How do "alternatives" in format strings work?

[JessiAuro]

I'm trying to create format strings that have a fallback value. For example with the Patreon extractor I want the filename to use a file's {hash}, but fall back to {num:>04} if a hash isn't available. From reading the relevant docs, it looks like I want to use alternatives.

As I understand it, I should use something like this:

{
    filename: "{id}_{num:>04|hash}-{type}.{extension}"
}

However, gallery-dl errors out:

[patreon][error] FilenameFormatError: Applying filename format string failed (ValueError: Invalid format specifier)

Am I misunderstanding the alternatives feature or do <format-specifiers> not work with alternatives?

[mikf]

"Alternatives" only work for the data selection portion, i.e. Field Names. The format specifiers have to be shared among all possible fields.

{hash|num:>04} should work here, since zero-filling the hash value until it is 4 characters long is a noop.

[mikf]

No, it will be applied, but it does nothing since hash is already longer than 4 characters.

:>04 works for integers, floats, and strings in Python, possibly more.

>>> "{v:>04}".format(v=1)
'0001'
>>> "{v:>04}".format(v="foobar")
'foobar'
>>> "{v:>04}".format(v="f"*32)
'ffffffffffffffffffffffffffffffff'

[JessiAuro]

Hmmm, I can see that causing problems in some cases.

As an cleaner alternative, I wanted to do something like this:

{
    filename: {
        "hash": "{id}_{hash}-{type}.{extension}",
        ""    : "{id}_{num:>04}-{type}.{extension}"
    }
}

However, I want my output files and metadata to follow the same naming scheme, and it seems that metadata.filename doesn't support an object in the same way extractor.filename does. Is this just an omission in the docs or would it be possible to allow passing objects to metadata.filename as well?

[mikf]

This basically comes back to "all files should be handled the same" (#1956), which is not the case at the moment. Any files created by post processors use their own code paths and don't have the same features as regular downloaded files.

[JessiAuro]

Oof. So the {hash|num:>04} is probably the smartest way to do this with the current codebase? Would it be somewhat easy for me to make a patched version of the patreon module that adds a {hash_or_num} value to the metadata dict and then force gallery-dl to use that for Patreon instead?

[JessiAuro]

As in, copy-paste the patreon module to patreon-custom so I could pull without having to merge.