Does Go have "if x in" construct similar to Python?

There is no built-in operator to do it in Go. You need to iterate over the array. You can write your own function to do it, like this:

func stringInSlice(a string, list []string) bool {
    for _, b := range list {
        if b == a {
            return true
        }
    }
    return false
}

If you want to be able to check for membership without iterating over the whole list, you need to use a map instead of an array or slice, like this:

visitedURL := map[string]bool {
    "http://www.google.com": true,
    "https://paypal.com": true,
}
if visitedURL[thisSite] {
    fmt.Println("Already been here.")
}

Another solution if the list contains static values.

eg: checking for a valid value from a list of valid values:

func IsValidCategory(category string) bool {
    switch category {
    case
        "auto",
        "news",
        "sport",
        "music":
        return true
    }
    return false
}