How can I install two versions of Python on a single Conda Environment?

Question

I need to have a conda environment with two versions of python (3.5.3 and 3.7.1).

Is it possible to have multiple python versions installed in a single conda environment? And if yes, how can I do so?

Answer 1

Create multiple environments

Please take time to read the Conda documentation, which covers using different Python versions.

One cannot install different Python versions in the same environment. As @Grismar commented, it is a central purpose of environments to isolate different Python installations from each other. Instead, create a separate environment for each Python version (or, better, for each project):

# v3.5.3 environment (name is arbitrary)
conda create -n python3_5_3 python=3.5.3  # also include additional packages here

# v3.7.1 env
conda create -n python3_7_1 python=3.7.1

Then activate to use the environment of choice:

conda activate python3_7_1
python

---

Running multiple Python processes

@Egret commented that OP may need to run code in two different versions in a single context. However, asking to co-install those two versions in one environment would make this an XY problem - rather than asking how to solve a problem, one has instead asked how to implement a (possibly inappropriate) solution to that problem.

If this is the case, then please see these related answers on how to execute within multiple environments via subprocesses:

• Running script in Conda environment from within script in another Conda environment
• Switching environments within a python script
• Executing a command out of conda env

That is, I'd regard the idiomatic solution as using multiple environments and orchestrating execution within them as appropriate. The hard part left out in those answers is exchanging non-trivial data types across processes (e.g., via serialization), but I think that is outside the scope of this thread.

---

Testing co-installation with Conda and Mamba

Simply trying to install both with Conda doesn't give a clear error, in my opinion:

$ conda create -n snakes python=3.5.3 python=3.7.1
Collecting package metadata (current_repodata.json): done
Solving environment: failed with repodata from current_repodata.json, will retry with next repodata source.
Collecting package metadata (repodata.json): done
Solving environment: failed

PackagesNotFoundError: The following packages are not available from current channels:

  - python[version='3.5.3.*,3.7.1.*']

However, Mamba seems to give more sensible feedback (and much faster):

$ mamba create -n snakes python=3.5.3 python=3.7.1

                  __    __    __    __
                 /  \  /  \  /  \  /  \
                /    \/    \/    \/    \
███████████████/  /██/  /██/  /██/  /████████████████████████
              /  / \   / \   / \   / \  \____
             /  /   \_/   \_/   \_/   \    o \__,
            / _/                       \_____/  `
            |/
        ███╗   ███╗ █████╗ ███╗   ███╗██████╗  █████╗
        ████╗ ████║██╔══██╗████╗ ████║██╔══██╗██╔══██╗
        ██╔████╔██║███████║██╔████╔██║██████╔╝███████║
        ██║╚██╔╝██║██╔══██║██║╚██╔╝██║██╔══██╗██╔══██║
        ██║ ╚═╝ ██║██║  ██║██║ ╚═╝ ██║██████╔╝██║  ██║
        ╚═╝     ╚═╝╚═╝  ╚═╝╚═╝     ╚═╝╚═════╝ ╚═╝  ╚═╝

        mamba (0.11.1) supported by @QuantStack

        GitHub:  https://github.com/mamba-org/mamba
        Twitter: https://twitter.com/QuantStack

█████████████████████████████████████████████████████████████


Looking for: ['python=3.5.3', 'python=3.7.1']

conda-forge/osx-64       Using cache
conda-forge/noarch       Using cache
pkgs/main/osx-64         Using cache
pkgs/main/noarch         Using cache
pkgs/r/osx-64            Using cache
pkgs/r/noarch            Using cache

Encountered problems while solving:
  - cannot install both python-3.7.1-h145921a_1000 and python-3.5.3-0