How to change default Anaconda python environment

If you just want to temporarily change to another environment, use

source activate environment-name

ETA: This may be deprecated. I believe the current correct command is:

source conda activate environment-name

(you can create environment-name with conda create)

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To change permanently, there is no method except creating a startup script that runs the above code.

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Typically it's best to just create new environments. However, if you really want to change the Python version in the default environment, you can do so as follows:

First, make sure you have the latest version of conda by running

conda update conda

Then run

conda install python=3.5

This will attempt to update all your packages in your root environment to Python 3 versions. If it is not possible (e.g., because some package is not built for Python 3.5), it will give you an error message indicating which package(s) caused the issue.

If you installed packages with pip, you'll have to reinstall them.

Overview
Some people have multiple Conda environments with different versions of Python for compatibility reasons. In this case, you should activate the desired default environment in the shell initialization file (e.g., .bashrc, .zshrc). With this method, you can preserve the versions of Python you use in your environments.

The following assumes environment_name is the name of your environment

Mac / Linux:
Edit your bash profile so that the last line is conda activate environment_name. In Mac OSX this is ~/.bash_profile, in other environments this may be ~/.bashrc

Example:
Here's how I did it on Mac OSX

1. Open Terminal and type:

   nano ~/.bash_profile

2. Go to end of file and type the following, where "p3.5" is my environment:

   conda activate p3.5

3. Exit File. Start a new terminal window.

4. Type the following to see what environment is active

   conda info -e

The result shows that I'm using my p3.5 environment by default.

For Windows:
Create a command file (.cmd) with activate environment_name and follow these instructions to have it execute whenever you open a command prompt

1. Create a batch file command, e.g. "my_conda.cmd", put it in the Application Data folder.
2. Configure it to be started automatically whenever you open cmd. This setting is in Registry:
   key: HKCU\SOFTWARE\Microsoft\Command Processor
   value: AutoRun
   type: REG_EXPAND_SZ
   data: "%AppData%\my_conda.cmd"

from this answer: https://superuser.com/a/302553/143794