2020-01-21 运行$ flake8 /path/to/file 报了如下错误：
"pyflakes" failed during execution due to "'FlakesChecker' object has no attribute 'NAMEDEXPR'"
Run flake8 with greater verbosity to see more details
原因是flake8依赖pyflake，而pyflake虽然修复了这个问题，但并未release到pypi
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解决：
pip install git+https://github.com/PyCQA/pyflakes.git --user --upgrade
如果github太慢可以换成gitee：
pip install git+https://gitee.com/waketzheng/pyflakes.git --user --upgrade