How to transform JSON strings in columns of dataframe in PySpark?

问题

I have a pyspark dataframe as shown below

+--------------------+---+
|            _c0|_c1|
+--------------------+---+
|{"object":"F...|  0|
|{"object":"F...|  1|
|{"object":"F...|  2|
|{"object":"E...|  3|
|{"object":"F...|  4|
|{"object":"F...|  5|
|{"object":"F...|  6|
|{"object":"S...|  7|
|{"object":"F...|  8|

The column _c0 contains a string in dictionary form.

'{"object":"F","time":"2019-07-18T15:08:16.143Z","values":[0.22124142944812775,0.2147877812385559,0.16713131964206696,0.3102800250053406,0.31872493028640747,0.3366488814353943,0.25324496626853943,0.14537988603115082,0.12684473395347595,0.13864757120609283,0.15222792327404022,0.238663449883461,0.22896413505077362,0.237777978181839]}'

How can I convert the above string to a dictionary form and fetch each key value pair and store it to a variables? I don't want to convert it to pandas as it is expensive.

回答1:

You should use the equivalents of Spark API for Scala's Dataset.withColumn and from_json standard function.

回答2:

Extending on @Jacek Laskowski's post: First create the schema of the struct column. Then use from_json to convert the string column to a struct. Lastly we use the nested schema structure to extract the new columns (we use the f-strings which need python 3.6). On the struct-type you can directly use .select to operate on the nested structure.

schema = StructType([StructField("object",StringType()),
                    StructField("time",StringType()),
                    StructField("values",ArrayType(FloatType()))])

df=df.withColumn('_c0',f.from_json('_c0', schema))

select_list = ["_c0","_c1"] + [f.col(f'_c0.{column}').alias(column) for column in ["object","time","values"]] 
df.select(*select_list).show()

Output (just first to rows)

+--------------------+---+------+--------------------+--------------------+
|                 _c0|_c1|object|                time|              values|
+--------------------+---+------+--------------------+--------------------+
|[F, 2019-07-18T15...|  0|     F|2019-07-18T15:08:...|[0.22124143, 0.21...|
|[F, 2019-07-18T15...|  1|     F|2019-07-18T15:08:...|[0.22124143, 0.21...|
+--------------------+---+------+--------------------+--------------------+

回答3:

df.rdd.map applies the given function to each row of data. I have not yet used the python variant of spark, but it could work like this:

import json

def wrangle(row):
   tmp = json.loads(row._c0)
   return (row._c1, tmp['object'], tmp['time'], tmp['values'])

df.rdd.map(wrangle).toDF()  # should yield a new frame/rdd with the object split

The question how to address the columns might work like that, but you seem to have figured that out already.

This loads the JSON-formatted string to a Python object and returns a tuple with the required elements. Maybe you need to return a Row object instead of a tuple, but, as above, I have not yet used the python part of spark.

来源：https://stackoverflow.com/questions/57108654/how-to-transform-json-strings-in-columns-of-dataframe-in-pyspark

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