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Molten is a fork of Magma, a plugin for running code interactively with the jupyter kernel. I owe a large portion of the functionality of this plugin to Magma. Standing on the shoulders of giants here.

Feature Highlights

  • Send code to run asynchronously in the jupyter kernel
  • Supports any language with a Jupyter Kernel (in theory, they haven't all been tested)
  • See output in a floating window right below the code
  • Send code from multiple buffers to the same kernel
  • Send code from the same buffer to multiple kernels
  • See output in real time, without flicker
  • Python virtual environment support
  • Renders images, plots, and LaTeX to the terminal


  • NeoVim 9.4+
  • Python 3.10+
  • image.nvim is only required if you want to render images
  • Required Python packages (can be installed in a venv. read more):
  • Optional Python packages:
    • cairosvg (for displaying SVG images with transparency)
      • If you don't need transparency, image.nvim can render svg images perfectly fine
    • pnglatex (for displaying TeX formulas)
    • plotly and kaleido (for displaying Plotly figures)
    • pyperclip if you want to use molten_copy_output

You can run :checkhealth to see what you have installed.

Note: Python packages which are used only for the display of some specific kind of output are only imported when that output actually appears.


Configuration information is located in this README, there is more information about getting started in these places:

Probably Too Quick Start Guide or Not So Quick Start Guide

The docs/ folder also contains more in depth information about different ways to use the plugin.


Start by initializing a kernel. This kernel will get a kernel_id which is most commonly just the name of the kernel. If you try to initialize two kernels with the same name, the second one will be named kernel_name_n where n is the total number of kernels that are already initialized.

You execute code by sending it to a kernel, specified by it's kernel_id (this is handled automatically if there is only one option).

When you execute some code, it will create a cell. You can recognize a cell because it will be highlighted when your cursor is in it.

A cell is delimited using two extmarks (see :h api-extended-marks), so each cell will adjust when editing text within it's boundaries.

When your cursor is in a cell (i.e., you have an active cell), a floating window may be shown below the cell, reporting output. This is the display window, or output window. (To see more about whether a window is shown or not, see :MoltenShowOutput and g:molten_auto_open_output). When you cursor is not in any cell, no cell is active.

Overlapping cells are not allowed. If you create an overlapping cell, the old cell will be deleted.

The output window has a header, containing the execution count and execution state (i.e., whether the cell is waiting to be run, running, has finished successfully or has finished with an error). Below the header output is shown.

Jupyter provides a rich set of outputs. To see what we can currently handle, see Output Chunks.


Molten provides a bunch of user commands as an interface to the user. It is recommended to map most of them to keys, as explained in Keybindings.

Here is a list of the commands and their arguments. Args in [] are optional, args in "" are literal.

When the kernel argument is specified as optional a command behaves in the following way:

  • if the kernel is specified, send the code to that kernel
  • else if there is only one active kernel for the current buffer, send the code to that kernel
  • else if there is more than one active kernel for the current buffer, prompt the user for the kernel
Command Arguments Description
MoltenInfo none Show information about the state of the plugin, initialization status, available kernels, and running kernels
MoltenInit ["shared"] [kernel] Initialize a kernel for the current buffer. If shared is passed as the first value, this buffer will use an already running kernel. If no kernel is given, prompts the user.
MoltenDeinit none De-initialize the current buffer's runtime and molten instance. (called automatically on vim close/buffer unload)
MoltenGoto [n] Go to the nth code cell n defaults to 1 (1 indexed)
MoltenNext [n] Go to the next code cell, or jump n code cells n defaults to 1. Values wrap. Negative values move backwards
MoltenPrev [n] like Next but backwards
MoltenEvaluateLine [kernel] Evaluate the current line
MoltenEvaluateVisual [kernel] Evaluate the visual selection (cannot be called with a range!)
MoltenEvaluateOperator [kernel] Evaluate text selected by the following operator. see Keybindings for useage
MoltenEvaluateArgument [kernel] code Evaluate given code in the given kernel
MoltenReevaluateCell none Re-evaluate the active cell (including new code) with the same kernel that it was originally evaluated with
MoltenDelete none Delete the active cell (does nothing if there is no active cell)
MoltenShowOutput none Shows the output window for the active cell
MoltenHideOutput none Hide currently open output window
MoltenEnterOutput none Move into the active cell's output window. Opens but does not enter the output if it's not open. must be called with noautocmd (see Keybindings for example)
MoltenInterrupt [kernel] Sends a keyboard interrupt to the kernel which stops any currently running code. (does nothing if there's no current output)
MoltenRestart [!] [kernel] Shuts down a restarts the kernel. Deletes all outputs if used with a bang
MoltenSave [path] [kernel] Save the current cells and evaluated outputs into a JSON file. When path is specified, save the file to path, otherwise save to g:molten_save_path. currently only saves one kernel per file
MoltenLoad ["shared"] [path] Loads cell locations and output from a JSON file generated by MoltenSave. path functions the same as MoltenSave. If shared is specified, the buffer shares an already running kernel.
MoltenExportOutput [!] [path] [kernel] Export outputs from the current buffer and kernel to a jupyter notebook .ipynb at the given path. read more


The commands above should be mapped to keys for the best experience. There are more detailed setups in the Docs, but here are some example bindings. Pay attention to MoltenEvaluateVisual and MoltenEnterOutput, as they need to be run in...odd ways.

