When you attempt to read from the pipe within a PySpark UDF, you encounter the [Errno 9] Bad file descriptor error. This occurs because the file descriptor created using os.pipe() in the main Python process is not accessible within the UDF.

Spark executors are separate processes on worker nodes. When you create a UDF, the Python code is executed within a new Python process spawned by the Spark executor. File descriptors are not inherited by child processes. This means the file descriptor created in the main process does not exist in the UDF's process.