.implode().list method is not valid before .over.

[PierXuY]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl

df = pl.DataFrame({
    'type': ['water','fire','water','earth']
})


out1 = df.select(
    pl.col('type').implode().over("type", mapping_strategy="explode").list.first()
)
print(out1)

# shape: (3, 1)
# ┌───────┐
# │ type  │
# │ ---   │
# │ str   │
# ╞═══════╡
# │ water │
# │ fire  │
# │ earth │
# └───────┘

# The code above works, but the expression below that I think is equivalent throws an error.

out2 = df.select(
    pl.col('type').implode().list.first().over("type", mapping_strategy="explode")
)
print(out2)
# InvalidOperationError: 'implode' followed by an aggregation is not allowed

Log output

No response

Issue description

The example is from another issue, and according to the issue, this expression was legal in earlier versions of polars.

In addition, the error reported in the current polars version is also confusing.
In code that works fine:

out1 = df.select(
    pl.col('type').implode().over("type", mapping_strategy="explode").list.first()
)
print(out1)

'implode' followed by an aggregation is allowed.

But another code pl.col('type').implode().list.first().over("type", mapping_strategy="explode") reports an error: InvalidOperationError: 'implode' followed by an aggregation is not allowed.

Expected behavior

I think both expressions should be able to execute normally. If the error code is unreasonable, there should be a more accurate error message.

Installed versions

--------Version info---------
Polars:               1.5.0
Index type:           UInt32
Platform:             Windows-10-10.0.22631-SP0
Python:               3.10.11 (tags/v3.10.11:7d4cc5a, Apr  5 2023, 00:38:17) [MSC v.1929 64 bit (AMD64)]

----Optional dependencies----
adbc_driver_manager:  1.1.0
cloudpickle:          3.0.0
connectorx:           0.3.3
deltalake:            0.19.0
fastexcel:            0.11.5
fsspec:               2024.6.1
gevent:               24.2.1
great_tables:         0.10.0
hvplot:               0.10.0
matplotlib:           3.9.2
nest_asyncio:         1.6.0
numpy:                2.0.1
openpyxl:             3.1.5
pandas:               2.2.2
pyarrow:              17.0.0
pydantic:             2.8.2
pyiceberg:            0.7.0
sqlalchemy:           2.0.32
torch:                <not installed>
xlsx2csv:             0.8.3
xlsxwriter:           3.2.0

[ritchie46]

It is not a bug, we don't allow this operation, hence the error.

[PierXuY]

It is not a bug, we don't allow this operation, hence the error.

It seems that #18228 is related to this reason.

[ritchie46]

Yes, that one is the samse story. Not allowed for now. There is complexity in that, that I don't want to get into for now.

[ritchie46]

I will close this as this is not a bug.