Develop Reference

Is there a way to create a virtual environment with an earlier version of python?

I have python 3.8 installed on my pc, but i need 3.7 for a specific task. When i try to set up a virtual environment via
virtualenv -p "my/path/to/python37.exe"
it calls the installer, so i am to install py37 manually, but then it ends up with an error:
Error 0x80070666: Cannot install a product when a newer version is installed
Is there a proper way to implement such a thing?
Thanks!

Have you tried anaconda or miniconda (the lighter version of anaconda)? Having more python versions in different environments and switching between them is quite easy.

I haven't installed Anaconda. If you want to have multiple different environments with pure Python you can do it like this:
1) Install the python versions that you want with the exe installer, ie Python 2.x.x, Python 3.7.x, Python 3.8.x etc, maybe inside a common folder like C:\Python
2) Then edit the System Variables path and pinpoint to the folder for the version you want to create a virtual environment. You need two entries here, one to python folder (for the python.exe) and one to the Scripts folder (for the pip.exe)
3) Open command prompt and hit python. You ll see that it's showing the apporpriate version. Install the virtual environment with "python -m venv name_of_env_you_want"
4) If you want to create a virtual environment with a different version change the paths

On Windows machine, installed Python 3.7 but it seem like 3.6 is running

Yesterday I uninstalled my Python 3.6 and installed Python 3.7. I simply ran the executable and followed the prompts. Here is a screen shot of my Programs and Features in my Control Panel.
However, when I check the version in the command prompt, it seems like version 3.6 is still running!

If you have an Anaconda Python installation, you can either upgrade it to your chosen version of Python or create a new conda environment with the version you want.
To see what environments you have set up, do conda env list
To upgrade Python in an environment myenv, do activate myenv then conda update python (to get the latest version) or conda install python=x.y.z (to install version x.y.z, assuming it's available from your conda channels)

You probably didn't completely uninstall your old python version and windows is still pointing to the old installation. Edit your PATH and point your new python 3.7 directory.
Go to My Computer/This PC --> right click --> properties --> advanced system settings --> environment variables --> PATH
You should see a path directory that looks something like this:
C:\Users\Programs\Python\Python36\
Change that directory and point to your python 3.7 installation.
If you want you can keep the old installation of python 3.6 on your computer, it's not necessary to uninstall it and you can work on projects using older python versions when needed.

It looks like you have a Python executable in your PATH from a Anaconda installation (hence the Anaconda Inc at the end of your version number).
You have a few options here:
You can either find the Anaconda installation and remove it (echo %path% into the command prompt may help).
Edit your PATH to make sure that your Python 3.7.3 installation is called first before the Anaconda install.
Upgrade your Anaconda installation to the latest version that uses Python 3.7.

When you install anaconda, a box is automatically checked to make your computer see the version of python installed with conda as the primary version, you could probably uninstall conda or change you system env path taking conda off it.

How can I use both Anaconda versions (2.7 & 3.5)?

I was using the Anaconda 3.5 distro in a Windows 10 machine. Due to dependencies in libraries that I want to work with, I had to have the 2.7 version installed as well.
The good news is that the libraries I needed can now work with the 2.7 version smoothly and Visual Studio 2015 automagically detected my new Python environment.
The problem comes when using the command line. Upon issuing the command
conda info --envs
I get
root * C:\Users\norah\Anaconda2
i.e. a single environment (to my understanding and search so far, according to this I should see two envs listed). This means I can't use conda to acquire new packages for my Python3.5 installation, at least not at the command line as I used to since conda only refers to Python2.7. The same goes for the GUI version, Anaconda navigator (I'm not very fond of the GUI version but I tried it out).
There's also no way of launching python3 from the command line since
$python
always fires up python2.7 and issuing $python3 or $python3.5 in the command line doesn't seem to work (nor would adding the path of python3 to the system since the actual executable has the same name as python2 i.e. python.exe)
Is my system taken over by Python2.7? Is anyone here using them both smoothly and if so could you please elaborate on how to do that? Was it a "no no" move to install both versions of Anaconda?

