Better error messages for parsing errors

[ck-schmidi]

We still have an old open issue for our CLI tool which is kind of a cosmetic thing, but it's still annoying and we would really like to have it fixed somehow.

If the parsing of a flag fails this error message gets created:

invalid argument "test" for "-w, --warning" flag: strconv.ParseInt: parsing "test": invalid syntax

For us, exposing internal function names to our users is not really an option.

There was also a discussion on the same topic for the golang stdlibrary. And they fixed the error messages

They went from:
invalid boolean value "3" for -debug: strconv.ParseBool: parsing "3": invalid syntax

to
invalid boolean value "3" for -debug: parse error

which doesn't expose any technical details to the users.

It would be nice if we could a similar way for the pflag package.

According to my example one proposal would be:
invalid argument "test" for "-w, --warning" flag: parse error

or even better (type in the front part of the message):
invalid integer value "test" for "-w, --warning" flag: parse error

Another approach could be providing a format function, so users of the pflag library can inject a format function for a customized formatting of parsing errors.

I checked all old issues and didn't find any similar problems. It's just a proposal and I would spend time providing a patch for it, but maybe some of you have thoughts on it.

[Emyrk]

@ck-schmidi I also would like this, but instead of imposing a formatted string, I would prefer a sentinel error I can check and format myself.

type FlagParseError struct {
  // The flag being parsed and all it's context.
  Flag *Flag
  // The error thrown by the parse/set function
  Err error
  // The input value
  Value string
}

Then the caller can check if errors.As() and do w/e they want.

[reuvenharrison]

I just bumped into this through https://github.com/Tufin/oasdiff which uses https://github.com/spf13/cobra.

Running:

oasdiff changelog data/common-params/params_in_path.yaml data/common-params/params_in_op.yaml --flatten-params=x

Returns:

Error: invalid argument "x" for "--flatten-params" flag: strconv.ParseBool: parsing "x": invalid syntax

For more details see Tufin/oasdiff#482.

[tomqwpl]

I second this. spf13 shouldn't be returning internal programmer targeted errors ("strconv.ParseBool..."). It should return a simpler end user targeted error.