Python 3.15 is out¶

Python 3.15.0 was released on 9 October 2026. It adds a lazy keyword for imports, a built-in frozendict, * unpacking inside comprehensions, and a new sampling profiler. It also makes UTF-8 the default text encoding on every platform, which is the change most likely to affect code you have already written.

Everything below comes from the official What's new in Python 3.15, which is the full list. This is the short one.

The parts you will notice¶

Lazy imports¶

Put lazy in front of an import and the module is not loaded until you first use the name (PEP 810).

lazy import json
lazy from pathlib import Path

print("Starting up...")  # json and pathlib are not fully imported yet

data = json.loads('{"key": "value"}')  # json gets fully imported here
p = Path(".")  # pathlib gets fully imported here

This is for programs with slow startup, mostly command-line tools that import a lot and use a little of it on any one run. The catch: if the module is missing, the error turns up at first use, not on the import line. A typo in a lazy import can sit there until that code path runs.

frozendict¶

A dictionary you can't change, and can therefore hash (PEP 814). It is a built-in, so there is nothing to import.

>>> a = frozendict(x=1, y=2)
>>> a
frozendict({'x': 1, 'y': 2})
>>> a['z'] = 3
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    a['z'] = 3
    ~^^^^^
TypeError: 'frozendict' object does not support item assignment
>>> b = frozendict(y=2, x=1)
>>> hash(a) == hash(b)
True
>>> a == b
True

Being hashable means it can be a dictionary key or a member of a set, which a dict can't. It keeps insertion order but ignores it when comparing, same as dict. It is not a subclass of dict, so isinstance(a, dict) is False.

Unpacking in comprehensions¶

* and ** now work inside comprehensions and generator expressions (PEP 798).

>>> lists = [[1, 2], [3, 4], [5]]
>>> [*L for L in lists]  # equivalent to [x for L in lists for x in L]
[1, 2, 3, 4, 5]

>>> dicts = [{'a': 1}, {'b': 2}, {'a': 3}]
>>> {**d for d in dicts}  # equivalent to {k: v for d in dicts for k,v in d.items()}
{'a': 3, 'b': 2}

Flattening a list of lists is one of the most-asked questions in the server. The old answer was a comprehension with two for clauses in an order nobody remembers. This one reads the way you would say it.

Error messages¶

AttributeError now recognises method names from other languages and tells you the Python one:

>>> [1, 2, 3].push(4)
AttributeError: 'list' object has no attribute 'push'. Did you mean '.append'?

>>> 'hello'.toUpperCase()
AttributeError: 'str' object has no attribute 'toUpperCase'. Did you mean '.upper'?

>>> {}.put("a", 1)
AttributeError: 'dict' object has no attribute 'put'. Use d[k] = v.

>>> (1, 2, 3).append(4)
AttributeError: 'tuple' object has no attribute 'append'. Did you mean to use a 'list' object?

If you came to Python from JavaScript or Java, this one is for you.

The part that can break things¶

Python now uses UTF-8 whenever you don't pass encoding (PEP 686). So open("notes.txt") reads UTF-8 on every platform.

On Linux and macOS that was almost always the case already. On Windows the default used to be the system code page, so a script that reads a file written by an older Windows program may now raise UnicodeDecodeError, or quietly read the wrong characters.

For people who measure things¶

There is a new profiling package (PEP 799). profiling.tracing is the deterministic profiler you know as cProfile, which stays as an alias. profiling.sampling is new. It is a sampling profiler called Tachyon that can attach to a process that is already running, by PID, and write flame graphs. The pure-Python profile module is deprecated and goes away in 3.17.

The experimental JIT compiler got a significant upgrade as well. It is still experimental.

Smaller things¶

• A sentinel built-in for making unique marker values, the job object() has been doing (PEP 661).
• re.prefixmatch() is the new name for what re.match() does. re.match() still works and is soft deprecated, meaning no removal is planned.
• The __cached__ module attribute is gone. Use __spec__.cached.
• Typing gained TypedDict with typed extra items (PEP 728) and TypeForm (PEP 747).
• The official macOS installer now includes the free-threaded build by default.

Should you upgrade?¶

Python 3.14 is supported until October 2030, so nobody has to hurry.

Trying it¶

Installers are on python.org. With uv, which keeps versions side by side without touching the one you already have:

$ uv self update
$ uv python install 3.15
$ uv run --python 3.15 python

Update uv first. A copy from before the release only knows about the release candidates.

Then type [1, 2, 3].push(4) and see what it says.