我安装了spark，在将pyspark模块加载到ipython时遇到了问题。我得到以下错误：

ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-2-49d7c4e178f8> in <module>
----> 1 import pyspark

/opt/spark/python/pyspark/__init__.py in <module>
     44 
     45 from pyspark.conf import SparkConf
---> 46 from pyspark.context import SparkContext
     47 from pyspark.rdd import RDD
     48 from pyspark.files import SparkFiles

/opt/spark/python/pyspark/context.py in <module>
     27 from tempfile import NamedTemporaryFile
     28 
---> 29 from py4j.protocol import Py4JError
     30 
     31 from pyspark import accumulators

ModuleNotFoundError: No module named 'py4j'