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Using both Python 2 and 3

I recently switch to Anaconda for convenient environment for studying machine learning, as suggested by CS231n.

Then I encounter the problem of switching back and forth between Python 2 and 3, again.

Fortunately, Anaconda provide a handy solution for this problem.

Then I go to this SO post and it works magically.


Both of the posts suggest using Conda.

Conda, the package manager for Anaconda, fully supports separated environments.

Create Environment

I first installed Anaconda for python3, so now I would like to create a new environment for python2. So

conda create -n python2 python=2.7 anaconda

will create such an environment named python2 for you.

Activate Environment


source activate python2

and now you can run any of your codes now in python, without the nuisance of using python2 or python3 to run your codes now (which I used, and is pretty error-prone).

import sys


2.7.13 |Anaconda 4.3.1 (x86_64)| (default, Dec 20 2016, 23:05:08) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)]

Deactivate Environment


source deactivate python2

and the (python2) in front of your command line should disappear and you are back to python3!

Anaconda directory and Normal Python directory

One thing to bear in mind is that the configuration for your python are now in anaconda directory.

This means you need to be careful when following instructions on installation or configuration, because methods normally suitable for non-anaconda users might not suit you anymore.

For example, I have installed extensions for Jupyter Notebook following the instructions on GitHub by installing into the user’s home jupyter directories

jupyter contrib nbextension install --user

When I started the Notebook, it warns me like

[jupyter_nbextensions_configurator] nbextension 'qtconsole/qtconsole' has duplicate listings in both '/Users/Me/anaconda3/envs/python2/share/jupyter/nbextensions/qtconsole/qtconsole.yaml' and '/Users/Me/Library/Jupyter/nbextensions/qtconsole/qtconsole.yaml'

To deal with this problem, I uninstall the original files by jupyter contrib nbextension uninstall --user and install the extension in /Users/Me/anaconda3/envs/python2/share/jupyter/nbextensions/

Notice that this notebook extension is now installed for python2, and you may still need to install it for python3.

Install opencv3 by anaconda

Look up the current available python versions.

install the 3.5.3 version since opencv is currently not supported by anaconda on Mac, and install it from conda-forge channel.

conda search python
conda install python=3.5.3
conda install -c conda-forge opencv
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