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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (A guide to Python virtual environments with virtualenvwrapper)
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[#]: via: (https://opensource.com/article/21/2/python-virtualenvwrapper)
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[#]: author: (Ben Nuttall https://opensource.com/users/bennuttall)
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A guide to Python virtual environments with virtualenvwrapper
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======
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Virtual environments are key to safely tinkering with different versions
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of Python and combinations of packages
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![Python in a coffee cup.][1]
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For some time, Python has included support for managing virtual environments. Python 3.3 even added the built-in **venv** module for creating environments without third-party libraries. Python programmers use several different tools to manage their environments, and the one I use is called [**virtualenvwrapper**][2].
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Virtual environments are a way of separating your Python project and its dependencies from your system-installed Python. If you use a macOS or Linux-based operating system, it very likely comes with a version of Python as part of the installation, and in fact, it will probably be dependent on that particular version of Python to function properly. But it's your computer, and you may want to use it for your own purposes. You may need to install another version of Python than the operating system provides. You may need to install some additional libraries, too. Although it's possible to upgrade your system Python, it's not recommended. It's also possible to install other libraries, but you must take care not to interfere with anything the system relies on.
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Virtual environments are key to creating the isolation you need to safely tinker with different versions of Python and different combinations of packages. They also allow you to install different versions of the same library for different projects, which resolves what would be impossible if all of your projects' requirements were installed in the same environment.
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Why virtualenvwrapper over other tools? In short:
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* Rather than having a `venv` directory inside or alongside your project directory, virtualenvwrapper keeps all your environments in one place: `~/.virtualenvs` by default.
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* It provides commands for creating and activating environments easily, and the activation doesn't rely on locating the right `activate` script. It's just `workon projectname` (from anywhere) rather than `source ~/Projects/flashylights-env/bin/activate`.
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### Getting started
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First of all, it's important to take the time to understand how your system Python is configured and a bit about how the **pip** tool works.
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To use the Raspberry Pi OS as an example, the operating system comes with both Python 2.7 and 3.7 installed. It also provides separate instances of **pip**, one for each version:
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* The command `python` runs Python 2.7 and is located at `/usr/bin/python`.
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* The command `python3` runs Python 3.7 and is located at `/usr/bin/python3`.
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* The command `pip` installs packages for Python 2.7 and is located at `/usr/bin/pip`.
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* The command `pip3` installs packages for Python 3.7 and is located at `/usr/bin/pip3`.
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![Python commands on Raspberry Pi][3]
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(Ben Nuttall, [CC BY-SA 4.0][4])
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It's useful to verify your own state of affairs when it comes to the `python` and `pip` commands before starting to use virtual environments. More information about your `pip` instances can be found by running the command `pip debug` or `pip3 debug`.
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The equivalent information on my Linux computer, which runs Ubuntu, is almost identical (except that it's Python 3.8); and it's very similar on my Macbook, except that the only system Python is 2.6, and I used `brew` to install Python 3.8, so it's located at `/usr/local/bin/python3` instead (along with `pip3`).
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### Installing virtualenvwrapper
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You'll need to install virtualenvwrapper using your system `pip` for Python 3:
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```
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`sudo pip3 install virtualenvwrapper`
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```
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The next step is to configure your shell to load the virtualenvwrapper commands. You do this by editing your shell's RC file (e.g. `.bashrc`, `.bash_profile`, or `.zshrc`) and adding the following lines:
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```
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export VIRTUALENVWRAPPER_PYTHON=/usr/bin/python3
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export VIRTUALENVWRAPPER_VIRTUALENV=/usr/local/bin/virtualenv
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source /usr/local/bin/virtualenvwrapper.sh
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```
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![bashrc][5]
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(Ben Nuttall, [CC BY-SA 4.0][4])
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If your Python 3 is located elsewhere, change the first line according to your setup.
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Close your terminal and reopen it for this to take effect. The first time you open the terminal, you should see some output from virtualenvwrapper. This will only happen once, as some directories are created as part of the setup.
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Now you should be able to type the command `mkvirtualenv --version` to verify that virtualenvwrapper is installed.
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### Creating a new virtual environment
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Say you're working on a project called **flashylights**. To create a virtual environment with this name, run the command:
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```
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`mkvirtualenv flashylights`
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```
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The environment has been created and activated, so you'll see that `(flashlylights)` appears before your prompt:
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![Flashylights prompt][6]
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(Ben Nuttall, [CC BY-SA 4.0][4])
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Now that the environment is activated, things have changed. The `python` now points at a completely different Python instance than the one(s) you identified on your system earlier. It's created a directory for your environment and placed a copy of the Python 3 binary, the pip command, and more inside it. Type `which python` and `which pip` to see where they're located:
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![Flashylights command][7]
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(Ben Nuttall, [CC BY-SA 4.0][4])
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If you run a Python program now, you can run it with `python` instead of `python3`, and you can use `pip` instead of `pip3`. Any packages you install using `pip` will be installed within this environment alone, and they will not interfere with your other projects, other environments, or your system installation.
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To deactivate the environment, run the command `deactivate`. To re-enable it, run `workon flashylights`.
