ExMonty

Elixir NIF wrapper for Monty, a minimal secure Python interpreter written in Rust.

Execute Python code from Elixir with microsecond startup, full sandboxing, resource limits, and interactive pause/resume for external function calls and filesystem access.

Features

• Fast --- no Python runtime required, microsecond startup
• Safe --- sandboxed execution with configurable memory, time, and recursion limits
• Interactive --- Python code pauses at external function calls, hands control to Elixir, and resumes with results
• Pseudo filesystem --- provide virtual files and environment variables to Python code without touching the real filesystem
• Host filesystem mounts --- map virtual sandbox paths to real host directories with read-only / read-write / overlay modes and symlink-escape protection
• Natural type mapping --- Python types map to Elixir types (dicts to maps, sets to MapSet, etc.)

Installation

def deps do
  [
    {:ex_monty, "~> 0.3"}
  ]
end

Requires Rust >= 1.90. Building also requires network access (the Monty Rust crate is a git dependency).

Quick Start

Simple Evaluation

{:ok, 4, ""} = ExMonty.eval("2 + 2")

{:ok, result, ""} = ExMonty.eval("[x**2 for x in range(10)]")
# result = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

With Inputs

{:ok, result, ""} = ExMonty.eval("x + y", inputs: %{"x" => 10, "y" => 20})
# result = 30

Print Capture

All print() output is captured and returned as the third element:

{:ok, nil, "hello world\n"} = ExMonty.eval("print('hello world')")

Compile Once, Run Many

{:ok, runner} = ExMonty.compile("x * 2", inputs: ["x"])

{:ok, 10, ""} = ExMonty.run(runner, %{"x" => 5})
{:ok, 20, ""} = ExMonty.run(runner, %{"x" => 10})
{:ok, 200, ""} = ExMonty.run(runner, %{"x" => 100})

Interactive Execution

Monty's killer feature is interactive execution: Python code pauses when it calls an external function, hands control back to Elixir, and resumes with the result.

Low-Level API

External functions are auto-detected at runtime — no upfront declaration needed. When Python code calls an undefined function, execution pauses with a :function_call progress tag. When code references an undefined name without calling it, execution pauses with :name_lookup.

{:ok, runner} = ExMonty.compile("result = fetch(url)\nresult", inputs: ["url"])

{:ok, progress} = ExMonty.start(runner, %{"url" => "https://example.com"})

case progress do
  {:function_call, call, snapshot, output} ->
    # call.name == "fetch", call.args == ["https://example.com"]
    response = do_fetch(call.args)
    {:ok, next} = ExMonty.resume(snapshot, {:ok, response})

  {:name_lookup, name, snapshot, output} ->
    # Provide a function object or value for the undefined name
    {:ok, next} = ExMonty.resume(snapshot, {:ok, {:function, name}})

  {:os_call, call, snapshot, output} ->
    # call.function == :read_text, call.args == [{:path, "/some/file"}]
    {:ok, next} = ExMonty.resume(snapshot, {:ok, file_content})

  {:complete, value, output} ->
    value
end

High-Level Sandbox

ExMonty.Sandbox automates the interactive loop:

{:ok, 42, ""} = ExMonty.Sandbox.run(
  "double(21)",
  functions: %{
    "double" => fn [x], _kwargs -> {:ok, x * 2} end
  }
)

With a handler module:

defmodule MyHandler do
  @behaviour ExMonty.Sandbox

  @impl true
  def handle_function("fetch", [url], _kwargs) do
    case Req.get(url) do
      {:ok, resp} -> {:ok, resp.body}
      {:error, _} -> {:error, :runtime_error, "fetch failed"}
    end
  end
end

{:ok, result, _output} = ExMonty.Sandbox.run(code, handler: MyHandler)

Pseudo Filesystem

Python code using pathlib.Path and the os module generates OS calls that pause execution just like external function calls. ExMonty.PseudoFS provides a complete in-memory virtual filesystem so Python code can read and write files without touching the real filesystem.

