## What changes were proposed in this pull request?
This is a followup of https://github.com/apache/spark/pull/23882 to handle binary math/string functions. For instance, see the cases below:
**Before:**
```python
>>> from pyspark.sql.functions import lit, ascii
>>> spark.range(1).select(lit('a').alias("value")).select(ascii("value"))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/.../spark/python/pyspark/sql/functions.py", line 51, in _
jc = getattr(sc._jvm.functions, name)(col._jc if isinstance(col, Column) else col)
File "/.../spark/python/lib/py4j-0.10.8.1-src.zip/py4j/java_gateway.py", line 1286, in __call__
File "/.../spark/python/pyspark/sql/utils.py", line 63, in deco
return f(*a, **kw)
File "/.../spark/python/lib/py4j-0.10.8.1-src.zip/py4j/protocol.py", line 332, in get_return_value
py4j.protocol.Py4JError: An error occurred while calling z:org.apache.spark.sql.functions.ascii. Trace:
py4j.Py4JException: Method ascii([class java.lang.String]) does not exist
at py4j.reflection.ReflectionEngine.getMethod(ReflectionEngine.java:318)
at py4j.reflection.ReflectionEngine.getMethod(ReflectionEngine.java:339)
at py4j.Gateway.invoke(Gateway.java:276)
at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:132)
at py4j.commands.CallCommand.execute(CallCommand.java:79)
at py4j.GatewayConnection.run(GatewayConnection.java:238)
at java.lang.Thread.run(Thread.java:748)
```
```python
>>> from pyspark.sql.functions import atan2
>>> spark.range(1).select(atan2("id", "id"))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/.../spark/python/pyspark/sql/functions.py", line 78, in _
jc = getattr(sc._jvm.functions, name)(col1._jc if isinstance(col1, Column) else float(col1),
ValueError: could not convert string to float: id
```
**After:**
```python
>>> from pyspark.sql.functions import lit, ascii
>>> spark.range(1).select(lit('a').alias("value")).select(ascii("value"))
DataFrame[ascii(value): int]
```
```python
>>> from pyspark.sql.functions import atan2
>>> spark.range(1).select(atan2("id", "id"))
DataFrame[ATAN2(id, id): double]
```
Note that,
- This PR causes a slight behaviour changes for math functions. For instance, numbers as strings (e.g., `"1"`) were supported as arguments of binary math functions before. After this PR, it recognises it as column names.
- I also intentionally didn't document this behaviour changes since we're going ahead for Spark 3.0 and I don't think numbers as strings make much sense in math functions.
- There is another exception `when`, which takes string as literal values as below. This PR doeesn't fix this ambiguity.
```python
>>> spark.range(1).select(when(lit(True), col("id"))).show()
```
```
+--------------------------+
|CASE WHEN true THEN id END|
+--------------------------+
| 0|
+--------------------------+
```
```python
>>> spark.range(1).select(when(lit(True), "id")).show()
```
```
+--------------------------+
|CASE WHEN true THEN id END|
+--------------------------+
| id|
+--------------------------+
```
This PR also fixes as below:
https://github.com/apache/spark/pull/23882 fixed it to:
- Rename `_create_function` to `_create_name_function`
- Define new `_create_function` to take strings as column names.
This PR, I proposes to:
- Revert `_create_name_function` name to `_create_function`.
- Define new `_create_function_over_column` to take strings as column names.
## How was this patch tested?
Some unit tests were added for binary math / string functions.
Closes#24121 from HyukjinKwon/SPARK-26979.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
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