Python versions

py2axum supports projects running on Python 3.12, 3.13, 3.14 and 3.15. Messages and behaviours that changed between these versions follow the Python the project runs on, so the binary answers what your Python answers: ZeroDivisionError texts, the TypeError of an unhashable dict key, base64 and base32 decoding, datetime.strptime, the MIME types FileResponse guesses, and more (the full list is below).

This page reviews every change of What's New in Python 3.14 and What's New in Python 3.15 that can show up in a FastAPI, Pydantic or SQLAlchemy application. It also lists the message changes those pages do not mention, found by running the same code on each Python. Python 3.15.0 was released on 9 October 2026; the latest 3.14 is 3.14.8.

Which Python the binary follows

In order:

1. --python X.Y (on py2axum, py2axum check and py2axum watch), or the PY2AXUM_TARGET_PYTHON environment variable;
2. the project's .python-version (as uv and pyenv write it), at --root or above;
3. the Python running py2axum, when the project's requires-python (in uv.lock or pyproject.toml) allows it. This is the usual case of a Docker build, where py2axum runs on the image's Python;
4. else, the lowest Python that requires-python allows.

The Python running py2axum must be at least as recent as the project's: the parser only knows its own syntax. A Python outside 3.12–3.15 is refused (P2A0601).

A few behaviours changed inside a minor: strptime('%e') is accepted from 3.13.6, and the security releases 3.13.13 and 3.14.4 rewrote base64's padding rules and a JSON escape message. Distributions backport such fixes without changing the version (Debian's 3.13.5-2+deb13u5 has the new base64 rules, not the other two). So when py2axum runs on the project's minor and no patch release is named, it probes these behaviours on itself and the binary reproduces what it found. Otherwise the binary follows the patch release named by --python 3.13.5 or .python-version, else the newest; PY2AXUM_PYTHON_VERSION (e.g. 3.13.5) overrides both when the binary starts. py2axum check prints the Python it targets and what it probed.

Probed behaviour	Changed in	Without a probe
base64.b64decode padding rules (data after = decoded)	3.13.13, 3.14.4	the named patch, else the newest
datetime.strptime accepts %e	3.13.6	the named patch, else the newest
json.loads('"\\u0041'): unterminated string (was "Invalid \uXXXX escape")	3.13.13, 3.14.4	the named patch, else the newest

The probe measures the Python that translates, not the one in production. Translate with the same Python as production: the same image (the builder stage of a Dockerfile FROM the production image), or the same python:3.13 official build. Translating on Debian's python3 for an application served by the official python:3.13 image (or the reverse) can pick the other side of these three behaviours; name the patch release then (--python 3.13.5 or .python-version).

sys.version_info

sys.version_info is read at translation time, against the project's Python:

• comparisons with a tuple of integers (sys.version_info >= (3, 14)), .major, .minor, [0], [1] and [:2] become constants;
• an if whose test is decided this way keeps only the branch taken. A module imported only under a test the project's Python fails is never translated, and never needs to parse. So a project can hold 3.15 syntax behind if sys.version_info >= (3, 15): and still be translated for 3.14;
• what depends on the patch level (.micro, sys.version_info >= (3, 14, 2) for a 3.14 project, sys.version, sys.hexversion) is refused with file:line.

Status

Status	Meaning
Supported	Translated with the behaviour of the project's Python; a conformance case proves it (fixtures/dynapp: pyver.py on every Python, py314.py and py315.py on the Pythons that have their syntax)
Refused	Refused at translation with file:line and an error code, or by a run-time TypeError of py2axum where the value is only known then (noted). --python-side leaves the route to Python
Difference	Translated, with a documented difference
N/A	Not observable by an HTTP client of a web API (packaging, interpreter internals, command-line tools, platforms), or concerns a library py2axum does not translate
To do	Not reproduced or not checked yet: listed for a later version

Python 3.14

Syntax and semantics

Change	Status	Notes
Template strings t"..." (PEP 750)	Refused (P2A0117)	No string.templatelib in the runtime; use an f-string
except A, B: without brackets (PEP 758)	Supported	/py314/except
return/break/continue leaving a finally (PEP 765)	Supported	Only a SyntaxWarning, same behaviour; /py314/finally
Deferred evaluation of annotations (PEP 649, 749)	Supported	A model can name a class defined after it, unquoted (/py314/forward); an annotation naming an undefined name is never evaluated (/py314/lazy-annotation)
annotationlib	Refused (P2A0201)	Not in the library map
from __future__ import annotations (deprecated, unchanged)	Supported	As before
More SyntaxErrors (__debug__, misplaced await)	N/A	The project does not parse on 3.14 either
Mixed real/complex arithmetic (C99 rules)	N/A	Complex arithmetic is not translated
json decoding is bounded by the C stack instead of 10 000 levels (Stack overflow (used 8144 kB) while decoding a JSON array… past ~50 000 levels on an 8 MiB stack)	Difference	The binary keeps 10 000 levels (RecursionError: maximum recursion depth exceeded while decoding a JSON array from a unicode string): a limit on untrusted input (Supported subset, Security). A document between the two depths passes json.loads in Python, then usually exceeds the recursion limit elsewhere (FastAPI's jsonable_encoder): the same 500, another traceback

