使用venv创建虚拟环境
python3.3 之后venv已经作为标准库嵌入到了python中，而之前的版本需要借助virtualenv这个第三方库来实现。
在终端中使用python -m venv -h可以显示venv工具的用法。
C:\Users\mango>python -m venv -h
usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear] [--upgrade] [--without-pip]
[--prompt PROMPT] [--upgrade-deps]
ENV_DIR [ENV_DIR ...]
Creates virtual Python environments in one or more target directories.
positional arguments:
ENV_DIR A directory to create the environment in.
optional arguments:
-h, --help show this help message and exit
--system-site-packages
Give the virtual environment access to the system site-packages dir.
--symlinks Try to use symlinks rather than copies, when symlinks are not the default for the platform.
--copies Try to use copies rather than symlinks, even when symlinks are the default for the platform.
--clear Delete the contents of the environment directory if it already exists, before environment
creation.
--upgrade Upgrade the environment directory to use this version of Python, assuming Python has been
upgraded in-place.
--without-pip Skips installing or upgrading pip in the virtual environment (pip is bootstrapped by default)
--prompt PROMPT Provides an alternative prompt prefix for this environment.
--upgrade-deps Upgrade core dependencies: pip setuptools to the latest version in PyPI
Once an environment has been created, you may wish to activate it, e.g. by sourcing an activate script in its bin
directory.
C:\Users\mango>
介绍的两个选项：
--system-site-packages代表使用全局环境中的第三方库（否则虚拟环境直接是纯洁的第三方库）
--without-pip代表不安装pip（一般都是要装的，所以默认就行）
创建虚拟环境
所以正常创建虚拟环境指令如下：python -m venv 安装路径
在当前目录下安装venvdemo这个虚拟环境
python -m venv venvtorch