alias ExMonty.PseudoFS

fs = PseudoFS.new()
  |> PseudoFS.put_file("/data/config.json", ~s({"model": "gpt-4", "temperature": 0.7}))
  |> PseudoFS.put_file("/data/prompt.txt", "Summarize the following text:")
  |> PseudoFS.mkdir("/output")
  |> PseudoFS.put_env("API_KEY", "sk-secret123")

code = """
from pathlib import Path
import os

config = Path('/data/config.json').read_text()
prompt = Path('/data/prompt.txt').read_text()
api_key = os.getenv('API_KEY')

Path('/output/result.txt').write_text(f'Read config: {config}')
Path('/output/result.txt').read_text()
"""

{:ok, result, _output} = ExMonty.Sandbox.run(code, os: fs)
# result = "Read config: {\"model\": \"gpt-4\", \"temperature\": 0.7}"

Supported Operations

Custom OS Handlers

For cases where PseudoFS isn't enough (e.g., proxying to the real filesystem with access controls), implement handle_os/3 in a handler module or pass a function map:

# Function map
{:ok, result, _} = ExMonty.Sandbox.run(code,
  os: %{
    read_text: fn [{:path, path}], _kwargs ->
      case File.read(path) do
        {:ok, content} -> {:ok, content}
        {:error, _} -> {:error, :file_not_found_error, "not found: #{path}"}
      end
    end
  }
)

# Handler module
defmodule MyOsHandler do
  @behaviour ExMonty.Sandbox

  @impl true
  def handle_function(_, _, _), do: {:error, :name_error, "not defined"}

  @impl true
  def handle_os(:read_text, [{:path, path}], _kwargs) do
    if String.starts_with?(path, "/allowed/") do
      case File.read(path) do
        {:ok, content} -> {:ok, content}
        {:error, _} -> {:error, :file_not_found_error, "not found"}
      end
    else
      {:error, :os_error, "access denied: #{path}"}
    end
  end
end

Clock Handlers (date.today() / datetime.now())

Python's date.today() and datetime.now() are surfaced as :date_today and :datetime_now os calls. Provide handlers to control what "now" means — useful for deterministic tests, time-travel, or pinning to a request timestamp:

{:ok, result, _} = ExMonty.Sandbox.run(
  """
  from datetime import date, datetime, timezone
  d = date.today()
  dt = datetime.now(tz=timezone.utc)
  (d.year, dt.hour, dt.tzinfo is not None)
  """,
  os: %{
    date_today: fn _args, _kwargs ->
      {:ok, {:date, %{year: 2026, month: 5, day: 1}}}
    end,
    datetime_now: fn _args, _kwargs ->
      {:ok,
       {:datetime,
        %{
          year: 2026, month: 5, day: 1,
          hour: 14, minute: 30, second: 0, microsecond: 0,
          offset_seconds: 0, tz_name: nil
        }}}
    end
  }
)
# result = {2026, 14, true}

The handler's first argument for :datetime_now is the requested timezone ({:timezone, %{offset_seconds: ..., name: ...}} for an aware datetime, or nil for a naive one). If you don't care, ignore it; if you do, branch on it.

Host Filesystem Mounts

ExMonty.PseudoFS is purely in-memory. When you need the sandbox to read real host directories — but with sandboxing guarantees — use ExMonty.Mount. A mount maps a virtual path inside the sandbox to a host directory with one of three access modes. Path canonicalisation, boundary checks, and symlink-escape detection are always enforced.

mounts =
  ExMonty.Mount.new!()
  |> ExMonty.Mount.add!("/data",    "/var/lib/myapp/data",    :read_only)
  |> ExMonty.Mount.add!("/scratch", "/tmp/sandbox-scratch",   :overlay)
  |> ExMonty.Mount.add!("/output",  "/var/lib/myapp/output",  :read_write,
       write_bytes_limit: 10_000_000)

ExMonty.Sandbox.run(code, mounts: mounts)

Modes

Examples

Read-only access to a data directory:

mounts = ExMonty.Mount.new!() |> ExMonty.Mount.add!("/data", "/var/lib/myapp/data", :read_only)

code = """
from pathlib import Path
Path("/data/users.csv").read_text()
"""