Builtins and error messages

Change	Status	Notes
Unhashable dict key / set element: cannot use 'list' as a dict key (unhashable type: 'list')	Supported	Since 0.3
math domain errors name the expected input (expected a nonnegative input, got -1.0)	Supported	Since 0.3
json errors carry notes (when serializing list item 1, when serializing dict item 'a')	Supported	/pyver/json_notes; __notes__, add_note() and the notes printed by traceback.format_exception* are supported on every Python (/pyver/notes)
_ and , separators in the fractional part of a format spec (f"{x:.6_f}")	Supported	/pyver/fmt_frac; a ValueError before 3.14, as in CPython
bytes.fromhex() accepts bytes	Refused (P2A0203)	bytes.fromhex is not in the runtime
float.from_number(), complex.from_number()	Refused (P2A0203)
map(..., strict=True)	Refused (P2A0209)
NotImplemented in a boolean context raises TypeError	Refused (P2A0105)	NotImplemented is not translated
int() no longer falls back to __trunc__()	N/A	Number protocols of project classes are not translated
Three-argument pow() tries __rpow__; functools.partial as a method descriptor; super objects copyable	N/A	Not translated
repr(Union[int, str]) is int \| str; Union and UnionType are one type	Refused (run time)	The text of a type expression (str(), repr(), f-string) raises a py2axum TypeError: it depends on the Python and on the classes' modules
Clearer SyntaxErrors (keyword typos, elif after else...)	N/A	The project parses
with/async with on a value without that protocol names the missing method and suggests the other statement	Supported	/pyver/cm/{kind}; the type is named like CPython (_thread.lock; a class written in Python with its module from 3.14, asyncio.locks.Semaphore). Difference: from 3.14, a project class is named without its module (A, where CPython says app.main.A)

Standard library

Change	Status	Notes
uuid.uuid7() (RFC 9562)	Supported	CPython's algorithm (48-bit ms time, 42-bit counter monotonic within a millisecond); /py314/uuid7; refused for an older project (P2A0603)
uuid.uuid6(), uuid.uuid8(), uuid.NIL, uuid.MAX	Refused (P2A0201)
contextvars.Token as a context manager (with var.set(x):)	Supported	/pyver/token_with; TypeError before 3.14, as in CPython
Path.copy(), copy_into(), move(), move_into()	Supported	/pyver/path_copy; follow_symlinks=False and preserve_metadata=True raise a py2axum TypeError. Path.info is refused
re: \z is \Z; \B matches an empty string	Supported	/pyver/re_z, /pyver/re_B_empty; \z is a bad escape before 3.14
mimetypes: many new types (fonts, .yaml, .7z, .docx...)	Supported	FileResponse guesses with CPython's built-in table of the project's Python, then the host's mime.types files, like mimetypes.init(); /pyver/mime/{name}. mimetypes.guess_type itself is refused (P2A0201)
pickle default protocol 5	Supported	Since 0.3
date.strptime(), time.strptime()	Refused (P2A0201)	datetime.strptime is supported
Decimal.from_number(), Fraction.from_number(), Fraction(obj with as_integer_ratio)	Refused
functools.Placeholder, reduce(initial=)	Refused (P2A0201)	functools.reduce is not in the library map
heapq max-heap functions	Refused (P2A0201)
operator.is_none(), is_not_none()	Refused (P2A0201)
os.reload_environ()	Refused (P2A0201)
compression.zstd, compression.gzip/zlib/bz2/lzma	Refused (P2A0201)	gzip and zlib under their usual names are supported
concurrent.interpreters, InterpreterPoolExecutor	Refused (P2A0201)
asyncio: create_task(**kwargs), call graphs, get_event_loop() raising without a loop	N/A	A route always runs inside the loop; the call-graph functions are not translated
logging.handlers.QueueListener as a context manager	N/A	Not translated (the logging work of 0.8)
inspect.signature(annotation_format=), io.Reader/Writer, ast additions	N/A	Not translated
unicodedata 16.0	Supported	unicodedata.normalize/combining and str.isalpha()... follow the project's Python: the runtime's tables (Unicode 17.0) less what that Python's Unicode does not assign (py2axum/ucdages.py, from tools/gen_ucdages.py), even when the translating Python is newer; /libs/unicodedata
hmac, ssl, http.server, tarfile, zipfile, configparser, argparse, sqlite3, unittest, getpass, faulthandler...	N/A	Not used by a request handler, or not translated

