如果env var为空，如何分配默认值？

Question

How do you assign a default value if an environment variable isn't set in Go?

In Python I could do mongo_password = os.getenv('MONGO_PASS', 'pass') where pass is the default value if MONGO_PASS env var isn't set.

I tried an if statement based on os.Getenv being empty, but that doesn't seem to work due to the scope of variable assignment within an if statement. And I'm checking for multiple env var's, so I can't act on this information within the if statement.

Answer 1

There's no built-in to fall back to a default value, so you have to do a good old-fashioned if-else.

But you can always create a helper function to make that easier:

func getenv(key, fallback string) string {
    value := os.Getenv(key)
    if len(value) == 0 {
        return fallback
    }
    return value
}

Note that as @michael-hausenblas pointed out in a comment, keep in mind that if the value of the environment variable is really empty, you will get the fallback value instead.

Even better as @ŁukaszWojciechowski pointed out, using os.LookupEnv:

func getEnv(key, fallback string) string {
    if value, ok := os.LookupEnv(key); ok {
        return value
    }
    return fallback
}