Virtual environments can be described as isolated installation directories. This isolation allows you to localized the installation of your project's dependencies, without forcing you to install them system-wide.
Imagine you have two applications App1 and App2. Both require the package Pak, but with different versions. If you install Pak version 2.3 for App1, you would not be able to run App2, because it requires version 3.1. Here is when virtual environments come in handy.
Benefits:
* You can have multiple environments, with multiple sets of packages, without conflicts among them. This way, different projects' requirements can be satisfied at the same time.
* You can easily release your project with its own dependent modules.
[`virtualenv`](https://virtualenv.pypa.io/en/stable/) is a tool used to create isolated Python environments. It creates a folder which contains all the necessary executables to use the packages that a Python project would need.
You can install it with `pip`:
```
pip install virtualenv
```
Verify the installation with the following command:
This creates a folder in the current directory with the name of the environment (`my-env/`). This folder contains the directories for installing modules and Python executables.
You can also specify the Python version you want to work with. Just use the argument `--python=/path/to/python/version`. For instance, `python2.7`:
This ensures that only packages under `my-env/` are used.
You will notice that the name of the environment is shown on the left of the prompt. This way you can see which is the active environment.
### Install Packages
You can install packages one by one, or by setting a `requirements.txt` file for your project.
```
pip install some-package
pip install -r requirements.txt
```
If you want to create a `requirements.txt` file from the already installed packages, run the following command:
```
pip freeze > requirements.txt
```
The file will contain the list of all the packages installed in the current environment, and their respective versions. This will help you release your project with its own dependent modules.
### Deactivate an Environment
If you are done working with the virtual environment you can deactivate it with:
```
deactivate
```
This puts you back to the system’s default Python interpreter with all its installed libraries.
[`pipenv`](https://pipenv.readthedocs.io/en/latest/) is a tool that automatically creates and track packges in your virtual environments. It combines `pip` and `virtualenv` together so you do need to use them separately.
You can install it with `pip`:
```
pip install pipenv
```
Verify the installation with the following command:
```
pipenv --version
```
### Create an Environment
To create a virtual environment, use `pipenv` and specify which version of Python you want to use:
```
pipenv --python 3.6
```
### Install Packages
Virtual environments created by pipenv do not need to be activated. Each project is managed independently. You simply go inside the project directory and install the packages you want:
```
pipenv install some-package
```
Pipenv will install the packge and create a `Pipfile` inside the directory. The Pipfile tracks all packages and dependencies that your project needs. `Pipfile` is the replacement of `requirements.txt` when you use pipenv.
### Uninstall Packages
To uninstall a package and remove it from Pipfile use:
```
pipenv uninstall some-package
```
If you want to uninstall all packages use:
```
pipenv uninstall --all
```
### Import requirements.txt to Pipfile
You can import the packages and dependencies from `requirements.txt` to your Pipfile using:
```
pipenv install -r requirements.txt
```
### Executing Python Scripts
To run Python scripts use:
```
pipenv run python main.py
```
`pipenv run` makes sure that packages installed with pipenv will be available to your program. You do not need to activate your virtual environment to do so.
### Remove an Environment
If you want to remove the virtual environment from pipenv use:
```
pipenv --rm
```
You can also delete the environment by deleting the project directory.
