[Bug]: Migration of pl.when chaining expressions

[jesusestevez]

Describe the bug

It seems that when trying to chain multiple expressions, Narwhals is not detecting the second one and is raising an error.

This becomes clearer when trying to chain multiple when expressions.

Steps or code to reproduce the bug

pl_df = pl.DataFrame({"foo": [1, 3, 4]})

df = nw.from_native(pl_df)
df.with_columns(
    nw.when(pl.col("foo") > 2)
    .then(1)
    .when(pl.col("foo") == 1)
    .then(0)
    .otherwise(-1)
    .alias("val")
)

>>> AttributeError: 'Then' object has no attribute 'when'

Expected results

Multiple when expressions can be chained, similar as polars:

pl_df = pl.DataFrame({"foo": [1, 3, 4]})

df = nw.from_native(pl_df)
pl_df.with_columns(
    pl.when(pl.col("foo") > 2)
    .then(1)
    .when(pl.col("foo") == 1)
    .then(0)
    .otherwise(-1)
    .alias("val")
)

shape: (3, 2)
┌─────┬─────┐
│ foo ┆ val │
│ --- ┆ --- │
│ i64 ┆ i32 │
╞═════╪═════╡
│ 1   ┆ 0   │
│ 3   ┆ 1   │
│ 4   ┆ 1   │
└─────┴─────┘

Actual results

AttributeError: 'Then' object has no attribute 'when'

Please run narwhals.show_version() and enter the output below.

System:
    python: 3.12.3 | packaged by conda-forge | (main, Apr 15 2024, 18:20:11) [MSC v.1938 64 bit (AMD64)]
executable: c:\Mambaforge\envs\myenv\python.exe
   machine: Windows-11-10.0.22621-SP0

Python dependencies:
     narwhals: 1.26.0
       pandas: 2.2.2
       polars: 1.22.0
         cudf: 
        modin: 
      pyarrow: 16.1.0
        numpy: 1.26.4

Relevant log output

[FBruzzesi]

Hey @jesusestevez thanks for reaching out.

We are aware of this issue - it's tracked in #668 and addressed in #669 (although we don't know when that's going to be merged).

We should improve documentation of when until such improvement happens.

It seems that when trying to chain multiple expressions, Narwhals is not detecting the second one and is raising an error.

This is not generally true for expressions, it applies only for chained when statements. You should expect any other expression to be chain-able. If not, please report it as a bug.

---

I am going to close the issue as duplicate for now 😇