_ side-effect import
sticks around even if the regular import is removed.
Reference a symbol
As an additional measure, include a reference to any of the side-effect
package’s symbols in your source file. If the expvar import is accidentally
removed, this line would fail to compile, making you consciously aware that
you’ve accidentally lost the side-effect import.
// NOTE: this exists to guard against accidental removal of
// the expvar import, which is imported for for its
var _ = expvar.Do