我有一个Dataframe，它有以下模式。这个 translation_version 场下 translations --> languages (no, pt,...) 列位于 null . 我想把所有的 translation_version 作为字符串。我有17种语言 translations ```
root
|-- translations: struct (nullable = true)
| |-- no: struct (nullable = true)
| | |-- Description: string (nullable = true)
| | |-- class: string (nullable = true)
| | |-- description: string (nullable = true)
| | |-- translation_version: null (nullable = true) // Want to cast as string
| |-- pt: struct (nullable = true)
| | |-- Description: string (nullable = true)
| | |-- class: string (nullable = true)
| | |-- description: string (nullable = true)
| | |-- translation_version: null (nullable = true)
| |-- fr: struct (nullable = true)
| | |-- Description: string (nullable = true)
| | |-- class: string (nullable = true)
| | |-- description: string (nullable = true)
| | |-- translation_version: null (nullable = true)

我试过了 `df = df.na.fill('null')` 但没有改变什么。还尝试用以下代码强制转换

df = df.withColumn("translations", F.col("translations").cast("struct<struct<translation_version: string>>"))

但这返回了以下错误

pyspark.sql.utils.ParseException: u"\nmismatched input '<' expecting ':'(line 1, pos 13)\n\n== SQL ==\nstruct<struct<translation_version: string>>\n-------------^^^\n"

你知道怎么投吗 `translation_version` 作为每种语言的字符串？