Example Run Binds

vim.keymap.set("n", "<localleader>R", ":MoltenEvaluateOperator<CR>",
    { silent = true, noremap = true, desc = "run operator selection" })
vim.keymap.set("n", "<localleader>rl", ":MoltenEvaluateLine<CR>",
    { silent = true, noremap = true, desc = "evaluate line" })
vim.keymap.set("n", "<localleader>rc", ":MoltenReevaluateCell<CR>",
    { silent = true, noremap = true, desc = "re-evaluate cell" })
vim.keymap.set("v", "<localleader>r", ":<C-u>MoltenEvaluateVisual<CR>gv",
    { silent = true, noremap = true, desc = "evaluate visual selection" })

Other example mappings

vim.keymap.set("n", "<localleader>rd", ":MoltenDelete<CR>",
    { silent = true, noremap = true, desc = "molten delete cell" })
vim.keymap.set("n", "<localleader>oh", ":MoltenHideOutput<CR>",
    { silent = true, noremap = true, desc = "hide output" })
vim.keymap.set("n", "<localleader>os", ":noautocmd MoltenEnterOutput<CR>",
    { silent = true, noremap = true, desc = "show/enter output" })


Configuration is done with variables. Below you'll find a table of all the potential configuration variable, their values, and a brief description.

the default value is wrapped in ()

Variable Values Description
g:molten_auto_open_output (true) | false Automatically open the output window when your cursor moves over a cell
g:molten_copy_output true | (false) Copy evaluation output to clipboard automatically (requires pyperclip)
g:molten_enter_output_behavior ("open_then_enter") | "open_and_enter" | "no_open The behavior of MoltenEnterOutput
g:molten_image_provider ("none") | "image_nvim" How image are displayed
g:molten_output_crop_border (true) | false 'crops' the bottom border of the output window when it would otherwise just sit at the bottom of the screen
g:molten_output_show_more true | (false) When the window can't display the entire contents of the output buffer, shows the number of extra lines in the window footer (requires nvim 10.0+ and a window border)
g:molten_output_virt_lines true | (false) Pad the main buffer with virtual lines so the output doesn't cover anything while it's open
g:molten_output_win_border ({ "", "━", "", "" }) | any value for border in :h nvim_open_win() Some border features will not work if you don't specify your border as a table. see border option of :h nvim_open_win()
g:molten_output_win_cover_gutter (true) | false Should the output window cover the gutter (numbers and sign col), or not. If you change this, you probably also want to change molten_output_win_style
g:molten_output_win_hide_on_leave (true) | false After leaving the output window (via :q or switching windows), do not attempt to redraw the output window
g:molten_output_win_max_height (999999) | int Max height of the output window
g:molten_output_win_max_width (999999) | int Max width of the output window
g:molten_output_win_style (false) | "minimal" Value passed to the style option in :h nvim_open_win()
g:molten_save_path (stdpath("data").."/molten") | any path to a folder Where to save/load data with :MoltenSave and :MoltenLoad
g:molten_use_border_highlights true | (false) When true, uses different highlights for output border depending on the state of the cell (running, done, error). see highlights
g:molten_limit_output_chars (1000000) | int Limit on the number of chars in an output. If you're lagging your editor with too much output text, decrease it
g:molten_virt_lines_off_by_1 true | (false) Allows the output window to cover exactly one line of the regular buffer when output_virt_lines is true, also effects virt_text_output. (useful for running code in a markdown file where that covered line will just be ```)
g:molten_virt_text_output true | (false) When true, show output as virtual text below the cell. When true, output window doesn't open automatically on run. Effected by virt_lines_off_by_1
g:molten_virt_text_max_lines (12) | int Max height of the virtual text
g:molten_wrap_output true | (false) Wrap text in output windows
[DEBUG] g:molten_show_mimetype_debug true | (false) Before any non-iostream output chunk, the mime-type for that output chunk is shown. Meant for debugging/plugin devlopment

Status Line

Molten provides a few functions that you can use to see information in your status line. These are listed below:

require('molten.status').initialized() -- "Molten" or "" based on initialization information
require('molten.status').kernels() -- "kernel1 kernel2" list of kernels attached to buffer or ""
require('molten.status').all_kernels() -- same as kernels, but will show all kernels

The way these are used will vary based on status line plugin. So please refer to your status line plugin to figure out how to use these.


