### What changes were proposed in this pull request?
This PR proposes to redesign pandas UDFs as described in [the proposal](https://docs.google.com/document/d/1-kV0FS_LF2zvaRh_GhkV32Uqksm_Sq8SvnBBmRyxm30/edit?usp=sharing).
```python
from pyspark.sql.functions import pandas_udf
import pandas as pd
pandas_udf("long")
def plug_one(s: pd.Series) -> pd.Series:
return s + 1
spark.range(10).select(plug_one("id")).show()
```
```
+------------+
|plug_one(id)|
+------------+
| 1|
| 2|
| 3|
| 4|
| 5|
| 6|
| 7|
| 8|
| 9|
| 10|
+------------+
```
Note that, this PR address one of the future improvements described [here](https://docs.google.com/document/d/1-kV0FS_LF2zvaRh_GhkV32Uqksm_Sq8SvnBBmRyxm30/edit#heading=h.h3ncjpk6ujqu), "A couple of less-intuitive pandas UDF types" (by zero323) together.
In short,
- Adds new way with type hints as an alternative and experimental way.
```python
pandas_udf(schema='...')
def func(c1: Series, c2: Series) -> DataFrame:
pass
```
- Replace and/or add an alias for three types below from UDF, and make them as separate standalone APIs. So, `pandas_udf` is now consistent with regular `udf`s and other expressions.
`df.mapInPandas(udf)` -replace-> `df.mapInPandas(f, schema)`
`df.groupby.apply(udf)` -alias-> `df.groupby.applyInPandas(f, schema)`
`df.groupby.cogroup.apply(udf)` -replace-> `df.groupby.cogroup.applyInPandas(f, schema)`
*`df.groupby.apply` was added from 2.3 while the other were added in the master only.
- No deprecation for the existing ways for now.
```python
pandas_udf(schema='...', functionType=PandasUDFType.SCALAR)
def func(c1, c2):
pass
```
If users are happy with this, I plan to deprecate the existing way and declare using type hints is not experimental anymore.
One design goal in this PR was that, avoid touching the internal (since we didn't deprecate the old ways for now), but supports type hints with a minimised changes only at the interface.
- Once we deprecate or remove the old ways, I think it requires another refactoring for the internal in the future. At the very least, we should rename internal pandas evaluation types.
- If users find this experimental type hints isn't quite helpful, we should simply revert the changes at the interface level.
### Why are the changes needed?
In order to address old design issues. Please see [the proposal](https://docs.google.com/document/d/1-kV0FS_LF2zvaRh_GhkV32Uqksm_Sq8SvnBBmRyxm30/edit?usp=sharing).
### Does this PR introduce any user-facing change?
For behaviour changes, No.
It adds new ways to use pandas UDFs by using type hints. See below.
**SCALAR**:
```python
pandas_udf(schema='...')
def func(c1: Series, c2: DataFrame) -> Series:
pass # DataFrame represents a struct column
```
**SCALAR_ITER**:
```python
pandas_udf(schema='...')
def func(iter: Iterator[Tuple[Series, DataFrame, ...]]) -> Iterator[Series]:
pass # Same as SCALAR but wrapped by Iterator
```
**GROUPED_AGG**:
```python
pandas_udf(schema='...')
def func(c1: Series, c2: DataFrame) -> int:
pass # DataFrame represents a struct column
```
**GROUPED_MAP**:
This was added in Spark 2.3 as of SPARK-20396. As described above, it keeps the existing behaviour. Additionally, we now have a new alias `groupby.applyInPandas` for `groupby.apply`. See the example below:
```python
def func(pdf):
return pdf
df.groupby("...").applyInPandas(func, schema=df.schema)
```
**MAP_ITER**: this is not a pandas UDF anymore
This was added in Spark 3.0 as of SPARK-28198; and this PR replaces the usages. See the example below:
```python
def func(iter):
for df in iter:
yield df
df.mapInPandas(func, df.schema)
```
**COGROUPED_MAP**: this is not a pandas UDF anymore
This was added in Spark 3.0 as of SPARK-27463; and this PR replaces the usages. See the example below:
```python
def asof_join(left, right):
return pd.merge_asof(left, right, on="...", by="...")
df1.groupby("...").cogroup(df2.groupby("...")).applyInPandas(asof_join, schema="...")
```
### How was this patch tested?
Unittests added and tested against Python 2.7, 3.6 and 3.7.
Closes#27165 from HyukjinKwon/revisit-pandas.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
ab0890bdb1