Bug: Change emptyValueInRead's effect to that any fields matching this string will be set as "" when reading csv files
Status: Resolved | Priority: Major | Resolution: Not A Problem
Created: 10/Dec/21 03:58
Affects Version/s: 2.4.0

The csv data format is imported from databricks [spark-csv|https://github.com/databricks/spark-csv] by issue SPARK-12833 with PR [10766|https://github.com/apache/spark/pull/10766] .

{*}For the nullValue option{*}, according to features described in spark-csv readme file, it's designed as:
{noformat}
When reading files:
nullValue: specifies a string that indicates a null value, any fields matching this string will be set as nulls in the DataFrame

When writing files:
nullValue: specifies a string that indicates a null value, nulls in the DataFrame will be written as this string.
{noformat}
For example, when writing:
{code:scala}
Seq(("Tesla", null:String)).toDF("make", "comment").write.option("nullValue", "NULL").csv(path){code}
The saved csv file is shown as:
{noformat}
Tesla,NULL
{noformat}
When reading:
{code:scala}
spark.read.option("nullValue", "NULL").csv(path).show()
{code}
The parsed dataframe is shown as:
||make||comment||
|Tesla|null|

We can find that null columns in dataframe can be saved as "NULL" strings in csv files and {color:#00875a}*"NULL" strings in csv files can be parsed as null columns*{color} in dataframe. That is:
{noformat}
When writing, convert null(in dataframe) to nullValue(in csv)
When reading, convert nullValue or nothing(in csv) to null(in dataframe)
{noformat}
But actually, the option nullValue in depended component univocity's {*}_CommonSettings_{*}, is designed as that:
{noformat}
when reading, if the parser does not read any character from the input, the nullValue is used instead of an empty string.
when writing, if the writer has a null object to write to the output, the nullValue is used instead of an empty string.{noformat}
{*}There is a difference when reading{*}. In univocity, nothing content will be convert to nullValue strings. But In Spark, we finally convert nothing content or nullValue strings to null in *_UnivocityParser_ _nullSafeDatum_* method:
{code:java}
private def nullSafeDatum(
     datum: String,
     name: String,
     nullable: Boolean,
     options: CSVOptions)(converter: ValueConverter): Any = {
  if (datum == options.nullValue || datum == null) {
    if (!nullable) {
      throw QueryExecutionErrors.foundNullValueForNotNullableFieldError(name)
    }
    null
  } else {
    converter.apply(datum)
  }
} {code}
 

From now, we start to talk about emptyValue.

{*}For the emptyValue option{*},  we add a emptyValueInRead option for reading and a emptyValueInWrite option for writing. I found that Spark keeps the same behaviors for emptyValue with univocity, that is:
{noformat}
When reading, if the parser does not read any character from the input, and the input is within quotes, the empty is used instead of an empty string.

When writing, if the writer has an empty String to write to the output, the emptyValue is used instead of an empty string.{noformat}
For example, when writing:
{code:scala}
Seq(("Tesla", "")).toDF("make", "comment").write.option("emptyValue", "EMPTY").csv(path){code}
The saved csv file is shown as:
{noformat}
Tesla,EMPTY {noformat}
When reading:
{code:scala}
spark.read.option("emptyValue", "EMPTY").csv(path).show()
{code}
The parsed dataframe is shown as:
||make||comment||
|Tesla|EMPTY|

We can find that empty columns in dataframe can be saved as "EMPTY" strings in csv files, *{color:#de350b}but "EMPTY" strings in csv files can not be parsed as empty columns{color}* in dataframe. That is:
{noformat}
When writing, convert "" empty(in dataframe) to emptyValue(in csv)
When reading, convert "\"\"" quoted empty strings to emptyValue(in dataframe)
{noformat}
 

There is an obvious difference between nullValue and emptyValue in read handling. For nullValue, we will convert nothing or nullValue strings to null in dataframe, but for emptyValue, we just try to convert "\"\""(quoted empty strings) to emptyValue strings rather than to convert both "\"\""(quoted empty strings) and emptyValue strings to ""(empty) in dataframe.

I think it's better that if we keep the similar behavior(try to recover emptyValue in csv files to "") for emptyValue as nullValue when reading. So, I suggest that the emptyValueInRead(in CSVOptions) should  be designed as that any fields matching this string will be set as empty values "" when reading.