Panic with multiple outputs in then()

[2-5]

Polars version checks

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• I have confirmed this bug exists on the latest version of Polars.

Issue description

then() typing seems to suggest that it supports multiple output expressions, but it panics if you try that.

def then(
    self,
    expr: (
        PolarsExprType
        | PythonLiteral
        | pli.Series
        | Iterable[PolarsExprType | PythonLiteral | pli.Series]
        | None
    ),
) -> WhenThen:

Reproducible example

import os

import polars as pl

os.environ["RUST_BACKTRACE"] = "full"

df = pl.DataFrame({
    "a": list(range(10)),
})
print(df)

df2 = df.with_columns([
    pl.when(pl.col("a") > 5).then([
        pl.lit(100).alias("b"),
        pl.lit(200).alias("c"),
    ]),
])
print(df2)

Outputs:

thread '<unnamed>' panicked at 'not implemented for dtype Object("object")', D:\a\polars\polars\polars\polars-core\src\series\ops\null.rs:59:17
stack backtrace:
   0:     0x7ff871a82310 - ffi_select_with_compiled_path
   1:     0x7ff8702f359b - BrotliDecoderVersion
   2:     0x7ff871a703ca - ffi_select_with_compiled_path
   3:     0x7ff871a83dbb - ffi_select_with_compiled_path
   4:     0x7ff871a839c8 - ffi_select_with_compiled_path
   5:     0x7ff871a84930 - ffi_select_with_compiled_path
   6:     0x7ff871a84386 - ffi_select_with_compiled_path
   7:     0x7ff871a842cf - ffi_select_with_compiled_path
   8:     0x7ff871a842a4 - ffi_select_with_compiled_path
   9:     0x7ff871b3e555 - ffi_select_with_compiled_path
  10:     0x7ff87049c51e - ffi_select_with_compiled_path
  11:     0x7ff8705085a2 - ffi_select_with_compiled_path
  12:     0x7ff8704ee4d4 - ffi_select_with_compiled_path
  13:     0x7ff8713170bd - ffi_select_with_compiled_path
  14:     0x7ff871369011 - ffi_select_with_compiled_path
  15:     0x7ff871bea8ca - ffi_select_with_compiled_path
  16:     0x7ff871987fa4 - ffi_select_with_compiled_path
  17:     0x7ff8719878ac - ffi_select_with_compiled_path
  18:     0x7ff871a8142c - ffi_select_with_compiled_path
  19:     0x7ff913bc7614 - BaseThreadInitThunk
  20:     0x7ff914aa26a1 - RtlUserThreadStart
Traceback (most recent call last):
  File "test.py", line 12, in <module>
    df2 = df.with_columns([
  File ".venv\lib\site-packages\polars\internals\dataframe\frame.py", line 6022, in with_columns
    self.lazy()
  File ".venv\lib\site-packages\polars\internals\lazyframe\frame.py", line 1148, in collect
    return pli.wrap_df(ldf.collect())
pyo3_runtime.PanicException: not implemented for dtype Object("object")

Expected behavior

Works? Or documented as not supporting multiple outputs.

Installed versions

---Version info---
Polars: 0.16.7
Index type: UInt32
Platform: Windows-10-10.0.19045-SP0
Python: 3.10.8 (tags/v3.10.8:aaaf517, Oct 11 2022, 16:50:30) [MSC v.1933 64 bit (AMD64)]
---Optional dependencies---
pyarrow: 11.0.0
pandas: 1.5.3
numpy: 1.23.5
fsspec: 2023.1.0
connectorx: <not installed>
xlsx2csv: <not installed>
deltalake: <not installed>
matplotlib: 3.7.0

[universalmind303]

This still seems relevant as of 0.19.19. The type hints don't explicitly state that iterables are accepted, but as mentioned in #7040, we should allow them.

[stinodego]

The type signature has been updated since. But this still panics when it shouldn't.