问题
Despite a ton of Googling, I can't find any docs for pip's command line options/arguments. What does pip install -U mean? Does anyone have a link to a list of pip's options and arguments?
回答1:
Type pip install -h to list help:
-U, --upgrade Upgrade all packages to the newest available version
So, if you already have a package installed, it will upgrade the package for you. Without the -U switch it'll tell you the package is already installed and exit.
Each pip subcommand has its own help listing. pip -h shows you overall help, and pip [subcommand] -h gives you help for that sub command, such as install.
You can also find the full reference documentation online; the General Options section covers switches available for every pip subcommand, while each subcommand has a separate Options section to cover subcommand-specific switches; see the pip install options section, for example.
来源：https://stackoverflow.com/questions/12435209/what-does-the-u-option-stand-for-in-pip-install-u