Preview: Python 3.15

How to try it with Pixi

You don't need to change your project to experiment. pixi exec creates a temporary, cached environment from a spec and runs a command in it:

# a throwaway 3.15 REPL
pixi exec --spec "python=3.15" python

# run one file with 3.15
pixi exec --spec "python=3.15" python lazy_demo.py

# check the version
pixi exec --spec "python=3.15" python -VV

To test a project against 3.15 alongside its 3.14 default, add a feature and an environment to pixi.toml. The default environment keeps Python 3.14; the py315 environment uses 3.15.

[dependencies]            # shared by every environment; no python pin here
pytest = "*"

[feature.py314.dependencies]
python = "3.14.*"

[feature.py315.dependencies]
python = "3.15.*"

[environments]
default = ["py314"]       # the course baseline
py315 = ["py315"]         # the preview

pixi install -e py315               # solve and install the 3.15 environment
pixi run -e py315 python -VV         # Python 3.15.x
pixi run -e py315 pytest             # your test suite on 3.15
pixi shell -e py315                  # a shell with 3.15 active

Explicit lazy imports (PEP 810)

A normal import runs the whole module right away. Large CLIs often import dozens of heavy modules they never use in a given run, so --help takes a second. The new lazy soft keyword binds the name immediately but defers the actual import until the name is first used.

def export(rows):
    import json        # hidden dependency
    return json.dumps(rows)

lazy import json

def export(rows):
    return json.dumps(rows)

• lazy is allowed only at module scope: not inside functions, classes, or try/except blocks (those raise SyntaxError).
• lazy from x import * is not allowed.
• Import errors surface at first use, not at the lazy import line. Keep that in mind when reading tracebacks.
• Measure the payoff with python -X importtime. M39 covers startup profiling.

frozendict built-in (PEP 814)

Python had frozenset but no immutable mapping in the builtins. frozendict is a read-only, hashable dict: it can be a dict key, a set member, a safe default value, or a constant configuration table that nobody can mutate by accident.

from types import MappingProxyType
DEFAULTS = MappingProxyType({"retries": 3})
# read-only view, but not hashable

DEFAULTS = frozendict(retries=3)
cache[DEFAULTS] = "ok"   # hashable

Taught alongside immutability in M18 Functional Python.

sentinel built-in (PEP 661)

A sentinel is a unique marker value meaning "nothing was passed", used when None is itself a valid argument. The old idiom _MISSING = object() has an ugly repr (<object object at 0x...>) and does not survive copying or pickling well.

_MISSING = object()

def get(key, default=_MISSING): ...

MISSING = sentinel("MISSING")

def get(key, default=MISSING): ...

UTF-8 is the default encoding (PEP 686)

Until now, open(path) without encoding= used the locale encoding, which is UTF-8 on macOS and Linux but often cp1252 on Windows. The same script could read a file correctly on one machine and produce mojibake (or UnicodeDecodeError) on another. In 3.15 UTF-8 mode is on by default, so text I/O defaults to UTF-8 everywhere.

Unpacking in comprehensions (PEP 798)

You can now use * and ** directly in a comprehension's output expression to flatten one level. It reads exactly like the unpacking you already use in list and dict displays.

flat = [x for xs in lists for x in xs]
merged = {k: v for d in dicts for k, v in d.items()}

flat = [*xs for xs in lists]
merged = {**d for d in dicts}

The profiling package (PEP 799)

Profilers get a single home. profiling.tracing is the deterministic profiler you know as cProfile (which remains as an alias). profiling.sampling is new: the "Tachyon" sampling profiler, which periodically records the stack of a running process from the outside. Sampling has almost no overhead, so you can point it at a production process, and it can produce flame graphs or a live, top-like view.

# deterministic, function-level counts (formerly cProfile)
pixi run -e py315 python -m profiling.tracing -s cumtime app.py

# sample a script and write an interactive flame graph
pixi run -e py315 python -m profiling.sampling run --flamegraph -o profile.html app.py

# attach to a running process and watch it live
pixi run -e py315 python -m profiling.sampling attach --live 12345

The old pure-Python profile module is deprecated (see migration). M39 teaches profiling workflow on 3.14 with cProfile.

math.integer (PEP 791)

The integer-only functions of math get their own module: math.integer holds gcd, lcm, isqrt, comb, perm, and factorial. Keeping them separate from the floating-point functions makes it clear which results are exact. The old names in math continue to work.

See M34 Strings, Bits & Number Theory.

Smaller library improvements

TOML 1.1 in tomllib

tomllib now parses TOML 1.1: inline tables may span several lines and end with a trailing comma, the \e and \xHH escapes are allowed, and the seconds in times are optional.

Counter symmetric difference

Counter gains ^: for each key, the absolute difference of the counts.

asyncio.TaskGroup.cancel()

A task group can now be cancelled as a whole, which stops every task in it and exits the async with block cleanly. Before, you had to raise a custom exception and catch it outside.

Better "did you mean" suggestions

AttributeError suggestions now know common names from other languages, so [].push(1) suggests .append.

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

Typing

Typing on 3.14 is taught in M16. Type checkers adopt new PEPs on their own schedule; check your mypy version's release notes.

Interpreter and performance

pixi exec --spec "python=3.15" python -c "import sys; print(sys._jit.is_available())"
PYTHON_JIT=1 pixi run -e py315 python bench.py

Migration notes: 3.14 → 3.15

Upgrade checklist

• Add a py315 feature and environment, and run pixi install -e py315
• Run the tests with warnings as errors: pixi run -e py315 python -W error -m pytest
• Grep for open( calls without encoding= and fix them
• Replace import profile with profiling.tracing or cProfile
• Benchmark memory and latency against 3.14
• Add 3.15 to the CI matrix before making it the default

Check yourself

Flashcards and further reading

Going further

• What's New In Python 3.15
• PEP 810, PEP 814, PEP 686, PEP 798, PEP 799
• Back to the baseline: What's New in Python 3.14