{:ok, csv, _} = ExMonty.Sandbox.run(code, mounts: mounts)

Overlay (sandbox writes are ephemeral):

mounts = ExMonty.Mount.new!() |> ExMonty.Mount.add!("/scratch", "/tmp/work", :overlay)

# Sandbox writes go into in-memory overlay storage on the mount object,
# not to /tmp/work. The host directory stays untouched.
code = """
from pathlib import Path
Path("/scratch/intermediate.json").write_text("{}")
Path("/scratch/intermediate.json").read_text()
"""

{:ok, _, _} = ExMonty.Sandbox.run(code, mounts: mounts)

Overlay state persists across runs against the same mount object. Construct a fresh mount to discard accumulated overlay writes.

Compose mounts with :os fallbacks (mounts handle FS calls; the :os map handles non-FS calls like getenv and datetime_now):

ExMonty.Sandbox.run(code,
  mounts: mounts,
  os: %{
    getenv:       fn _args, _kwargs -> {:ok, "from-host"} end,
    datetime_now: fn _args, _kwargs -> {:ok, fixed_datetime()} end
  }
)

Unmounted paths

Filesystem operations on paths that don't fall under any mount raise PermissionError:

Path("/etc/passwd").read_text()
# PermissionError: Permission denied: '/etc/passwd'

This is upstream monty's OsFunction::on_no_handler behaviour for filesystem operations. Non-filesystem operations (like os.getenv) raise RuntimeError if no fallback handler is configured.

Cumulative limits

write_bytes_limit is cumulative on the mount object, not per-run:

mounts = ExMonty.Mount.new!() |> ExMonty.Mount.add!("/o", tmp, :overlay, write_bytes_limit: 16)

ExMonty.Sandbox.run(write_10_bytes, mounts: mounts)  # OK: 10/16 used
ExMonty.Sandbox.run(write_10_bytes, mounts: mounts)  # FAILS: 20 > 16

Construct a fresh mount to reset the counter.

Concurrency

A mount can only serve one run at a time. Calling Sandbox.run against a mount that's already in a run returns {:error, :mount_in_use}. If you need parallel runs with the same host directory, create separate mount objects.

:read_write is a footgun

Sandbox code in :read_write mode can modify real host files. Use sparingly. Most sandboxed-execution use cases want :read_only (provide data) or :overlay (let the sandbox scribble freely without touching disk).

Resource Limits

Control memory, execution time, allocations, and recursion depth:

{:ok, runner} = ExMonty.compile(code)

{:ok, result, output} = ExMonty.run(runner, %{}, limits: %{
  max_duration_secs: 5.0,       # wall-clock timeout
  max_memory: 10_000_000,       # ~10MB memory limit
  max_allocations: 100_000,     # heap allocation count limit
  max_recursion_depth: 100      # call stack depth limit
})

When a limit is exceeded, execution stops and an error is returned:

{:error, %ExMonty.Exception{type: :recursion_error}} =
  ExMonty.eval("def f(): return f()\nf()", limits: %{max_recursion_depth: 50})

Serialization

Runners and snapshots can be serialized to binary for storage or transfer:

# Serialize a compiled runner
{:ok, runner} = ExMonty.compile("x + 1", inputs: ["x"])
{:ok, binary} = ExMonty.dump(runner)

# Restore and use
{:ok, restored} = ExMonty.load_runner(binary)
{:ok, 2, ""} = ExMonty.run(restored, %{"x" => 1})

# Serialize a paused snapshot (for long-running workflows)
{:ok, {:function_call, _call, snapshot, _}} = ExMonty.start(runner_with_ext_fns)
{:ok, snap_binary} = ExMonty.dump_snapshot(snapshot)

# Later: restore and resume
{:ok, restored_snap} = ExMonty.load_snapshot(snap_binary)
{:ok, {:complete, result, _}} = ExMonty.resume(restored_snap, {:ok, value})

Python Support

Monty does not implement all of CPython. It targets a useful subset of the language and standard library — enough to evaluate expressions, scripts, and data transformations safely. This section calls out features that frequently trip people up.

