Variable-shadowing

Variable shadowing is re-declaring a variable in an inner block and can result in unexpected outcomes.

Examples

Problem

In this case, the client looks as if it is being assigned, but it is just the shadow variable that is being set.

var client *http.Client
if tracing {
    // Shadowed client
	client, err := NewTracingClient()
	if err != nil {
	    return err
	}
} else {
    // Shadowed client
	client, err := NewClient()
	if err != nil {
	    return err
	}
}
// Client is still nil at this point

Solutions

In this case, a temporary variable is being declared and then is assigned to the client

var client *http.Client
if tracing {
	c, err := NewTracingClient()
	if err != nil {
	    return err
	}
    client = c
} else {
	c, err := NewClient()
	if err != nil {
	    return err
	}
    client = c
}

It also makes sense to avoid declaring the error inside the conditional scope.

var client *http.Client
var err error
if tracing {
	c, err = NewTracingClient()
} else {
	c, err = NewClient()
}

if err != nil {
    return err
}

References

100 Go Mistakes