flag package in Go - do I have to always set default value?
Solution 1
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
.
Solution 2
When I have a flag that cannot have a default value I often use the value REQUIRED
or something similar. I find this makes the --help
easier to read.
As for why it wasn't baked in, I suspect it just wasn't considered important enough. The default wouldn't fit every need. However, the --help
flag is similar; it doesn't fit every need, but it's good enough most of the time.
That's not to say the required flags are a bad idea. If you're passionate enough a flagutil
package could be nice. Wrap the current flag
api, make Parse
return an error that describes the missing flag and add a RequiredInt
and RequiredIntVar
etc. If it turns out to be useful / popular it could be merged with the official flag
package.
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Blaszard
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Updated on June 03, 2022Comments
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Blaszard about 2 years
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 doingflag.Parse()
, and if that value istest.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.
-
Kissaki almost 11 yearsPlease only ask one question per question.
-
-
Blaszard almost 11 yearsThanks for the
flagutil
package. I'll give it a shot. -
Kiril about 10 yearsDoes the
flag
package provide a way to check whether a flag is set? I'm not talking about default values, or no values. I mean, whether a flag is present in the command line. -
Rewanth Tammana almost 5 years@Kiril, you can use the Bool option. If its value is true, then the flag is set. Example