ModuleNotFoundError: No module named ‘_bz2‘

记录一个可能会再次遇到又容易忘记的灯下黑的问题备查，以免以后再遇到浪费时间。

安装模型的支持环境时可能有时会遇到ModuleNotFoundError: No module named '_bz2'这样问题，比如说yolov5第六版的export脚本所需的pandas包就需要bzip2的支持，可能会报下面这样的错误:

Traceback (most recent call last):
  File "export.py", line 40, in 
    from models.common import Conv
  File "/workspace/yolov5-v6/models/common.py", line 13, in 
    import pandas as pd
  File "/usr/local/lib/python3.6/site-packages/pandas/__init__.py", line 52, in 
    from pandas.core.api import (
  File "/usr/local/lib/python3.6/site-packages/pandas/core/api.py", line 29, in 
    from pandas.core.groupby import Grouper, NamedAgg
  File "/usr/local/lib/python3.6/site-packages/pandas/core/groupby/__init__.py", line 1, in 
    from pandas.core.groupby.generic import DataFrameGroupBy, NamedAgg, SeriesGroupBy
  File "/usr/local/lib/python3.6/site-packages/pandas/core/groupby/generic.py", line 57, in 
    from pandas.core.aggregation import (
  File "/usr/local/lib/python3.6/site-packages/pandas/core/aggregation.py", line 27, in 
    from pandas.core.series import FrameOrSeriesUnion, Series
  File "/usr/local/lib/python3.6/site-packages/pandas/core/series.py", line 68, in 
    from pandas.core import algorithms, base, generic, nanops, ops
  File "/usr/local/lib/python3.6/site-packages/pandas/core/generic.py", line 102, in 
    from pandas.io.formats import format as fmt
  File "/usr/local/lib/python3.6/site-packages/pandas/io/formats/format.py", line 71, in 
    from pandas.io.common import stringify_path
  File "/usr/local/lib/python3.6/site-packages/pandas/io/common.py", line 3, in 
    import bz2
  File "/usr/local/lib/python3.6/bz2.py", line 23, in 
    from _bz2 import BZ2Compressor, BZ2Decompressor
ModuleNotFoundError: No module named '_bz2'

这时去安装bzip2，却发现bzip2已经安装了:

 查对应的so文件也确实存在:

/usr/lib/python3.6/lib-dynload/_bz2.cpython-36m-x86_64-linux-gnu.so

怎么就是报错呢？开始没仔细看环境，没注意看所执行的python所在的路径，以为是bzip2版本不对，就匆忙去下载bzip2的源码进行编译和安装，结果装完发现还是报上面的错，郁闷一阵后再仔细看一下恍然大悟，原来我的环境里安装了多个python，执行脚本里使用的python是使用的手工安装在/usr/local/bin/python而不是默认的/usr/bin/python，这样的话，创建个链接把那个so挂到/usr/local/开头的路径下去就行了:

ln -s /usr/lib/python3.6/lib-dynload/_bz2.cpython-36m-x86_64-linux-gnu.so  /usr/local/lib/python3.6/lib-dynload/_bz2.cpython-36m-x86_64-linux-gnu.so

另外，注意在arm64环境下因为平台不同名字稍有差异，对应的so是 /usr/lib/python3.6/lib-dynload/_bz2.cpython-36m-aarch64-linux-gnu.so