Deprecations, removals, runtime

Change	Status	Notes
Removals (asyncio child watchers, ast.Num/Str, pathlib extra arguments, urllib URLopener, itertools pickling, email.utils.localtime(isdst=)...)	N/A	None of them is translated
Deprecations (asyncio.iscoroutinefunction, the policy system, codecs.open...)	N/A	A warning only
Free-threaded build officially supported (PEP 779)	N/A	The binary has no GIL either; an endpoint's results do not depend on it
Incremental GC in 3.14.0–3.14.4 (back to generational in 3.14.5)	N/A
Tail-call interpreter, JIT, -X context_aware_warnings, -X thread_inherit_context, remote debugging	N/A

Python 3.15

Syntax and semantics

Change	Status	Notes
Unpacking in comprehensions: [*x for x in xs], {*x for ...}, (*x for ...), {**m for m in ms} (PEP 798)	Supported	/py315/unpack, with CPython's errors (Value after * must be an iterable, not int, 'int' object is not a mapping); ** of a mapping that is not a dict raises a py2axum TypeError
lazy import x, lazy from m import n (PEP 810)	Supported for libraries; Refused (P2A0117) for project modules	A library import has no observable start-up order; a project module's top-level code would run at start-up in the binary, at its first use in Python. __lazy_modules__ and sys.set_lazy_imports() are not translated
Unary plus in match literal patterns (case +1:)	Supported	/py315/match
UTF-8 is the default encoding (PEP 686)	Supported	The binary always reads and writes UTF-8 without encoding=
module.__cached__ removed, ImportError repr, import locks	N/A

Builtins and error messages

Change	Status	Notes
frozendict (PEP 814)	Refused (P2A0105)
sentinel (PEP 661)	Refused (P2A0105)
bytes.replace(..., count=) as a keyword	Supported	/pyver/bytes_replace_kw (a TypeError before 3.15, as in CPython); bytes.replace itself was missing from the runtime and is now supported on every Python
bytearray.take_bytes(), slice[...], memoryview complex formats, __slots__ changes	N/A	Not translated
Timestamps and timeouts accept any real number (Decimal, Fraction)	Supported	datetime.fromtimestamp (/pyver/fromtimestamp_decimal, with 3.14's reworded error before: argument must be int or float, not decimal.Decimal) and socket.create_connection(timeout=) (/pyver/timeout_*: must be real number, not str). The other C functions taking a timestamp or a timeout (time.sleep, time.localtime, date.fromtimestamp, threading, select...) are not translated (P2A0201). asyncio's delays are Python code, unchanged: asyncio.sleep/wait_for add the delay to the loop's float time (a Decimal fails, except asyncio.sleep on uvloop), /pyver/aio/*
AttributeError hints from other languages (Did you mean '.append'?)	Difference	str(exc) is unchanged; traceback.format_exception_only adds the hint in Python (on every Python for a close name, from 3.15 for these), not in the binary. Reading an attribute no runtime type has is refused anyway (P2A0204)

Standard library

Change	Status	Notes
base64.urlsafe_b64decode no longer requires padding; padded=	Supported	/pyver/b64_unpadded, /py315/padded; python-jose and PyJWT follow it (their base64url_decode)
base64.b32decode rewritten in C (new messages: Only base32 data is allowed, Excess padding not allowed...)	Supported	/pyver/b32_*; pyotp follows it
Other new base64/binascii parameters (wrapcol=, ignorechars=, canonical=), binascii base32/85 functions	Refused (P2A0209, P2A0201)	The base64 functions accept only the parameters the runtime reads
re.prefixmatch(), Pattern.prefixmatch()	Supported	/py315/prefixmatch; refused for an older project (P2A0603)
datetime.strptime: %d without a year is an error	Supported	/pyver/strptime_d (and %e, accepted from 3.13.6)
os.urandom(-1) (behind secrets.token_*): size cannot be negative	Supported	/pyver/token_negative
json.loads(array_hook=), json accepting frozendict	Refused (P2A0209)
urllib.parse.urlsplit(missing_as_none=), urlunsplit(keep_empty=)	Refused (P2A0209)
math.integer, math.isnormal/issubnormal/fmax/fmin/signbit	Refused (P2A0201)
asyncio.TaskGroup.cancel()	Refused (P2A0201)	asyncio.TaskGroup is not translated
contextlib.ContextDecorator keeps the context open across a decorated generator or coroutine	N/A	@contextmanager used as a decorator is not translated
collections.Counter ^ (symmetric difference)	Supported	/pyver/counter_xor; before 3.15 the TypeError names Counter (the runtime said dict on every Python)
functools.singledispatchmethod, threading iterator helpers, pprint(expand=)	Refused
typing: TypeForm, TypedDict(closed=, extra_items=) (PEP 728), @disjoint_base	Refused	A TypedDict as a request or response type is refused (P2A0410); elsewhere it is a dict, unchanged
email refuses to flatten a non-ASCII address it cannot encode (HeaderWriteError)	N/A	Only under a policy with header objects (EmailMessage, policy.default), not translated (P2A0201). The compat32 messages the runtime builds (MIMEText, MIMEMultipart...) encode such an address whole on every Python, unchanged (/mail/build?to=...)
unicodedata 17.0 and its new functions	Refused (P2A0201) for the new functions; Supported for normalize, combining	See 3.14
importlib.metadata, hashlib, http.client, http.cookies, ssl, sqlite3, tomllib 1.1, xml, zlib, os.statx, Path.mkdir(parent_mode=)...	N/A	Not used by a request handler, or not translated