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You can list all available environments with `workon` or use `lsvirtualenv`. You can delete an environment with `rmvirtualenv flashylights`.
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Adding virtual environments to your development routine is a sensible thing to do. In my experience, it keeps me from installing libraries I'm experimenting with system-wide, which can lead to problems. I find virtualenvwrapper the easiest way for me to get into that routine and manage my project environments hassle-free without thinking too much or remembering too many commands.
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### Advanced features
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* You can install multiple Python versions on your system (e.g., using the [deadsnakes PPA][8] on Ubuntu) and create a virtual environment with that particular version using, for example, `mkvirtualenv -p /usr/bin/python3.9 myproject`.
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* You can automate activation/deactivation upon entering/leaving a directory.
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* You can use the `postmkvirtualenv` hook to install common tools every time a new environment is created.
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See more tips in [the docs][9].
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* * *
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_This article is based on Ben Nuttall's [Tooling Tuesday post][10] on virtualenvwrapper and is reused with permission._
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/2/python-virtualenvwrapper
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作者：[Ben Nuttall][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/bennuttall
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/coffee_python.jpg?itok=G04cSvp_ (Python in a coffee cup.)
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[2]: https://virtualenvwrapper.readthedocs.io/en/latest/index.html
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[3]: https://opensource.com/sites/default/files/uploads/pi-python-cmds.png (Python commands on Raspberry Pi)
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[4]: https://creativecommons.org/licenses/by-sa/4.0/
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[5]: https://opensource.com/sites/default/files/uploads/bashrc.png (bashrc)
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[6]: https://opensource.com/sites/default/files/uploads/flashylights-activated-prompt.png (Flashylights prompt)
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[7]: https://opensource.com/sites/default/files/uploads/flashylights-activated-cmds.png (Flashylights command)
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[8]: https://tooling.bennuttall.com/deadsnakes/
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[9]: https://virtualenvwrapper.readthedocs.io/en/latest/tips.html
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[10]: https://tooling.bennuttall.com/virtualenvwrapper/
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@ -0,0 +1,144 @@
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (A guide to Python virtual environments with virtualenvwrapper)
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[#]: via: (https://opensource.com/article/21/2/python-virtualenvwrapper)
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[#]: author: (Ben Nuttall https://opensource.com/users/bennuttall)
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使用 virtualenvwrapper 构建 Python 虚拟环境指南
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======
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虚拟环境是安全地使用不同版本的 Python 和软件包组合的关键。
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![Python in a coffee cup.][1]
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一段时间以来，Python 已经包含了对管理虚拟环境的支持。Python 3.3 甚至增加了内置的 **venv** 模块，用于创建没有第三方库的环境。Python 程序员使用几种不同的工具来管理他们的环境，我使用的工具叫做 [**virtualenvwrapper**][2]。
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虚拟环境是将你的 Python 项目和它的依赖关系与你的系统安装的 Python 分离的一种方式。如果你使用的是基于 macOS 或 Linux 的操作系统，它很可能在安装中附带了一个 Python 版本，事实上，它很可能依赖于那个特定版本的 Python 才能正常运行。但这是你的计算机，你可能想用它来达到自己的目的。你可能需要安装另一个版本的 Python，而不是操作系统提供的版本。你可能还需要安装一些额外的库。尽管你可以升级你的系统 Python，但不推荐这样做。也可以安装其他库，但你必须注意不要干扰系统所依赖的任何东西。
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虚拟环境是创建隔离的关键，你需要安全地修改不同版本的 Python 和不同组合的包。它们还允许你为不同的项目安装同一库的不同版本，这解决了在相同环境满足所有项目需求这个不可能的问题。
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为什么选择 virtualenvwrapper 而不是其他工具？简而言之：
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* 与 `venv` 需要在项目目录内或旁边有一个 `venv` 目录不同，virtualenvwrapper 将所有环境保存在一个地方：默认在 `~/.virtualenvs` 中。
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* 它提供了用于创建和激活环境的命令，而且激活环境不依赖于找到正确的 `activate` 脚本。它只需要 `workon projectname`（从任何地方)而不需要 `source ~/Projects/flashylights-env/bin/activate`。
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### 开始使用
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首先，花点时间了解一下你的系统 Python 是如何配置的，以及 **pip** 工具是如何工作的。
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以树莓派系统为例，该系统同时安装了 Python 2.7 和 3.7。它还提供了单独的 **pip** 实例，每个版本一个：
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* 命令 `python` 运行 Python 2.7，位于 `/usr/bin/python`。
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* 命令 `python3` 运行 Python 3.7，位于 `/usr/bin/python3`。
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* 命令 `pip` 安装 Python 2.7 的软件包，位于 `/usr/bin/pip`。
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* 命令 `pip3` 安装 Python 3.7 的包，位于 `/usr/bin/pip3`。
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![Python commands on Raspberry Pi][3]
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（Ben Nuttall, [CC BY-SA 4.0][4]）
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在开始使用虚拟环境之前，验证一下使用 `python` 和 `pip` 命令的状态是很有用的。关于你的 `pip` 实例的更多信息可以通过运行 `pip debug` 或 `pip3 debug` 命令找到。