Standard Library

import math, json, re      # multi-module import works
math.pi                     # 3.14159...
json.dumps([1, 2, 3])       # '[1, 2, 3]'
json.loads('{"a": 1}')      # {'a': 1}  → Elixir %{"a" => 1}
re.match(r"\d+", "42")      # match object

Available modules: math, json, re, os (host-mediated), pathlib (host-mediated), datetime (host-mediated for today() / now()).

Syntax

# Multi-module import
import a, b, c

# Chain assignment
a = b = c = 7

# Nested subscript assignment
d["k"][1] = 99
matrix[i][j] = 0

# Generalised unpacking (PEP 448)
[*a, *b]
{**defaults, **overrides}

# Augmented subscript assignment
counts[k] += 1

Class definitions are not supported — use @dataclass or pass objects in from Elixir as %ExMonty.Dataclass{}. hasattr and setattr work on those host-provided objects:

hasattr(user, "email")     # works on dataclasses passed from Elixir
setattr(user, "name", "x") # works if dataclass is mutable (frozen=False)

Built-ins

zip([1,2,3], [4,5], strict=True)   # raises ValueError on length mismatch
"a\tb\tc".expandtabs(4)             # "a   b   c"
list(filter(None, items))           # filter, map, sorted, max, min, ...

int() Parse Limits

CPython's INT_MAX_STR_DIGITS guard is enforced (default 4300 digits). This prevents quadratic-time DoS via huge numeric strings:

int("1" * 5000)
# ValueError: Exceeds the limit (4300 digits) for integer string conversion

Type Mapping

Input Direction (Elixir to Python)

Native Elixir types are auto-detected. Use tagged tuples for ambiguous cases:

# Automatic
ExMonty.eval("x", inputs: %{"x" => 42})         # int
ExMonty.eval("x", inputs: %{"x" => "hello"})     # str
ExMonty.eval("x", inputs: %{"x" => [1, 2, 3]})   # list
ExMonty.eval("x", inputs: %{"x" => %{"a" => 1}}) # dict

# Tagged
ExMonty.eval("x", inputs: %{"x" => {:bytes, <<1, 2, 3>>}}) # bytes
ExMonty.eval("x", inputs: %{"x" => {:path, "/tmp/file"}})   # Path

# datetime types — same shape on input and output
ExMonty.eval("x.year", inputs: %{"x" => {:date, %{year: 2026, month: 5, day: 1}}})
# {:ok, 2026, ""}

ExMonty.eval("x.tzinfo is not None",
  inputs: %{"x" => {:datetime, %{
    year: 2026, month: 5, day: 1,
    hour: 12, minute: 0, second: 0, microsecond: 0,
    offset_seconds: 0, tz_name: nil
  }}})
# {:ok, true, ""}

ExMonty.eval("x.days", inputs: %{"x" => {:timedelta, %{days: 7, seconds: 0, microseconds: 0}}})
# {:ok, 7, ""}

Error Handling

Errors are returned as {:error, %ExMonty.Exception{}}:

{:error, %ExMonty.Exception{
  type: :zero_division_error,
  message: "division by zero",
  traceback: [%ExMonty.StackFrame{filename: "main.py", line: 1, ...}]
}} = ExMonty.eval("1 / 0")

Exception types are atoms matching Python exception names in snake_case: :value_error, :type_error, :key_error, :index_error, :name_error, :attribute_error, :runtime_error, :syntax_error, :file_not_found_error, :zero_division_error, :recursion_error, etc.

Architecture

ExMonty (Elixir API)
  |
  +-- ExMonty.Sandbox (handler behaviour, interactive loop)
  |     +-- ExMonty.PseudoFS (in-memory virtual filesystem)
  |     +-- ExMonty.Mount (host filesystem mounts with sandboxing)
  |
  +-- ExMonty.Native (NIF bindings via Rustler)
        |
        +-- Rust NIF crate (type conversion, resource management)
              |
              +-- monty crate (Python interpreter; fs::MountTable for mounts)

License

MIT