Partially unknown type for some Chart methods.

[alec-bike]

What happened?

I'm getting type checking (basedpyright) reportUnknownMemberType warnings for some Chart methods, such as mark_* and save. Others, like encode, are fine. Does not affect functionality, just trips up the type-checker.

Example:

from altair import Chart

Chart(df.unpivot(on=matches(regex), index=idx, value_name=ylab))
        .mark_line()
        .encode(x=idx, y=ylab, color="variable")

basedpyright output:

Type of "mark_line" is partially unknown
  Type of "mark_line" is "(align: Parameter | SchemaBase
          ...
          **kwds: Unknown) -> Chart" (basedpyright reportUnknownMemberType)

What would you like to happen instead?

No warnings.

Which version of Altair are you using?

6.0.0

[mattijn]

Do you know what we can do to fix this?

[alec-bike]

I used the basedpyright lsp to examine some altair functions that are triggering these warnings.

For Chart.save in api.py:

fp: str | Path | IO,
                 ^^ missing type arguments
embed_options: dict | None = None,
               ^^^^ missing type arguments
json_kwds: dict | None = None,
           ^^^^ missing type arguments

For Chart.mark_line in mixins.py¹:

copy = self.copy(deep=False)  # type: ignore[attr-defined]
            ^^^^ unknown type

Running basedpyright or ty on api.py reveals more type warnings and errors. None of these have caused any technical issues, they just get flagged by the type checkers.

Footnotes

1. The mixins.py file is excluded from pyright (also this line ignores type errors) but the basedpyright lsp still reports a warning. ↩

[alec-bike]

Looking at pyproject.toml, I see that type-checkers (pyright and mypy) are already configured¹.

I am using basedpyright and I see multiple issues in api.py... a similar number using ty. When I use pyright I see no issues in Chart functions.

Do you know what we can do to fix this?

If you are fine with the existing type-checkers... not much to do. However, if you wanted to avoid downstream type-checking issues, you could run ty or basedpyright in altair/vegalite/v6.

For example:

> cd altair/altair/vegalite/v6
> uv tool run basedpyright api.py
...
31 errors, 1005 warnings, 0 notes

Footnotes

1. The pyright options ignore v6/schema/mixins.py (presumably because this python file is auto-generated by uv run task generate-schema-wrapper). Fixing Chart.mark_line in mixins.py may require a different approach to Chart.save in v6/api.py. ↩

[mattijn]

I'm not a type expert, but we are welcoming PRs that improve the situation for Altair if you are much into types. Especially with new type tooling like ty and basedpyright, I'm sure we can improve!