Python language / parser

Monty parses Python source with Ruff's parser but rejects several constructs at parse time. Anything listed below raises NotImplementedError (prefixed with "The monty syntax parser does not yet support ") at compile time, before any code runs.

Statements rejected at parse time

class definitions — bare class Foo: ... is not supported. There is no in-sandbox class factory: @dataclass, typing.NamedTuple, and collections.namedtuple are all unavailable inside the sandbox (and collections is not importable). Host-supplied dataclass / namedtuple values can be passed in and used; use a plain function or a host-defined type for new structured data. See classes.md.
with / async with statements — no context manager protocol. This means no with open(...) as f: (call f.close() explicitly). See open.md.
yield / yield from expressions — no generator functions. Generator expressions ((x for x in ...)) parse but currently materialize to a list rather than a lazy iterator (this is a known temporary divergence; see iter__generator_expr_type.py).
match statements — structural pattern matching is not supported.
del statements — neither del x nor del d[k] parse.
try* / except* exception groups — PEP 654 syntax rejected.
type aliases (PEP 695 type Foo = int).
async for loops and async comprehensions.
Wildcard imports (from m import *) — raises ImportError: "Wildcard imports (\from ... import *`) are not supported"`.

Expressions rejected at parse time

Starred expressions in expression position (e.g. [*xs, *ys], f(*args)). Function calls with *args unpacking are not supported.
Multiple **kwargs unpacking in a single call (f(**a, **b)).
Complex number literals (1j, 2+3j).
Template strings (t-strings) — PEP 750.
Walrus operator (:=) — also rejected.

Source nesting depth

AST nesting is capped at 200 levels (30 in debug builds); exceeding it raises SyntaxError: Source is too deeply nested.
The budget is shared across every nesting-producing construct (parens, calls, subscripts, attribute chains, operators, comprehensions, control-flow blocks, with, etc.), including the synthetic nesting from a flat multi-item with — see with.md.
The message differs from CPython, which uses construct-specific wording (too many nested parentheses, too many statically nested blocks, …).

Imports

Only the bundled stdlib modules listed in modules.md can be imported. Importing anything else raises ModuleNotFoundError.
Relative imports (from . import x) raise ImportError: "attempted relative import with no known parent package" — there is no package system.
__import__ is not defined.

What does work

Functions (def, async def), nested functions, closures, decorators.
List / dict / set comprehensions (generator comprehensions degrade to lists — see above).
try / except / else / finally, raise ... from ....
for / while / if / elif / else, break, continue, pass, assert, global, nonlocal, return.
import x, import x.y, from x import y, z as w.
f-strings including = debug specifier, !r/!s/!a conversions, and format specs.