问题
How to parse and transform json string from spark dataframe rows in pyspark?
I'm looking for help how to parse:
- json string to json struct
output 1
- transform json string to columns a, b and id
output 2
Background: I get via API json strings with a large number of rows (jstr1
, jstr2
, ...), which are saved to spark df
. I can read schema for each row separately, but this is not the solution as it is very slow as schema has a large number of rows. Each jstr
has the same schema, columns/keys a and b stays the same, just id
and values in columns change.
EDIT: blackbishop solution to use MapType schema works like a charm schema = "map<string, array<struct<a:int,b:int>>>"
Question was extended to: How to transform JSON string with multiple keys, from spark data frame rows in pyspark?
from pyspark.sql import Row
jstr1 = '{"id_1": [{"a": 1, "b": 2}, {"a": 3, "b": 4}]}'
jstr2 = '{"id_2": [{"a": 5, "b": 6}, {"a": 7, "b": 8}]}'
df = sqlContext.createDataFrame([Row(json=jstr1),Row(json=jstr2)])
schema = F.schema_of_json(df.select(F.col("json")).take(1)[0].json)
df2 = df.withColumn('json', F.from_json(F.col('json'), schema))
df2.show()
Current output:
+--------------------+
| json|
+--------------------+
|[[[1, 2], [3, 4]]] |
| []|
+--------------------+
Required output 1:
+--------------------+-------+
| json | id |
+--------------------+-------+
|[[[1, 2], [3, 4]]] | id_1 |
|[[[5, 6], [7, 8]]] | id_2 |
+--------------------+-------+
Required output 2:
+---------+----------+-------+
| a | b | id |
+--------------------+-------+
| 1 | 2 | id_1 |
| 3 | 4 | id_1 |
| 5 | 6 | id_2 |
| 7 | 8 | id_2 |
+---------+----------+-------+
回答1:
You're getting null for the second row because you're using only the schema of the first row which is different from the second one. You can parse the JSON to a MapType instead, where the keys are of type string and values of type array of structs :
schema = "map<string, array<struct<a:int,b:int>>>"
df = df.withColumn('json', F.from_json(F.col('json'), schema))
df.printSchema()
#root
# |-- json: map (nullable = true)
# | |-- key: string
# | |-- value: array (valueContainsNull = true)
# | | |-- element: struct (containsNull = true)
# | | | |-- a: integer (nullable = true)
# | | | |-- b: integer (nullable = true)
Then, with some simple transformations, you get the expected outputs:
- The
id
column represents the key in the map, you get it withmap_keys
function - The structs
<a:int, b:int>
represents the values that you get usingmap_values
function
output1 = df.withColumn("id", F.map_keys("json").getItem(0)) \
.withColumn("json", F.map_values("json").getItem(0))
output1.show(truncate=False)
# +----------------+----+
# |json |id |
# +----------------+----+
# |[[1, 2], [3, 4]]|id_1|
# |[[5, 6], [7, 8]]|id_2|
# +----------------+----+
output2 = output1.withColumn("attr", F.explode("json")) \
.select("id", "attr.*")
output2.show(truncate=False)
# +----+---+---+
# |id |a |b |
# +----+---+---+
# |id_1|1 |2 |
# |id_1|3 |4 |
# |id_2|5 |6 |
# |id_2|7 |8 |
# +----+---+---+
来源:https://stackoverflow.com/questions/65956766/how-to-parse-and-transform-json-string-from-spark-data-frame-rows-in-pyspark