Use a Python environment

Molt runs the bundled kernel server with one configured Python executable. It does not install packages, create environments, or switch interpreters per tab. Any package importable by that executable is available to all four kernels.

Check the current default

Molt’s default interpreter setting is:

python3

Resolve that command in Terminal:

command -v python3
python3 --version

Create a dedicated virtual environment

Choose a durable location outside the Molt application bundle. For example:

mkdir -p "$HOME/.virtualenvs"
python3 -m venv "$HOME/.virtualenvs/molt"
"$HOME/.virtualenvs/molt/bin/python" -m pip install --upgrade pip

Install the packages you need through that interpreter:

"$HOME/.virtualenvs/molt/bin/python" -m pip install numpy pandas

Confirm the environment before assigning it to Molt:

"$HOME/.virtualenvs/molt/bin/python" -c "import numpy, pandas; print(numpy.__version__, pandas.__version__)"

Configure Molt

1. Open File → Settings….

2. In Python → Interpreter, enter the absolute executable path, such as:

   /Users/your-name/.virtualenvs/molt/bin/python

3. Select Save.

4. Quit Molt completely.

5. Relaunch Molt.

6. Run an import in a cell:

   import numpy as np
   np.arange(5)

Expected output resembles:

array([0, 1, 2, 3, 4])

The settings hint is inaccurate in this release

The settings window says an interpreter change takes effect on the next kernel restart. The backend actually reads and stores the interpreter only when Molt launches. A full app relaunch is required.

Use a project environment

You can point Molt at a project’s virtual environment:

/Users/your-name/projects/example/.venv/bin/python

This is useful for quick checks against the same dependencies as that project. Remember that all four tabs use the same interpreter path. Changing or deleting the environment while Molt is running can cause future kernel starts to fail.

Change the file directly

The runtime configuration file is:

~/Library/Application Support/molt/config.toml

A minimal valid configuration is:

config.toml

[python]
interpreter = "/Users/your-name/.virtualenvs/molt/bin/python"

[app]
auto_launch = false
tab_count = 4
native_effects = true

Quit Molt before editing the file, then relaunch.

Troubleshoot an environment

Molt reports that the interpreter is not found

Run the exact configured path with --version:

"/absolute/path/to/python" --version

Correct permissions or the path, save it, and relaunch Molt.

A package imports in Terminal but not Molt

Confirm that Terminal and Molt use the same executable:

"/absolute/path/to/python" -c "import sys; print(sys.executable)"

Then relaunch Molt. Restarting only the tab is not sufficient after a configuration change.

Package installation from a cell

Molt has no package-management UI. Running pip as Python code is not a supported workflow. Install through Terminal with the selected interpreter’s -m pip form, then restart the relevant kernel or relaunch Molt as needed.