pip install --index-url https://mirrors.aliyun.com/ERROR: You must give at least one requirement to

在用阿里云镜像安装pip安装第三方包时报错：ERROR: You must give at least one requirement to install (see “pip help install”)

我用了如下命令
pip install --index-url https://mirrors.aliyun.com/pypi/simple/pyecharts
但是报错：
ERROR: You must give at least one requirement to install (see “pip help install”)

原因是/simple/后面要有个空格在加包名称

如/simple/ pyecharts

pip install --index-url https://mirrors.aliyun.com/pypi/simple/ pyecharts