You can make Python 3.5 environment with your Anaconda 2.7:
conda create -n py35 python=3.5
Now, activate it:
activate py35
Finally you can install the desired packages:
conda install numpy
or, the whole anaconda:
conda install anaconda
The advantage of this approach is that you can also create Python 3.4 or 3.6 environments. Furthermore, you can create environments with different combinations and versions of libraries.
Actually, it makes sense to create a new environment for each larger project.

I also despise the virtual environment switch that Anaconda tries to force on us. I prefer to have both executables always instantly available from the command line. I'm pretty sure I had this working on a Windows machine once:
Install Anaconda2 and Anaconda3 to the C:\ drive as "C:\Anaconda2\" and "C:\Anaconda3\" respectively.
Edit your "Path" environment variable (Control Panel -> System and Security -> System -> Advanced system settings -> Environment Variables) and make sure that "C:\Anaconda2;C:\Anaconda2\Scripts;C:\Anaconda2\Library\bin" is in front of "C:\Anaconda3;C:\Anaconda3\Scripts;C:\Anaconda3\Library\bin".
Copy and rename the file "C:\Anaconda3\python.exe" to "C:\Anaconda3\python3.exe".
Copy and rename the file "C:\Anaconda3\Scripts\conda.exe" to "C:\Anaconda3\Scripts\conda3.exe"
Copy and rename any other scripts you might use in "C:\Anaconda3\Scripts\", such as "pip.exe" to "pip3.exe", etc.
Now, when you type "python" or "conda" at the command line you will get the python2 version, and when you type "python3" or "conda3", etc. at the command line you will get the python3 version.

The best way to use both Python 2.7x and Python 3.5x together is Jupyter Notebook.
http://jupyter.org/
You will be able to work on your browser with IPython Notebook style interface which is great for working with scripting languages.
I found some of these videos on YouTube very informative:
1) https://www.youtube.com/watch?v=HW29067qVWk
2) https://www.youtube.com/watch?v=e9cSF3eVQv0
Besides Python 2.7 and 3.5, there are a bunch of other languages that you will be able run with your Jupyter Notebook. The various Kernels are available in this link below:
https://github.com/jupyter/jupyter/wiki/Jupyter-kernels
After installation, as you start your Jupyter Notebook, your browser will open up a new page showing your Home Directory. Among the 4 tabs (Files|Running|Clusters|Conda):
1) The Files tab shows all the files in your Home Directory.
2) Running tab shows all the Notebooks that are running.
3) Clusters tab is provided by IPython parallel.
4) Conda tab is where you need to add the Python version 3.5 (and other languages if needed) to your Jupyter Notebook (If Python 2.7 is default).
If you are interested to try C++ with your Jupyter Notebook, there are a couple of Kernels available.
1) JupyRoot - https://github.com/root-mirror/root/tree/master/bindings/pyroot/JupyROOT
2) Cling - https://github.com/root-mirror/cling

Anaconda overriding python as the default site-packages directory windows 7

from what I remember of middle school, anacondas and pythons are large snakes that overpower their prey, but anacondas are much bigger and heavier, which may be how it is overtaking my python pathway:
I installed psycopg2 with an easy .exe on windows 7 last night, and it worked (to the anaconda path)- but when I tried to start a Django app, it is looking in Python27\lib\site-packages. I allowed Anaconda to add itself to the pathway when I installed it because a post recommending it said to, and I also have Python set up correctly with a windows path variable.
This makes me wonder, should I actually uninstall python2.7.10, since Anaconda came with python 2.7.9 built in, and use the anaconda prompt for everything, and also delete python 2.7 from my path variable in windows? So,exactly
What is the best way to use Anaconda with Django?

I currently spent the weekend trying to tackle a similar issue. I have Anaconda (Miniconda3) installed for analytical work, however I also wanted to work on a Django project. In the end I ended up using an Anaconda virtual environment to work on my project. Here is basically what i did in cmd:
>mkdir mysite
>cd mysite
>conda create -n mysite-env python=3
>activate mysite-env
>conda install django
Basically this created a virtualenv in the envs folder in my Anaconda installation and I can create/work with django without having to worry about multiple python installations. Not sure if this is the best way, but its the way that works for me.

