How can I turn off rounding in Spark?

I have a dataframe and I’m doing this:

df = dataframe.withColumn("test", lit(0.4219759403))

I want to get just the first four numbers after the dot, without rounding.

When I cast to DecimalType, with .cast(DataTypes.createDecimalType(20,4) or even with round function, this number is rounded to 0.4220.

The only way that I found without rounding is applying the function format_number(), but this function gives me a string, and when I cast this string to DecimalType(20,4), the framework rounds the number again to 0.4220.

I need to convert this number to DecimalType(20,4) without rounding, and I expect to see 0.4219.

Answer

If you have numbers with more than 1 digit before the decimal point, the substr is not adapt. Instead, you can use a regex to always extract the first 4 decimal digits (if present).
You can do this using regexp_extract

df = dataframe.withColumn('rounded', F.regexp_extract(F.col('test'), 'd+.d{0,4}', 0))

---

Example

import pyspark.sql.functions as F

dataframe = spark.createDataFrame([
    (0.4219759403, ),
    (0.4, ),
    (1.0, ),
    (0.5431293, ),
    (123.769859, )
], ['test'])
df = dataframe.withColumn('rounded', F.regexp_extract(F.col('test'), 'd+.d{0,4}', 0))
df.show()

+------------+--------+
|        test| rounded|
+------------+--------+
|0.4219759403|  0.4219|
|         0.4|     0.4|
|         1.0|     1.0|
|   0.5431293|  0.5431|
|  123.769859|123.7698|
+------------+--------+