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在我运行 Ubuntu Linux 的电脑上几乎是相同的信息（除了它是 Python 3.8）。在我的 Macbook 上也很相似，除了唯一的系统 Python 是 2.6，而我用 `brew` 安装 Python 3.8，所以它位于 `/usr/local/bin/python3` (和 `pip3` 一起)。
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### 安装 virtualenvwrapper
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你需要使用系统 Python 3 的 `pip` 安装 virtualenvwrapper：
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```
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`sudo pip3 install virtualenvwrapper`
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```
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下一步是配置你的 shell 来加载 virtualenvwrapper 命令。你可以通过编辑 shell 的 RC 文件（例如 `.bashrc`、`.bash_profile` 或 `.zshrc`）并添加以下几行：
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```
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export VIRTUALENVWRAPPER_PYTHON=/usr/bin/python3
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export VIRTUALENVWRAPPER_VIRTUALENV=/usr/local/bin/virtualenv
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source /usr/local/bin/virtualenvwrapper.sh
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```
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![bashrc][5]
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（Ben Nuttall, [CC BY-SA 4.0][4]）
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如果你的 Python 3 位于其他地方，请根据你的设置修改第一行。
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关闭你的终端，然后重新打开它，这样才能生效。第一次打开终端时，你应该看到 virtualenvwrapper 的一些输出。这只会发生一次，因为一些目录是作为设置的一部分被创建的。
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现在你应该可以输入 `mkvirtualenv --version` 命令来验证 virtualenvwrapper 是否已经安装。
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### 创建一个新的虚拟环境
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假设你正在进行一个名为 **flashylights** 的项目。要用这个名字创建一个虚拟环境，请运行该命令：
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```
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`mkvirtualenv flashylights`
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```
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环境已经创建并激活，所以你会看到 `(flashlylights)` 出现在你的提示前：
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![Flashylights prompt][6]
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（Ben Nuttall, [CC BY-SA 4.0][4]）
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现在环境被激活了，事情发生了变化。`python` 现在指向一个与你之前在系统中识别的 Python 实例完全不同的 Python 实例。它为你的环境创建了一个目录，并在其中放置了 Python 3 二进制文件、pip 命令等的副本。输入 `which python` 和 `which pip` 来查看它们的位置。
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![Flashylights command][7]
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（Ben Nuttall, [CC BY-SA 4.0][4]）
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如果你现在运行一个 Python 程序，你可以用 `python` 代替 `python3` 来运行，你可以用 `pip` 代替 `pip3`。你使用 `pip`安装的任何包都将只安装在这个环境中，它们不会干扰你的其他项目、其他环境或系统安装。
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要停用这个环境，运行 `deactivate` 命令。要重新启用它，运行 `workon flashylights`。
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你可以用 `workon` 或使用 `lsvirtualenv` 列出所有可用的环境。你可以用 `rmvirtualenv flashylights` 删除一个环境。
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在你的开发流程中添加虚拟环境是一件明智的事情。根据我的经验，它可以防止我在系统范围内安装我正在试验的库，这可能会导致问题。我发现 virtualenvwrapper 是最简单的可以让我进入流程的方法，并无忧无虑地管理我的项目环境，而不需要考虑太多，也不需要记住太多命令。
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### 高级特性
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* 你可以在你的系统上安装多个 Python 版本（例如，在 Ubuntu 上使用 [deadsnakes PPA][8]），并使用该版本创建一个虚拟环境，例如，`mkvirtualenv -p /usr/bin/python3.9 myproject`。
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* 可以在进入/离开目录时自动激活/停用。
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* 你可以使用 `postmkvirtualenv` 钩子在每次创建新环境时安装常用工具。
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更多提示请参见[文档][9]。
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* * *
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_本文基于 Ben Nuttall 在 [Tooling Tuesday 上关于 virtualenvwrapper 的帖子][10]，经许可后重用。_
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--------------------------------------------------------------------------------
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||||
via: https://opensource.com/article/21/2/python-virtualenvwrapper
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作者：[Ben Nuttall][a]
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选题：[lujun9972][b]
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译者：[geekpi](https://github.com/geekpi)
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||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/bennuttall
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/coffee_python.jpg?itok=G04cSvp_ (Python in a coffee cup.)
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[2]: https://virtualenvwrapper.readthedocs.io/en/latest/index.html
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[3]: https://opensource.com/sites/default/files/uploads/pi-python-cmds.png (Python commands on Raspberry Pi)
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[4]: https://creativecommons.org/licenses/by-sa/4.0/
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[5]: https://opensource.com/sites/default/files/uploads/bashrc.png (bashrc)
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[6]: https://opensource.com/sites/default/files/uploads/flashylights-activated-prompt.png (Flashylights prompt)
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[7]: https://opensource.com/sites/default/files/uploads/flashylights-activated-cmds.png (Flashylights command)
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[8]: https://tooling.bennuttall.com/deadsnakes/
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[9]: https://virtualenvwrapper.readthedocs.io/en/latest/tips.html
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[10]: https://tooling.bennuttall.com/virtualenvwrapper/
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