mojo-python-interop

var py_list = Python.list(1, 2.5, "three")
var py_tuple = Python.tuple(1, 2, 3)
var py_dict = Python.dict(name="value", count=42)
 
# Python.dict() is generic over a single value type V for all kwargs.
# Mixed types fail because the compiler can't infer one V.
# WRONG:  Python.dict(flag=my_bool, count=42)
# CORRECT: Python.dict(flag=PythonObject(my_bool), count=PythonObject(42))
 
# Literal syntax also works:
var list_obj: PythonObject = [1, 2, 3]
var dict_obj: PythonObject = {"key": "value"}

# Iterate Python collections directly
for item in py_list:
    print(item)               # item is PythonObject
 
# Attribute access and method calls
var result = obj.method(arg1, arg2, key=value)
 
# None
var none_obj = Python.none()
var obj: PythonObject = None      # implicit conversion works

# Expression
var result = Python.evaluate("1 + 2")
 
# Multi-line code as module (file=True)
var mod = Python.evaluate("def greet(n): return f'Hello {n}'", file=True)
var greeting = mod.greet("world")
 
# Add to Python path for local imports
Python.add_to_path("./my_modules")
var my_mod = Python.import_module("my_module")

def use_python() raises:
    try:
        var result = Python.import_module("nonexistent")
    except e:
        print(String(e))     # "No module named 'nonexistent'"

# Environment variables
# WRONG — using Python os module for env vars
# var os = Python.import_module("os")
# var val = os.environ.get("MY_VAR")
 
# CORRECT — Mojo has native env var access via std.os
from std.os import getenv
var val = getenv("MY_VAR")  # returns Optional[String]

# Sorting with custom key
# WRONG — Mojo has no lambda syntax
# var sorted = my_list.sort(key=lambda x: x["score"])
 
# CORRECT — Python.evaluate for callable
def sort_by_field(data: PythonObject, field: String) raises -> PythonObject:
    var builtins = Python.import_module("builtins")
    var key_fn = Python.evaluate("lambda x: x['" + field + "']")
    return builtins.sorted(data, key=key_fn)

# Dict .get() works on PythonObject
var name = data.get("name", PythonObject("unknown"))
var count = Int(py=data.get("count", PythonObject(0)))

from std.os import abort
from std.python import PythonObject
from std.python.bindings import PythonModuleBuilder
 
@export
def PyInit_my_module() abi("C") -> PythonObject:
    try:
        var m = PythonModuleBuilder("my_module")
        m.def_function[add]("add")
        m.def_function[greet]("greet")
        return m.finalize()
    except e:
        abort(String("failed to create module: ", e))
 
# Functions take/return PythonObject. Up to 6 args with def_function.
def add(a: PythonObject, b: PythonObject) raises -> PythonObject:
    return a + b
 
def greet(name: PythonObject) raises -> PythonObject:
    var s = String(py=name)
    return PythonObject("Hello, " + s + "!")

@fieldwise_init
struct Counter(Defaultable, Movable, Writable):
    var count: Int
 
    def __init__(out self):
        self.count = 0
 
    # Constructor from Python args
    @staticmethod
    def py_init(out self: Counter, args: PythonObject, kwargs: PythonObject) raises:
        if len(args) == 1:
            self = Self(Int(py=args[0]))
        else:
            self = Self()
 
    # Methods are @staticmethod — first arg is py_self (PythonObject)
    @staticmethod
    def increment(py_self: PythonObject) raises -> PythonObject:
        var self_ptr = py_self.downcast_value_ptr[Self]()
        self_ptr[].count += 1
        return PythonObject(self_ptr[].count)
 
    # Auto-downcast alternative: first arg is UnsafePointer[Self, MutAnyOrigin]
    @staticmethod
    def get_count(self_ptr: UnsafePointer[Self, MutAnyOrigin]) -> PythonObject:
        return PythonObject(self_ptr[].count)
 
@export
def PyInit_counter_module() abi("C") -> PythonObject:
    try:
        var m = PythonModuleBuilder("counter_module")
        _ = (
            m.add_type[Counter]("Counter")
            .def_py_init[Counter.py_init]()
            .def_method[Counter.increment]("increment")
            .def_method[Counter.get_count]("get_count")
        )
        return m.finalize()
    except e:
        abort(String("failed to create module: ", e))

from std.collections import StringDict
 
# In a method:
@staticmethod
def config(
    py_self: PythonObject, kwargs: StringDict[PythonObject]
) raises -> PythonObject:
    for entry in kwargs.items():
        print(entry.key, "=", entry.value)
    return py_self

import mojo.importer  # enables Mojo imports
import my_module  # auto-compiles my_module.mojo
 
print(my_module.add(1, 2))

# Wrap a Mojo value as a Python object (for bound types)
return PythonObject(alloc=my_mojo_value^)    # transfer ownership with ^
 
# Recover the Mojo value later
var ptr = py_obj.downcast_value_ptr[MyType]()
ptr[].field    # access fields via pointer