我相信这是目前正在解决的一个众所周知的问题.一周前的消息:http://pyfound.blogspot.com/2020/03/new-pip-resolver-to-roll-out-this-year.html

I believe this is a well known problem that is currently being worked on. Message from one week ago: http://pyfound.blogspot.com/2020/03/new-pip-resolver-to-roll-out-this-year.html

与此同时，可以采取一些措施来尝试缓解此类问题:

In the meantime there are some measures that can be taken to try and mitigate this kind of issues:

• 恢复依赖项的顺序(在您的示例中，a 可以在 b 之前列出 c)
• 使用额外的requirements.txt 或 constraints.txt 文件
• 根据实际需要，另一种工具可以提供帮助(我相信 poetry、pipenv，很可能其他人也可能有更好的依赖解析器，但它们不是 pip) 的一对一替代品

• Revert the order of the dependencies (in your example a could list c before b)
• Use an additional requirements.txt or constraints.txt file
• Depending on the actual needs, an alternative tool could help (I believe poetry, pipenv, and most likely others as well might have better dependency resolvers, but they are not a one-to-one replacement for pip)

今天似乎已经可以测试 pip 的未来依赖解析器:

It appears to be already possible to test pip's future dependency resolver today:

• 从源代码安装pip
• 运行 path/to/python -m pip install --unstable-feature=resolver ...

在某种程度上，似乎也可以通过 pip check 命令在当前版本或 pip 中测试此依赖项解析器.

In a way it also seems to be possible to somewhat test this dependency resolver in current releases or pip via the pip check command.

有关该主题的更多参考资料:

Some more references on the topic:

• https://pradyunsg.me/blog/2020/03/27/pip-resolver-testing/
• https://discuss.python.org/t/an-update-on-pip-and-dependency-resolution/1898/2