Python Interop

Nim to Python Interoperability Skill

This skill covers the main approaches for integrating Nim with Python: nimpy (call Python from Nim and export Nim to Python), nimporter (package Nim as Python modules), and cffi/ctypes (call Nim from Python).

Overview

There are two primary integration directions:

1. Nim → Python: Use nimpy to call Python libraries from Nim code
2. Python → Nim: Use nimpy with {.exportpy.} to expose Nim functions to Python, or use nimporter for packaging

Common use cases:

• Extending Nim with Python scientific computing (Scipy, Numpy)
• Adding scripting capability to Nim applications
• Speeding up Python code by rewriting hot paths in Nim
• Packaging Nim modules for Python distribution

nimpy: Calling Python from Nim

nimpy allows calling Python code and libraries directly from Nim. It provides ABI compatibility - compiled modules don't depend on a particular Python version.

Basic Usage

import nimpy

let os = pyImport("os")
echo "Current dir is: ", os.getcwd().to(string)

# sum(range(1, 5))
let py = pyBuiltinsModule()
let s = py.sum(py.range(0, 5)).to(int)
assert s == 10

Type Conversion

• Nim → Python: Automatic via Nimpy templates
• Python → Nim: Manual conversion using .to(T) API

let np = pyImport("numpy")
let arr = np.array(@[1.0, 2.0, 3.0].toNdArray)
discard py.print(arr)

nimpy: Exporting Nim to Python

nimpy can export Nim functions as a Python module. This is how you create Python extensions in Nim.

Basic Export

# mymodule.nim - filename MUST match the module name
import nimpy

proc greet*(name: string): string {.exportpy.} =
  return "Hello, " & name & "!"

Compilation

# Windows:
nim c --app:lib --out:mymodule.pyd --threads:on --tlsEmulation:off --passL:-static mymodule

# Linux/macOS:
nim c --app:lib --out:mymodule.so --threads:on mymodule

Using from Python

# test.py
import mymodule
assert mymodule.greet("world") == "Hello, world!"
assert mymodule.greet(name="world") == "Hello, world!"

Returning Complex Types (Tables, JsonNode)

nimpy supports returning Nim complex types that Python can use:

import nimpy, tables, json

proc getTable*(): Table[string, int] {.exportpy.} =
  result = {"Hello": 0, "SomeKey": 10}.toTable

proc getJsonAsDict*(): JsonNode {.exportpy.} =
  result = %*{
    "SomeKey": 1.0,
    "Another": 5,
    "Foo": [1, 2, 3.5, {"InArray": 5}],
    "Bar": {"Nested": "Value"}
  }

import mymodule

# Table becomes dict
table = mymodule.getTable()
assert table["Hello"] == 0
assert table["SomeKey"] == 10

# JsonNode becomes dict
json_obj = mymodule.getJsonAsDict()
assert json_obj["SomeKey"] == 1.0
assert json_obj["Foo"] == [1, 2, 3.5, {"InArray": 5}]

Type Mapping for Exports

Exporting Nim Types as Python Classes (Experimental)

nimpy can export Nim types as Python classes. This is useful for creating Python-native objects that wrap Nim state and behavior.

Requirements

• An exported type must be a ref object that inherits from PyNimObjectExperimental (directly or indirectly)
• At least one exported proc must have self as first argument to trigger type export
• Procs with self as first argument become methods on the Python type
• The type is only exported if requirements are met

Simple Example

# mymodule.nim
import nimpy

type
  TestType* = ref object of PyNimObjectExperimental
    myField*: string

proc setMyField*(self: TestType, value: string) {.exportpy.} =
  self.myField = value

proc getMyField*(self: TestType): string {.exportpy.} =
  self.myField

proc newTestType*(): TestType {.exportpy.} =
  TestType()

# test.py
import mymodule

tt = mymodule.newTestType()
tt.setMyField("Hello")
assert tt.getMyField() == "Hello"

__init__ and __repr__

nimpy provides special handling for initialization and representation:

__init__ - Export a proc as Python __init__ method when:

1. Proc name matches pattern init##TypeName
2. Has at least one argument
3. First argument named self
4. First argument type is ##TypeName
5. No return type

__repr__ - Export a proc as Python __repr__ method when:

1. Proc name is $
2. Has exactly one argument
3. First argument named self
4. First argument type is ##TypeName
5. Return type is string

