深入理解Virtual Environment Virtualenv in Python_it is recommended to use a virtual environment-优快云博客

virtual environment

PEP405 – Python Virtual Environments

The utility of Python virtual Environments has already been well established by the popularity of existing third-party virtual-environment tools, primarily $I a n B i c k i n g$ 's virtualenv.

Virtual environments are already widely used for dependency management and isolation.

由于缺乏Python的原生支持,早期的虚拟环境工具 don’t copy the Python binary into the virtual environment, cannot provide reliable isolation from system site directories.

Virtualenv which does copy the Python binary, is forced to dumplicate much of Python’s site module and manually symlink/copy an ever-changing set of standard-library modules into the virtual environment in order to perform a delicate boot-strapping dance at every startup.(virtualenv must copy the binary in order to provide isolation,as Python dereferences a symlinked executable before searching for sys.prefix)

By default, a virtual environment is entirely isolated from the system-level site-packages directories.
virtualenv & virtuanlenvwrapper & Virtualenv Burrito
Virtualenv

virtualenv is a tool to create isolated Python environments written by Mozilla’s own Ian Bicking(author of pip too).

The functionality is now going to be built more directly into Python itself, but virtualenv remains very popular for managing projects.

It’s just the right level of hack to get everything to work consistently and well.

Since Python 3.3, a subset of it has been integrated into the standard library under the venv module.

The venv module does not offer all features of this library, to name just a few more prominent:
- is slower (by not having the app-data seed method)
- is not as extendable
- cannot create virtual environments for arbitrarily installed python versions (and automatically discover these)
- is not upgrade-able via pip
- does not have as rich programmatic API (describe virtual environments without creating them)
virtualenv creates an environment that has its own installation directories, that doesn’t share libraries with other virtualenv environments(and optionally doesn’t access the globally installed libraries either)
virtualenvwrapper

virtualenvwrapper is a set of extensions to lan Bicking’s virtualenv tool.
Virtualenv Burrito

With one command, have a working Python virtualenv + virtualenvwrapper environment, 两年前已停止更新。
Virtualenv 开发起源

site-packages Considered Harmful

[Distutils] API for finding plugins

From Ian Bicking, he think site-packages is a bad idea, and that is should be empty and maybe not even on sys.path at all.

There are a lot of different things you can do with $PYTHONPATH, sitecustomize, .pth files, and other sys.path modifications.

The order in which you do things can matter a lot. This all sucks.

Instead there should be a global installation of the standard library, plus those boring/stable libraries that can practically be considered “standard”. And that’s the only globally installed thing at all. Everything else is only installed in a specific context.
安装

关于pipx参见《理解pipx VS. pip end-user application.md》

virtualenv is a CLI tool that needs a Python interpreter to run.

The best is to use pipx to install virtualenv into an isolated environment.

This has the added benefit that later you’ll be able to upgrade virtualenv without affecting other parts of the system.
```
python -m pip install --user pipx 
python -m pipx ensurepath
pipx install virtualnev 
```
使用

virtualenv command flags

How to use Python virtualenv
```
virtual venv
cd venv
source bin/activate
deactivate
rm -rf venv/
```
- 关于删除virtual environment
  
  There is no command for deleting your virtual enviroment. Simply deactivate it and rid your application of its arifacts by recursively removing it.
  
  This is the same regardless of what kind of virtual environment you are using. virtualenv, venv, Anaconda environment, pyenv, pipenv are all based the same priciple here.
References