Can't pass empty int slices

[gbl08ma]

I have a IntSliceVar flag called groups. I can't seem to be able to pass empty slices in any way:

If I use --groups=[],
Error: invalid argument "[]" for "--groups" flag: strconv.Atoi: parsing "[]": invalid syntax

If I use --groups="",
Error: invalid argument "" for "--groups" flag: strconv.Atoi: parsing "": invalid syntax

Is there another syntax I'm missing?
This is part of a command-line program that uses cobra but I believe the problem to be within pflag (even though the error handling, that is printing the above errors and returning before I can get to my command handler, appears to be within cobra).

[hoshsadiq]

What is the use-case here? Why do you need to pass in an empty integer? That would just be 0 wouldn't it?

[gbl08ma]

It's not an empty integer, it's an empty integer slice.

[hoshsadiq]

Oh right. I misread. I'm still curious what the use-case is. Why do you need to pass in an empty slice? Can you not set empty slice as a default and avoid passing in the flag at all?

[gbl08ma]

The use case is implementing the following interface on a command to edit a resource:

• Absence of flag: the field is not edited at all
• Presence of flag with empty slice: the field is edited to become the empty list
• Presence of flag with non-empty slice: the field is edited to become the passed list

[zerkms]

Happy to deliver a PR with implementation if dev team agrees this feature is welcome :-)

Proposed behaviour: --groups= (assuming it's defined as a IntSliceVar) should result to []int{}.

[tomasaschan]

This issue has not been updated in a long time. Is it still relevant?

(If I receive no answer before the end of the summer, I'll close it. Feel free to re-open if you come here after that happens and it is relevant to you - but if you do, please also comment and specify what outcome you're interested in!)

[zerkms]

It is still relevant - there is still no way to pass an empty slice value.

Eg:

for

cmd.Flags().IntSliceVar(&data, "data", []int{}, "data values to look for")

and --data= passed as command line arguments it returns

Error: invalid argument "" for "--data" flag: strconv.Atoi: parsing "": invalid syntax

error. --data=[] returns Error: invalid argument "[]" for "--data" flag: strconv.Atoi: parsing "[]": invalid syntax.

Version used for repro:

github.com/spf13/pflag v1.0.7 h1:vN6T9TfwStFPFM5XzjsvmzZkLuaLX+HS+0SeFLRgU6M=
github.com/spf13/pflag v1.0.7/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=

[tomasaschan]

I think allowing --data= to represent an empty slice sounds reasonable, since --data= currently results in an error (so the syntax is available, so to speak) and the current syntax for providing a non-empty slice is a list of comma-separated values (so the empty string is a clean extension of the existing functionality). Feel free to send a PR!

I think a few tests that should be present in that PR are

• passing --data= results in the expected (non-nil) empty slice
• not passing the flag results in a preserved default value, regardless of whether that default value is nil, an empty slice, or a non-empty slice

[zerkms]

Feel free to send a PR!

That's what I suggested 2 years ago too :-D #222 (comment)

I'm still keen to do that, but please nobody please take my word on it and if someone needs it quicker and/or has some free time before me - please go ahead and don't wait.

[tomasaschan]

Addendum to my above statement: it seems I was wrong that --data= produces an error - it does when data is an int slice, but not for the general slice case. It parses a raw value of [""] (since splitting the empty string results in a single-element slice with an empty string as the only value, rather than an empty slice) and then subsequently errors trying to strconv.Atoi("") to convert to an int slice.

I think a change to pflag for the semantics of passing empty slices needs to take into account that there are many value types, and ensure the same syntax behaves the same way for int slices, string slices, and any other type of slice.