Documentation - Set module and type docstrings:

setModuleDocString("This is a test module")
setDocStringForType(MyType, "This is a test type")

Complete Example

# simple.nim
import nimpy
import strformat

pyExportModule("simple")  # Only needed if filename differs from module name

type
  SimpleObj* = ref object of PyNimObjectExperimental
    a*: int

## Export as __init__ (tp_init)
proc initSimpleObj*(self: SimpleObj, a: int = 1) {.exportpy.} =
  echo "Calling initSimpleObj for SimpleObj"
  self.a = a

## Export as __repr__ (tp_repr)
proc `$`*(self: SimpleObj): string {.exportpy.} =
  &"SimpleObj(a={self.a})"

setModuleDocString("This is a test module")
setDocStringForType(SimpleObj, "This is a test type")

Compile:

nim c --app:lib -o:./simple.so ./simple.nim

Use in Python:

import simple

print(simple.__doc__)          # This is a test module
print(simple.SimpleObj.__doc__)  # This is a test type

obj = simple.SimpleObj(a=2)
print(obj)                    # SimpleObj(a=2)

Full Test Example

# texport_pytype.nim
import nimpy
import nimpy/py_types
import nimpy/py_lib as lib
import strformat

type
  PyCustomType* = ref object of PyNimObjectExperimental
    a*: int
    b*: float
    c*: string

proc initPyCustomType*(self: PyCustomType, aa: int = 1, bb: float = 2.0, cc: string = "default") {.exportpy} =
  self.a = aa
  self.b = bb
  self.c = cc

proc destroyPyCustomType*(self: PyCustomType) {.exportpy} =
  discard  # Cleanup if needed

proc `$`*(self: PyCustomType): string {.exportpy} =
  &"a: {self.a}, b: {self.b}, c: {self.c}"

proc get_a*(self: PyCustomType): int {.exportpy} =
  self.a

proc set_a*(self: PyCustomType, val: int) {.exportpy} =
  self.a = val

setModuleDocString("Test module for exported Python types")
setDocStringForType(PyCustomType, "Custom type with a, b, c fields")

import unittest

suite "Test Exported Python Types":
  let m = pyImport("texport_pytype")

  test "Test __doc__":
    check getAttr(m, "__doc__").`$` == "Test module for exported Python types"
    check getAttr(getAttr(m, "PyCustomType"), "__doc__").`$` == "Custom type with a, b, c fields"

  test "Test __init__ and methods":
    let constructor = getAttr(m, "PyCustomType")
    let obj = callObject(constructor, 99, 3.14, "hello")
    check obj.get_a().to(int) == 99
    obj.set_a(42)
    check obj.get_a().to(int) == 42

  test "Test __repr__":
    let constructor = getAttr(m, "PyCustomType")
    let obj = callObject(constructor, 99, 3.14, "hello")
    check obj.`$` == "a: 99, b: 3.14, c: hello"

Method Signatures

Type Mapping for Exports

Note: For Python class exports, the Nim type must inherit from PyNimObjectExperimental. See "Exporting Nim Types as Python Classes" above.

nimporter: Packaging Nim as Python Modules

nimporter builds on nimpy to provide seamless import and packaging for distribution.

Installation

pip install nimporter

Quick Start

# main.py
import nimporter  # Must import before Nim modules
import mymodule   # Compiled automatically!

result = mymodule.greet("world")

Two Concepts: Modules vs Libraries

Extension Modules (simple, direct import):

• Single .nim file
• No dependencies other than nimpy
• Cannot customize compiler switches
• Cannot import other Nim modules in same directory

Extension Libraries (full project):

• Folder with libname.nim, libname.nim.cfg, libname.nimble
• Can have external Nim dependencies
• Full folder structure supported
• CLI switches can be customized

Library Folder Structure

mylibrary/
    mylibrary.nim      # Must be present
    mylibrary.nim.cfg  # Must be present (can be empty)
    mylibrary.nimble   # Must contain `requires "nimpy"`

Distribution

# setup.py
import setuptools
from nimporter import get_nim_extensions, WINDOWS, MACOS, LINUX

setuptools.setup(
    name='mylibrary',
    install_requires=['nimporter'],
    py_modules=['mylibrary.py'],
    ext_modules=get_nim_extensions(platforms=[WINDOWS, LINUX, MACOS])
)

# Source distribution (end users need Nim compiler)
python setup.py sdist

# Binary distribution
python setup.py bdist_wheel

CLI Commands

nimporter clean      # Remove cached builds
nimporter compile    # Precompile all extensions
nimporter list       # List detected extensions

Docker Usage

# Precompile for Docker (no Nim needed in container)
nimporter compile

Ensure __pycache__ directories are included in Docker image.

cffi: Calling Nim from Python

cffi provides a Python interface to call compiled Nim shared libraries.

