Differences from CPython¶

petitpy (developed as nanopython) runs most everyday Python unchanged, but it is a MicroPython descendant, and 0.8 is an alpha. This page lists what behaves differently from CPython 3.12+, so you can tell a bug from a known limit. Each entry was checked against CPython and the build it names (2026-10).

There are two builds:

native (pip install petitpy, uvx petitpy; the command is petitpy or ppy): the full profile, with arbitrary-precision integers, Unicode strings, files, os, subprocess and sockets;
browser (npm install petitpy, the playground): the nano profile, smaller, with direct DOM access through the js module.

Language and built-in types¶

Area	nanopython	CPython
dict order	Iteration follows hash order: `list({"b": 1, "a": 2})` is `['a', 'b']`. Affects printing, `json.dumps`, `**kwargs`. Insertion order is planned after 0.8.	Insertion order.
`str.upper()` / `lower()`	ASCII letters only; no `unicodedata`.	Full Unicode case mapping.
Integers (browser)	64-bit: `2**100` raises `OverflowError`. Native has arbitrary precision.	Arbitrary precision.
Float literals (browser)	A literal with 17 significant digits can parse one ulp off. Native parses correctly rounded; `repr` is the shortest round-trip form on both.	Correctly rounded.
Complex numbers	`2j` does not compile.	Supported.
`slice(a, b)`	The constructor is unavailable; slice syntax, including steps, works.	Available.
Implicit exception chaining	`__context__` is always `None`; `raise ... from` works, with `__cause__` and a chained traceback.	Recorded and printed.
Class `__annotations__`	An `OrderedDict` of the annotations' source text, as under `from __future__ import annotations`.	Evaluated objects (or lazy, 3.14).
`__mro__`	Not available on classes; multiple inheritance works.	Available.
Exception text	MicroPython's: `ValueError('x',)` as repr, `KeyError: x` unquoted, `divide by zero`.	`ValueError('x')`, `KeyError: 'x'`, `division by zero`.
Tracebacks	File names and functions, no line numbers or source lines.	Line numbers and source.

Library and platform¶

Area	nanopython	CPython
Standard library	A subset: `json`, `re`, `math`, `itertools`, `functools`, `collections`, `dataclasses`, `datetime`, `pathlib`, `typing`, `enum`, `random`, `string`, `struct`, … (what runs). No `asyncio` (coroutines run, there is no event loop), `csv`, `decimal`, `fractions`, `unicodedata`, `sqlite3`, `ssl`.	The full library.
`sys.platform`	`"petitpy"`.	`"linux"`, `"darwin"`, …
`json.dumps`	Does not escape non-ASCII (`ensure_ascii` is not applied).	Escapes to `\u00e9` by default.
Files (native)	A write is visible to a reread only after `close()` or `flush()`.	Same rule, but CPython closes a dropped file at once (reference counting).
Files and `os` (browser)	No `open()` or `os`; programs see the files the page passes in.	Full file system and `os` module.
Threads	`_thread` is a stub.	Available.
`import os.nope`	Raises `AttributeError`.	Raises `ModuleNotFoundError`.

Reporting a difference¶

If something else behaves differently from CPython, it is a bug or an unknown gap: please report it with the smallest program that shows it.