Standard library

The modules of the standard library that the runtime implements, and their limits. Anything not listed here is refused at compile time with file:line.

datetime (with time zones, zoneinfo, Pydantic and isoformat formats, strptime without %f/%Z, combine, fromtimestamp), decimal (_pydecimal rules: 28 digits, ROUND_HALF_EVEN, exact add/mul then rounding, CPython's division, quantize, round, formatting; mixing with float refused), uuid, json, re (fancy-regex with Python syntax translated; consecutive empty matches follow Rust, not Python 3.7+; pos/endpos of a compiled pattern's methods refused), csv (simplified Sniffer; DictReader(restval=, restkey=); a positional dialect refused), io.StringIO/BytesIO, pathlib/open() (UTF-8, POSIX; Path.read_text/write_text(encoding=, errors=) with the codecs below), os.environ/os.getenv (read-only), os.path, os.remove/os.unlink (str or Path; OSErrors carry errno, strerror, filename), math, random (OS-seeded), secrets, hashlib, hmac, base64, urllib.parse, string constants, time.time/monotonic/perf_counter, statistics.median, logging (loggers, handlers, formatters, filters, basicConfig, dictConfig, LoggerAdapter, tracebacks: see Logging), python-json-logger's JsonFormatter, email.mime (incl. MIMEBase with set_payload and email.encoders.encode_base64)/email.utils (formataddr, formatdate, make_msgid), html.escape, html.unescape (CPython's HTML5 entity table, longest-prefix rule and numeric references), ipaddress.ip_address/ip_network (prefix length, strict=; membership, str/repr; no netmask form, no IPv6 scope id), socket.getaddrinfo (the C library's answer; family and kind are plain ints, not AddressFamily/SocketKind members; gaierror carries only its message), codecs of str.encode, bytes.decode and str(b, encoding): utf-8, utf-8-sig, latin-1 and ascii with their CPython aliases (errors= strict, ignore or replace when encoding, strict when decoding; any other codec raises), unicodedata.normalize/combining (the Unicode version of the Python that ran the translation: code points it leaves unassigned are left alone, as CPython does), bytes(), pickle (see below), functools.wraps, inspect.iscoroutinefunction, importlib.metadata.version("literal") (resolved at compile time from what uv sync --frozen installs: the project of the pyproject.toml next to a uv.lock at --root or above, or a package of that lock; any other name raises PackageNotFoundError; refused without such a lock), typing.get_args/get_origin/get_type_hints, collections.defaultdict with a builtin type factory (int, list, str...) or deque (type() of it reports dict), zipfile.ZipFile(io.BytesIO(), "w", ZIP_STORED | ZIP_DEFLATED, compresslevel=) with writestr(name, str | bytes, compress_type=, compresslevel=), namelist, close/with (CPython's bytes: same headers, 0o600 permissions, local time of the call, zlib raw deflate; a mode other than a literal "w" is refused; write(path), ZipInfo, ZIP64 and a file object other than io.BytesIO raise), socket.create_connection((host, port), timeout=) (blocking connect, the last failure raised as CPython does without all_errors; the socket supports with, close, getpeername, getsockname), cryptography's RSA keys (rsa.generate_private_key(65537 or 3, key_size >= 1024), private_bytes in PEM or DER, PKCS8 or TraditionalOpenSSL, NoEncryption() only; public_key().public_bytes as SubjectPublicKeyInfo or PKCS1; key_size), ... as a value (a "not given" sentinel default compared with is), collections.Counter (from an iterable, a mapping or keywords; a missing key reads 0 and is not stored; most_common, update/subtract, elements, total, copy, + - | & between Counters, CPython's repr; type() of it reports dict, and == compares like a dict, where CPython 3.10+ ignores zero counts), collections.deque (maxlen, append/pop on both ends, extend(left), rotate, remove, index, count, indexing; JSON-encoded as a list by FastAPI), string.Formatter().vformat/format, string.Template (substitute/safe_substitute, CPython's KeyError and invalid-placeholder ValueError). str.format and Formatter support {}/{0}/{name}, !r/!s and format specs; attribute/index fields ({a.b}, {a[0]}), nested specs and !a raise. Not yet: other Formatter methods.

pickle

pickle.dumps/loads use CPython's format (protocol 5 when the project targets Python ≥ 3.14, else 4): bytes written by the binary are read by CPython and the other way round (e.g. a Redis cache shared with Python workers). Supported: scalars, str/bytes, containers (shared references kept), datetime types, time zones, Decimal, UUID, enums, project class instances (__dict__ or __slots__), dataclasses, Pydantic models. Refused: mapped (ORM) objects, other globals, out-of-band buffers. Integers beyond 64 bits read back as integral Decimals.