Cannot instantiate pl.lit with a Decimal value

[alexander-beedie]

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

from decimal import Decimal as D
import polars as pl

with pl.Config( activate_decimals=True ):
    decimal_zero = pl.lit( D(0) )

Log output

Traceback (most recent call last):
  File "<ipython-input-13-ebe78b664c56>", line 1, in <module>
    pl.lit(D(0))
  File "/.../polars/py-polars/polars/functions/lit.py", line 150, in lit
    return wrap_expr(plr.lit(item, allow_object))
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

TypeError: invalid literal value: '0'

Issue description

We cannot instantiate a decimal literal, irrespective of whether activate_decimals is True/False.

Expected behavior

We can create decimal Series; should also be able to create literals, so we can filter/operate on those Series, eg:

df = pl.DataFrame({
    "dec": [
        D("1.6666666666666666666"), 
        D("2.123456789"),
    ],
})

df.filter( dec=D("1.6666666666666666666") )

This filter should succeed; instead we get...

# TypeError: invalid literal value: '1.6666666666666666666'

Note that if we try and filter with an equivalent float it will fail to match, as the target decimal value cannot be exactly represented (the closest float is given as "1.66666666666666674068...")

Installed versions

--------Version info---------
Polars:               0.20.3-rc.2
Index type:           UInt32
Platform:             macOS-14.2.1-arm64-arm-64bit
Python:               3.11.6 (main, Oct  2 2023, 13:45:54) [Clang 15.0.0 (clang-1500.0.40.1)]
----Optional dependencies----
adbc_driver_manager:  0.8.0
cloudpickle:          3.0.0
connectorx:           0.3.2
deltalake:            0.14.0
fsspec:               2023.12.2
gevent:               23.9.1
matplotlib:           3.8.2
numpy:                1.26.2
openpyxl:             3.1.2
pandas:               2.1.4
pyarrow:              14.0.2
pydantic:             2.5.3
pyiceberg:            0.5.1
pyxlsb:               1.0.10
sqlalchemy:           2.0.24
xlsx2csv:             0.8.1
xlsxwriter:           3.1.9

[deanm0000]

The filter fails even if I add that as a column before the filter

### this works (as you say)
df.with_columns(zz=pl.Series([D("1.6666666666666")]))
### this fails
df.with_columns(zz=pl.Series([D("1.6666666666666")])).filter(pl.col('dec')>pl.col('zz'))
PanicException: not implemented

[cgevans]

There are two problems here:

• pl.lit doesn't support Python Decimals (this seems relatively easy to fix, and I have something that seems to work).
• Equality and comparisons aren't implemented for (Polars) Decimals (Implement equality operation for Decimal types #12118).