The issue you are having is that you may not have looked into what Anaconda is used for or what it solves. This step was pointed out in the installation steps when it adds Anaconda to your PATH. Being that Anaconda installs Python and has "environments" there is no reason to have 2 versions of the Python installation on your system, but Anaconda allows you to have different versions of Python with environments. Only one can use the "Python" command unless you execute it from the python.exe directory or make a separate alias as the python command and point it to the Anaconda or other Python installation. Again this is pointless as you can create a new environment with your Django project and requirements.
In short, having 2 separate Python installations is pretty much taking away what Anaconda helps solve.

In Windows 10, open Anaconda Navigator, search Not installed for 'django'. Click the check box, then hit Apply.

Using (Ana)conda within PyCharm

I've got Pycharm 4 running on my Linux (Ubuntu 14.04) machine. In addition to the system python, I've also got Anaconda installed. Getting the two to play nicely together seems to be a bit of a problem... PyCharm provides some interesting integration for virtualenvs and pip, but the Anaconda Python distribution seems to prefer using its own conda tool for both activities.
Is there a relatively simple/painless way to be able to use conda in conjunction with PyCharm? Not just as an alternative interpreter i.e. point PyCharm at the Anaconda Python binary for a project interpreter, but to be able to create, source/activate and deactivate virtual envs, add/remove packages in those virtual envs, etc.
Or am I going to have to choose between using Anaconda (and having a more recent and up-to-date python than may come with the system), and being able to use PyCharm's features to their fullest extent?

I know it's late, but I thought it would be nice to clarify things: PyCharm and Conda and pip work well together.
The short answer
Just manage Conda from the command line. PyCharm will automatically notice changes once they happen, just like it does with pip.
The long answer
Create a new Conda environment:
conda create --name foo pandas bokeh
This environment lives under conda_root/envs/foo. Your python interpreter is conda_root/envs/foo/bin/pythonX.X and your all your site-packages are in conda_root/envs/foo/lib/pythonX.X/site-packages. This is same directory structure as in a pip virtual environement. PyCharm sees no difference.
Now to activate your new environment from PyCharm go to file > settings > project > interpreter, select Add local in the project interpreter field (the little gear wheel) and hunt down your python interpreter. Congratulations! You now have a Conda environment with pandas and bokeh!
Now install more packages:
conda install scikit-learn
OK... go back to your interpreter in settings. Magically, PyCharm now sees scikit-learn!
And the reverse is also true, i.e. when you pip install another package in PyCharm, Conda will automatically notice. Say you've installed requests. Now list the Conda packages in your current environment:
conda list
The list now includes requests and Conda has correctly detected (3rd column) that it was installed with pip.
Conclusion
This is definitely good news for people like myself who are trying to get away from the pip/virtualenv installation problems when packages are not pure python.
NB: I run PyCharm pro edition 4.5.3 on Linux. For Windows users, replace in command line with in the GUI (and forward slashes with backslashes). There's no reason it shouldn't work for you too.
EDIT: PyCharm5 is out with Conda support! In the community edition too.

Change the project interpreter to ~/anaconda2/python/bin by going to File -> Settings -> Project -> Project Interpreter. Also update the run configuration to use the project default Python interpreter via Run -> Edit Configurations. This makes PyCharm use Anaconda instead of the default Python interpreter under usr/bin/python27.

as per #cyberbikepunk answer pycharm supports Anaconda since pycharm5!
Have a look how easy is to add an environment:

Continuum Analytics now provides instructions on how to setup Anaconda with various IDEs including Pycharm here. However, with Pycharm 5.0.1 running on Unbuntu 15.10 Project Interpreter settings were found via the File | Settings and then under the Project branch of the treeview on the Settings dialog.

this might be repetitive.
I was trying to use pycharm to run flask - had anaconda 3, pycharm 2019.1.1 and windows 10. Created a new conda environment - it threw errors.
Followed these steps -
Used the cmd to install python and flask after creating environment as suggested above.
Followed this answer.
As suggested above, went to Run -> Edit Configurations and changed the environment there as well as in (2).
Obviously kept the correct python interpreter (the one in the environment) everywhere.