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Created July 17, 2020 18:35
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Hello virtual environments (for NU)
Raw
venvs.md

Virtual environments ("venvs") make setting up Python projects easy, especially for projects multiple people work on. First, we need to create the venv:

joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ python3 -m venv ./venv
joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ ls
venv

As you can see, running the command created a folder venv containing the virtual environment. This folder contains a basic Python3 interpreter and packages, and is ready to go. To get started, we activate it as follows:

joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ source venv/bin/activate
(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶

Notice how my shell is now prefixed with (venv) - this his how venvs signal that they are currently active. I can exit the virtual environment as follows:

(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ deactivate
joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶

Notice what happens with which python when I'm in the venv vs not:

joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ which python
/usr/bin/python
joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ source venv/bin/activate
(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ which python
/home/joshua.goller/Code/skip-example/venv/bin/python

As you can see, the default Python setup the shell uses is changed; as such, any python commands you run while in the venv will use the venv's python installation. Let's now install the requests library to the venv (you may be prompted to run pip install --upgrade pip, which you should do but isn't required here):

(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ pip3 install requests
Collecting requests
  Downloading https://files.pythonhosted.org/packages/45/1e/0c169c6a5381e241ba7404532c16a21d86ab872c9bed8bdcd4c423954103/requests-2.24.0-py2.py3-none-any.whl (61kB)
     |████████████████████████████████| 71kB 2.2MB/s 
Collecting urllib3!=1.25.0,!=1.25.1,<1.26,>=1.21.1 (from requests)
  Using cached https://files.pythonhosted.org/packages/e1/e5/df302e8017440f111c11cc41a6b432838672f5a70aa29227bf58149dc72f/urllib3-1.25.9-py2.py3-none-any.whl
Collecting idna<3,>=2.5 (from requests)
  Downloading https://files.pythonhosted.org/packages/a2/38/928ddce2273eaa564f6f50de919327bf3a00f091b5baba8dfa9460f3a8a8/idna-2.10-py2.py3-none-any.whl (58kB)
     |████████████████████████████████| 61kB 9.7MB/s 
Collecting certifi>=2017.4.17 (from requests)
  Downloading https://files.pythonhosted.org/packages/5e/c4/6c4fe722df5343c33226f0b4e0bb042e4dc13483228b4718baf286f86d87/certifi-2020.6.20-py2.py3-none-any.whl (156kB)
     |████████████████████████████████| 163kB 17.3MB/s 
Collecting chardet<4,>=3.0.2 (from requests)
  Using cached https://files.pythonhosted.org/packages/bc/a9/01ffebfb562e4274b6487b4bb1ddec7ca55ec7510b22e4c51f14098443b8/chardet-3.0.4-py2.py3-none-any.whl
Installing collected packages: urllib3, idna, certifi, chardet, requests
Successfully installed certifi-2020.6.20 chardet-3.0.4 idna-2.10 requests-2.24.0 urllib3-1.25.9

As you can see, we've now installed the requests package and can use it from our venv's python:

(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ python3
Python 3.7.7 (default, Dec 27 2004, 00:00:16) 
[GCC 10.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import requests
>>> requests
<module 'requests' from '/home/joshua.goller/Code/skip-example/venv/lib/python3.7/site-packages/requests/__init__.py'>

Watch what happens if we try to do the same thing from the default system python:

(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ deactivate
joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ python3
Python 3.7.7 (default, Dec 27 2004, 00:00:16) 
[GCC 10.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import requests
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'requests'
>>>

So as you can see, venvs let you manage your setup in a self contained way. One last tip - to see all the packages currently installed in your venv, do the following:

(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ pip freeze
certifi==2020.6.20
chardet==3.0.4
idna==2.10
requests==2.24.0
urllib3==1.25.9

This is highly useful, because then you can save it to a file, and use that file to install every package you need instead of having to install them one by one (the > character redirects the output of pip freeze to a file, which is created):

(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ pip freeze > requirements.txt 
(venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ cat requirements.txt 
certifi==2020.6.20
chardet==3.0.4
idna==2.10
requests==2.24.0
urllib3==1.25.9

Then, someone else (including you, if you want to set the project up on a new machine) can come by on their system with that file and do this:

(some-other-venv) joshua.goller @ joshua-goller ~/Code/skip-example
└─ $ ▶ pip install -r requirements.txt 
Collecting certifi==2020.6.20 (from -r requirements.txt (line 1))
  Using cached https://files.pythonhosted.org/packages/5e/c4/6c4fe722df5343c33226f0b4e0bb042e4dc13483228b4718baf286f86d87/certifi-2020.6.20-py2.py3-none-any.whl
Collecting chardet==3.0.4 (from -r requirements.txt (line 2))
  Using cached https://files.pythonhosted.org/packages/bc/a9/01ffebfb562e4274b6487b4bb1ddec7ca55ec7510b22e4c51f14098443b8/chardet-3.0.4-py2.py3-none-any.whl
Collecting idna==2.10 (from -r requirements.txt (line 3))
  Using cached https://files.pythonhosted.org/packages/a2/38/928ddce2273eaa564f6f50de919327bf3a00f091b5baba8dfa9460f3a8a8/idna-2.10-py2.py3-none-any.whl
Collecting requests==2.24.0 (from -r requirements.txt (line 4))
  Using cached https://files.pythonhosted.org/packages/45/1e/0c169c6a5381e241ba7404532c16a21d86ab872c9bed8bdcd4c423954103/requests-2.24.0-py2.py3-none-any.whl
Collecting urllib3==1.25.9 (from -r requirements.txt (line 5))
  Using cached https://files.pythonhosted.org/packages/e1/e5/df302e8017440f111c11cc41a6b432838672f5a70aa29227bf58149dc72f/urllib3-1.25.9-py2.py3-none-any.whl
Installing collected packages: certifi, chardet, idna, urllib3, requests
Successfully installed certifi-2020.6.20 chardet-3.0.4 idna-2.10 requests-2.24.0 urllib3-1.25.9
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