fillna does not work with decimals

[StevenLaan]

Description

The fillna function does not support the decimal type. If you have a column of DecimalType, which gets converted to decimal.Decimal (and is shown as object type by koalas), the straightforward way of using fillna will give you an error TypeError: Unsupported type Decimal

Example code:

df = pd.DataFrame({"test": [1.23, 2.34, None, 4.56, 5.67]})
sdf = spark.createDataFrame(df).select(f.col("test").cast("decimal(4,2)"))
kdf = sdf.to_koalas()

m = kdf["test"].mean()  # -> Decimal('3.450000')

kdf["test"].fillna(m)  # -> TypeError

Workaround

Currently you can work around this by filling with a float or a double and then recast the column to the decimaltype. This is however cumbersome and you cannot reuse the same code for different types, because you need to explicitly check if you're dealing with decimal types to prevent the error.

kdf["test"] = kdf["test"].fillna(float(m)).astype(decimal.Decimal)  # No TypeError

Suggested solution

It seems there are hardcoded type checks in the _fillna implementation (here and here). Add the decimal.Decimal type to that check to prevent the error. I'm not qualified to assess whether this solution is appropriate, hence this issue ticket :)

[HyukjinKwon]

Thanks for reporting this. Koalas has been migrated to Apache Spark. Would you mind reporting the issue to https://issues.apache.org/jira/projects/SPARK/issues please?