Limitations

Monty is not a Python implementation aiming for completeness. It implements enough Python for a model to express what it wants to do, and deliberately stops there. Everything it does implement is meant to behave exactly like CPython 3.14; everywhere it does not, the divergence is written down.

This page gives you the shape of the subset. The other pages in this section are the exhaustive record, one per builtin, module or construct. They list every known divergence, including the ones that feel obvious, so a behaviour missing from them can be assumed to match CPython 3.14. They exist for development and for agents debugging code that runs on Monty; most users need only this page.

!!! tip

Turn on [type checking](../type-checking.md) rather than memorising this page.
Unsupported APIs generally fail before they run; see [the caveats](../type-checking.md#caveats) for the exceptions.

Language features

Supported:

def, async def, nested functions, closures, lambda
Decorators on functions and classes
Simple classes: instance methods, __init__, __repr__/__str__, __eq__/__hash__, __iter__/__next__, __contains__, __index__, class variables
@dataclass, with the eq= and frozen= options only (every other option raises NotImplementedError, and there is no field(), fields() or asdict()), plus host class instances passed in and out (and host classes the sandbox may instantiate when granted)
List, dict and set comprehensions
try / except / else / finally, raise ... from ...
for, while, if / elif / else, break, continue, pass, assert, global, nonlocal, return
with statements, for files and for classes implementing __enter__ / __exit__
f-strings (including the = debug form) and str.format(), with !r / !s / !a conversions, format specs and nested replacement fields
async / await, and asyncio.run / asyncio.gather
import x, import x.y, from x import y, z as w
Starred unpacking everywhere CPython allows it

Rejected at parse time, with NotImplementedError before any code runs:

Class inheritance and metaclasses (class Foo(Bar):)
Decorators on methods — so no @classmethod, @staticmethod, @property
yield / yield from — there are no generator functions. Generator expressions parse, but currently materialise to a list
match statements
del, both del x and del d[k]
try* / except* exception groups
PEP 695 type aliases
async with, async for and async comprehensions
Wildcard imports (from m import *)
Complex literals (1j) and t-strings

Missing in other ways:

User-defined exception classes. The built-in exception types are a fixed set, and without inheritance you cannot add to it.
Function attributes. fn.__name__, fn.__doc__ and friends raise AttributeError, and new attributes cannot be set — so functools.wraps-style metadata copying and registries keyed on fn.__name__ have no equivalent.
eval, exec, compile, globals, locals, __import__ and super — all raise NameError.
Third-party packages. There is no sys.path and no site-packages.

Standard library

The following modules are present:

Module	Divergences
`asyncio`	asyncio.md
`base64`	base64.md
`binascii`	base64.md
`collections`	collections.md
`dataclasses`	dataclasses.md
`datetime`	datetime.md
`functools`	functools.md
`itertools`	itertools.md
`json`	json.md
`math`	math.md
`os`	os.md
`pathlib`	pathlib.md
`re`	re.md
`sys`	sys.md
`typing`	typing.md
`unicodedata`	unicodedata.md

Each covers only part of its CPython surface — often a small part. The absent names are missing from the module namespace rather than stubbed, so they fail type checking as well as raising AttributeError at runtime.

Notably absent: enum, contextlib, random, time, io, copy, string, struct, operator, inspect, logging, traceback, hashlib, uuid, urllib. Some of those are absent by design — socket, subprocess, multiprocessing, threading and ctypes would breach the sandbox — and others are simply not implemented yet.

The authoritative list is modules.md.

Things that work but not quite like CPython

A few divergences are worth knowing up front because they change how code behaves rather than whether it runs. Each links to the page that owns it, which is where the full account lives:

assert failures get pytest-style messages. assert 2 == 5 raises AssertionError: assert 2 == 5, not CPython's empty AssertionError. Turn it off with assert_message_annotations=False on checkout() (assert.md).
enumerate, zip, map, filter and reversed are eager, not lazy. So map(f, itertools.count()) runs until a resource limit trips (builtins.md).
re is backed by Rust's fancy-regex, not CPython's engine: no bytes patterns, no VERBOSE flag, and some error messages differ (re.md).
Only the class dunders listed above are dispatched. __lt__, __len__, __getitem__, __call__ and the arithmetic dunders raise TypeError as if undefined, while __bool__ and the __getattr__ family are ignored silently, so an instance is always truthy (classes.md).
There is no event loop inside the sandbox. async / await work, and asyncio exposes exactly two functions: run and gather, the latter running host calls concurrently. create_task, sleep and everything else do not exist (asyncio.md).
The format() builtin and %-formatting are not implemented. Use str.format() or f-strings (format.md).
Only UTF-8, ASCII, UTF-16 and UTF-32 codecs exist. latin-1 and friends raise LookupError (encoding.md).

How to go deeper

For a specific feature, open the page in this section named after the builtin, module or construct. The pages are the limitations/ directory of the repository, published verbatim. If you hit something that is neither in the subset nor on these pages, open an issue.