Conda 4: Extra material

The following extra material contains some more advanced things you can do with Conda/Mamba and the command line in general, which is not part of the main course materials. All the essential skills of are covered by the previous section: the material here should be considered tips and tricks from people who use Conda/Mamba as part of their daily work. You thus don’t need to use these things unless you want to, and you can even skip this part of the lesson if you like!

Configuration

The behaviour of your Conda installation can be changed using an optional configuration file .condarc. On a fresh Conda install no such file is included but it’s created in your home directory as ~/.condarc the first time you run conda config.

You can edit the .condarc file either using a text editor or by way of the conda config command. To list all config parameters and their settings run:

conda config --show

Similar to Conda environment files, the configuration file is in YAML syntax. This means that the config file is structured in the form of key:value pairs where the key is the name of the config parameter (e.g. auto_update_conda) and the value is the parameter setting (e.g. True).

Adding the name of a config parameter to conda config --show will show only that parameter, e.g. conda config --show channels.

You can change parameters with the --set, --add, --append and --remove flags to conda config.

If you for example want to enable the ‘Always yes’ behaviour which makes Conda automatically choose the yes option, such as when installing, you can run:

conda config --set always_yes True

To see details about a config parameter you can run conda config --describe parameter. Try running it on the channels parameter:

conda config --describe channels

In the beginning of this tutorial we added Conda channels to the .condarc file using conda config --add channels. To remove one of the channels from the configuration file you can run:

conda config --remove channels conda-forge

Check your .condarc file to see the change. To add the conda-forge channel back to the top of the channels simply run:

conda config --add channels conda-forge

To completely remove a parameter and all its values run:

conda config --remove-key parameter

For a list of Conda configuration parameters see the Conda configuration Links to an external site. page.

Managing Python versions

With Conda environments it’s possible to keep several different versions of Python on your computer at the same time, and switching between these versions is very easy. However, a single Conda environment can only contain one version of Python.

Your current Python installation

The base environment has its own version of Python installed. When you open a terminal (after having installed Conda/Mamba on your system) this base environment is activated by default (as evidenced by (base) prepended to your prompt). You can check what Python version is installed in this environment by running python --version. To see the exact path to the Python executable type which python.

In addition to this your computer may already have Python installed in a separate (system-wide) location outside of the Conda/Mamba installation. To see if that is the case type mamba deactivate until your prompt is not prepended with a Conda environment name. Then type which python. If a path was printed to the terminal (e.g. /usr/bin/python) that means some Python version is already installed in that location. Check what version it is by typing python --version.

Now activate the base environment again by typing mamba activate (or the equivalent mamba activate base) then check the Python installation path and version using which and python --version as above. See the difference? When you activate an environment your $PATH variable is updated so that when you call python (or any other program) the system first searches the directory of the currently active environment.

Different Python versions

When you create a new Conda environment you can choose to install a specific version of Python in that environment as well. As an example, create an environment containing Python version 3.5 by running:

mamba create -n py35 python=3.5

Here we name the environment py35 but you can choose whatever name you want.

To activate the environment run:

mamba activate py35

You now have a completely separate environment with its own Python version.

Let’s say you instead want an environment with Python version 2.7 installed. You may for instance want to run scripts or packages that were written for Python 2.x and are thus incompatible with Python 3.x. Simply create the new Conda environment with:

mamba create -n py27 python=2.7

Activate this environment with:

mamba activate py27

Now, switching between Python versions is as easy as typing mamba activate py35 / mamba activate py27.

Note
If you create an environment where none of the packages require Python, and you don’t explicitly install the python package then that new environment will use the Python version installed in your base environment.

Decorating your prompt

By default, the name of the currently activated environment is added to your command line prompt. This is a good thing, as it makes it easier to keep track of what environment and packages you have access to. The way this is done in the default implementation becomes an issue when using absolute paths for environments (specifying mamba env create -p path/to/environment, though, as the entire path will be added to the prompt. This can take up a lot of unnecessary space on your screen, but can be solved in a number of ways.

The most straightforward way to solve this is to change the Conda configuration file, specifically the settings of the env_prompt configuration value which determines how Conda modifies your command line prompt. For more information about this setting you can run conda config --describe env_prompt and to see your current setting you can run conda config --show env_prompt.

By default env_prompt is set to ({default_env}) which modifies your prompt with the active environment name if it was installed using the -n flag or if the environment folder has a parent folder named envs/. Otherwise the full environment path (i.e. the ‘prefix’) is displayed.

If you instead set env_prompt to ({name}) Conda will modify your prompt with the folder name of the active environment. You can change the setting by running conda config --set env_prompt '({name}) '

If you wish to keep the ({default_env}) behaviour, or just don’t want to change your Conda config, an alternative is to keep Conda environment folders within a parent folder called envs/. This will make Conda only add the folder name of the Conda environment to your prompt when you activate it.

As an example, say you have a project called project_a with the project path ~/myprojects/project_a. You could then install the environment for project_a into a folder ~/myprojects/project_a/envs/project_a_environment. Activating the environment by pointing Conda to it (e.g. mamba activate ~/myprojects/project_a/envs/project_a_environment) will only cause your prompt to be modified with project_a_environment.

Bash aliases for conda

Some programmers like to have aliases (i.e. shortcuts) for common commands. Two aliases that might be useful for you are alias coac='conda activate' and alias code='conda deactivate'. Don’t forget to add them to your ~/.bash_profile if you want to use them!

Rolling back to an earlier version of the environment

The history of the changes to an environment are automatically tracked. You can see revisions to an environment by using:

mamba list --revisions

Which shows each revision (numbered) and what’s installed.

You can revert back to particular revision using:

mamba install --revision 5