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gocommand-line-argumentsflags

flag package in Go - do I have to always set default value?

Is it possible not to set default value in flag package in Go? For example, in flag package you can write out the following line:

filename := flag.String("file", "test.csv", "Filename to cope with")

In the above code, I don't want to necessarily set default value, which is test.csv in this case, and instead always make users specify their own filename, and if it's not specified then I want to cause an error and exit the program.

One of the way I came up with is that I first check the value of filename after doing flag.Parse(), and if that value is test.csv then I have the program exits with the appropriate error message. However, I don't want to write such redundant code if it can be evaded - and even if it can't, I'd like to hear any better way to cope with the issue here.

You can do those kind of operations in Python's argparse module by the way - I just want to implement the similar thing if I can...

Also, can I implement both short and long arguments (in other words both -f and -file argument?) in flag package?

Thanks.

Solution

  • I think it's idiomatic to design your flag values in such a way which implies "not present" when equal to the zero value of their respective types. For example:

    optFile := flag.String("file", "", "Source file")
flag.Parse()
fn := *optFile
if fn == "" {
        fn = "/dev/stdin"
}
f, err := os.Open(fn)
...

    Ad the 2nd question: IINM, the flag package by design doesn't distinguish between -flag and --flag. IOW, you can have both -f and --file in your flag set and write any version of - or -- before both f and file. However, considering another defined flag -g, the flag package will not recognize -gf foo as being equvalent of -g -f foo.