Nim Side: Export Functions

# called.nim
proc nim_add*(num1: int, num2: int): int {.exportc.} =
    return num1 + num2

Compile as a shared library:

nim c --app:lib called.nim
# Creates libcalled.so (or .pyd on Windows, .dylib on macOS)

Python Side: Call via cffi

from cffi import FFI

ffi = FFI()

ffi.cdef("""
    int nim_add(int num1, int num2);
""")

lib = ffi.dlopen("./libcalled.so")
result = lib.nim_add(5, 10)
print(result)  # 15

Type Mapping (Nim to C)

ctypes: Calling Nim from Python

ctypes is Python's built-in FFI library. Similar to cffi but uses stdlib only.

Nim Side: Export with Dynamic Library

# partitions.nim
proc partitions*(cards: var array[0..9, int], subtotal: int): int {. exportc, dynlib .} =
    var total: int
    result = 0

    for i in 0..9:
        if cards[i] > 0:
            total = subtotal + i + 1
            if total < 21:
                result += 1
                cards[i] -= 1
                result += partitions(cards, total)
                cards[i] += 1
            elif total == 21:
                result += 1

    return result

Compile:

nim c --app:lib --dynlib:yes partitions.nim

Python Side: Call via ctypes

#!/usr/bin/env python
from ctypes import *
import os

lib = cdll.LoadLibrary(os.path.abspath("libpartitions.so"))
lib.partitions.argtypes = [POINTER(c_long), c_long]
lib.partitions.restype = c_long

deck = [4] * 9
deck.append(16)

for i in range(10):
    deck[i] -= 1
    p = 0
    for j in range(10):
        deck[j] -= 1
        nums_arr = (c_long * len(deck))(*deck)
        n = lib.partitions(nums_arr, c_long(j + 1))
        deck[j] += 1
        p += n
    print(f'Dealer showing {i} partitions = {p}')
    deck[i] += 1

Comparison Reference

When to Use Which

• nimpy (call Python): Need Python libraries (Numpy, Scipy) in Nim application
• nimpy (export to Python): Create Python extension in Nim
• nimporter: Distribute Nim code to Python users with easy packaging
• cffi: Simple Nim→Python calls, want lightweight solution
• ctypes: stdlib-only solution, no extra dependencies

Common Patterns

Performance Optimization Pattern

# Identify hot path in Python
def slow_function():
    for i in range(1000000):
        # compute-intensive work

# Rewrite in Nim with {.exportpy.}, package with nimporter

Scientific Computing Pattern (nimpy)

import nimpy
import arraymancer
import scinim/numpyarrays  # For efficient numpy interop

let np = pyImport("numpy")
let scipy = pyImport("scipy")

# Use numpy/scipy directly
let result = scipy.special.gamma(nim_array.toNdArray)

Scripting Pattern

import nimpy

proc calculate*(x: float): float {.exportpy.} =
  result = x * 2.0

Troubleshooting

nimpy Import Error

If you get dynamic module does not define module export function:

• Ensure the Nim file name matches the Python module name exactly

nimpy libpython Not Found

pip install find_libpython
python3 -c 'import find_libpython; print(find_libpython.find_libpython())'

Then set nimpy.py_lib.pyInitLibPath.

GC Issues with Multiple Modules

For multiple nimpy modules, consider moving Nim runtime to a separate shared library. See Nim docs on DLL generation.

Windows Threads with MinGW

Use --tlsEmulation:off and link statically with --passL:-static on Windows.

Installation Quick Reference

# Nim compiler
# https://nim-lang.org/install.html

# Python packages
pip install nimporter    # For packaging Nim as Python modules
pip install cffi         # For cffi approach (optional, ctypes is stdlib)
pip install find_libpython  # For debugging libpython issues

# Nim packages
nimble install nimpy    # For calling Python from Nim and exporting to Python

Resources

• nimpy GitHub
• nimporter GitHub
• Nim for Python Programmers
• SciNim/numpyarrays for performance-critical numpy interop