Deprecations, removals, runtime

Change	Status	Notes
Removals (typing.NamedTuple keyword syntax, TypedDict("TD"), sre_*, glob0, CGI server, PurePath.is_reserved, Loader.load_module...)	N/A	None of them is translated
Deprecations (profile, -b, .pth imports, hashlib(string=)...)	N/A
profiling package, upgraded JIT, mimalloc, frame pointers, free-threading by default on macOS installers	N/A

Changes the What's New pages do not list

Found by running the same code on 3.12, 3.13, 3.14 and 3.15 (/pyver/{name} reproduces each one):

Change	From	Status
ZeroDivisionError says division by zero for every operator and type (integer modulo by zero, float floor division by zero, float divmod()... before; float modulo became float modulo by zero in 3.13)	3.14	Supported
0.0 ** -1: zero to a negative power (was 0.0 cannot be raised to a negative power)	3.14	Supported
too many values to unpack (expected 2, got 3) for a list, tuple or dict (a str or an iterator: unchanged)	3.14	Supported
dict([1]): TypeError: object is not iterable with the note Cannot convert dictionary update sequence element #0 to a sequence	3.14	Supported
date/time/datetime range errors name the value (month must be in 1..12, not 13, day 29 must be in range 1..28 for month 2 in year 2025)	3.14	Supported
list.index: list.index(x): x not in list	3.14	Supported
in on a non-container: argument of type 'int' is not a container or iterable	3.14	Supported
str/bytes methods called without their argument: count expected at least 1 argument, got 0 (was count() takes at least 1 argument (0 given))	3.13	Supported (and bytes from 3.15 for replace)
str.replace(old, new, count=n) accepted as a keyword	3.13	Supported (it was ignored by the runtime before)
datetime.strptime accepts %e	3.13.6	Supported
base64.b64decode: data after the padding is decoded, excess = skipped (non-strict)	3.13.13, 3.14.4	Supported (/pyver/b64_after_padding)
%-formatting errors name the argument and the position (format argument 2: %d requires a real number, not str, not enough arguments for format string (got 1), stray % at position 3)	3.15	Supported (/pyver/pct_*)
socket.getaddrinfo (and create_connection, asyncio.open_connection): the idna codec's check of the host raises UnicodeEncodeError naming the position (label too long, label empty), where 3.12 raises UnicodeError: label empty or too long	3.13	Supported (/libs/ssrf, /stdmore/sock); before, the binary passed such a host to the C library
re.error is named re.PatternError (re.error an alias)	3.13	Supported: type(e).__name__ and tracebacks follow the project's Python (/pyver/re_error_name, /py314/pattern-error); re.PatternError is refused for a 3.12 project (P2A0603)
socket.create_connection(timeout=) (C settimeout): 'Fraction' object cannot be interpreted as an integer (3.13: integer, 3.14: integer or float and fractions.Fraction); out of range: timestamp out of range for platform time_t for a float, too large to convert to C _PyTime_t for an int (PyTime_t from 3.13), then timestamp out of range for C PyTime_t for both from the security releases 3.13.15 and 3.14.7 (probed on a distribution's Python: pytime_range)	3.13, 3.14	Supported (/pyver/timeout_*); before, a NaN or out-of-range timeout aborted the request

Differences found and fixed on every Python

The review also found places where the binary differed from Python whatever its version, now fixed: format() (the builtin was not translated; f"{n:_}" grouped with commas; Unknown format code lacked the code), bytes.replace (missing), dict(seq) element errors, divmod() by zero, list.count()/"a".count() without an argument (a crash of the request), base64.b32decode(map01=) (ignored), the type named by an operator error on a Counter or a defaultdict, the type named by a context-manager error (_thread.lock), the datetime.fromtimestamp error, and the MIME types of FileResponse (a 20-entry table instead of CPython's, and text/plain for an unknown type where Starlette 1.1+ says application/octet-stream).