You can change highlights like so:

-- see :h nvim_set_hl for the values of opts
-- I would recommend using the `link` option to link the values to colors from your color scheme
vim.api.nvim_set_hl(0, "MoltenOutputBorder", { opts })

Here is a complete list of the highlight groups that Molten uses, and their default values

  • MoltenOutputBorder = FloatBorder: default output window border
  • MoltenOutputBorderFail = MoltenOutputBorder: border of a failed output window
  • MoltenOutputBorderSuccess = MoltenOutputBorder: border of a successfully run output window
  • MoltenOutputWin = NormalFloat: the innards of the output window
  • MoltenOutputWinNC = MoltenOutputWin: a "Non-Current" output window
  • MoltenOutputFooter = FloatFooter: the "x more lines" text
  • MoltenCell = CursorLine: applied to code that makes up a cell
  • MoltenVirtualText = Comment: output that is rendered as virtual text


We provide some User autocommands (see :help User) for further customization. They are:

  • MoltenInitPre: runs right before MoltenInit initialization happens for a buffer
  • MoltenInitPost: runs right after MoltenInit initialization happens for a buffer
  • MoltenDeinitPre: runs right before MoltenDeinit de-initialization happens for a buffer
  • MoltenDeinitPost: runs right after MoltenDeinit de-initialization happens for a buffer

There isn't very good documentation (at the time of writing) on using User Autocommands in lua, so here is an example of attaching molten specific mappings to the buffer after initialization

vim.api.nvim_create_autocmd("User", {
  pattern = "MoltenInitPost",
  callback = function()
    vim.keymap.set("v", "<localleader>r", ":<C-u>MoltenEvaluateVisual<CR>gv",
      { desc = "execute visual selection", buffer = true, silent = true })
    -- more mappings here

Similarly, you could remove these mappings on MoltenDeinitPost


Molten exposes some functionality through vim functions.

There is a provided function MoltenEvaluateRange(start_line, end_line, [start_col, end_col]) which evaluates the code between the given line numbers (inclusive). This is intended for use in scripts.

-- run lines 1 through 23 (inclusive):
vim.fn.MoltenEvaluateRange(1, 23)

-- run code starting with col 4 on line 1, and ending with col 20 on line 3
vim.fn.MoltenEvaluateRange(1, 3, 4, 20)

Additionally, this function can take a string as the first argument. When a string is specified, it's assumed to be a kernel_id.

-- run lines 1 through 23 (inclusive) with the python3 kernel
vim.fn.MoltenEvaluateRange("python3", 1, 23)

-- run code starting with col 4 on line 1, and ending with col 20 on line 3 with the R kernel
vim.fn.MoltenEvaluateRange("ir", 1, 3, 4, 20)

When there are multiple kernels attached to the buffer, and this function is called without a kernel_id, the user will be prompted for a kernel with

Because Molten is a remote plugin, options are loaded and cached at initialization. This avoids making an unnecessary number of RPC calls if we were to fetch configuration values every time we needed to use them. This comes with the trade-off of not being able to update config values on the fly... can you see where this is going.

This function lets you update a configuration value after initialization, and the new value will take effect immediately.

You can specify option names with or without the "molten" prefix.

-- these are the same!
vim.fn.MoltenUpdateOption("auto_open_output", true)
vim.fn.MoltenUpdateOption("molten_auto_open_output", true)

Takes in a start line, and end line, and a kernel and creates a code cell in the current buffer associated with that kernel. Does not run the code or create/open an output window.

for compatibility reasons, if there is only one active kernel, you do not need to pass the kernel argument

-- Creates a cell from line 5 to line 10 associated with the python3 kernel
vim.fn.MoltenDefineCell(5, 10, 'python3')


Output Chunks

In the Jupyter protocol, most output-related messages provide a dictionary of mime-types which can be used to display the data. Theoretically, a text/plain field (i.e., plain text) is always present, so we (theoretically) always have that fallback.

Here is a list of the currently handled mime-types:

  • text/plain: Plain text. Shown as text in the output window's buffer.
  • image/*: Molten attempts to render any image mimetype by sending it to image.nvim. In theory, this means that Molten can handle any image format that ImageMagick supports, though I've only tested common formats
  • application/vnd.plotly.v1+json: A Plotly figure. Rendered into a PNG with Plotly + Kaleido
  • text/latex: A LaTeX formula. Rendered into a PNG with pnglatex

This already provides quite a bit of basic functionality, but if you find a use case for a mime-type that isn't currently supported, feel free to open an issue and/or PR!