Python 3.15: frozendict, UTF-8 by Default, and More

1 minute read

Python 3.15 is shaping up to be a significant release. The feature set is now frozen as the second beta landed in June, and the final release is expected this fall.

frozendict is Here

After years of discussion, PEP 814 delivers a built-in frozendict — an immutable dictionary that’s hashable and can be used as a dictionary key or set member.

from frozendict import frozendict

config = frozendict({"host": "localhost", "port": 8080})
# config["port"] = 9090  # TypeError: 'frozendict' object does not support item assignment

cache = {config: "cached_value"}  # Works because frozendict is hashable

UTF-8 Mode is Now the Default

PEP 686 makes UTF-8 mode the default on all platforms. No more locale.getpreferredencoding() returning something surprising. If you need the old behavior for compatibility, set PYTHON_UTF8=0.

Sentinel Values

PEP 661 adds a standard way to create sentinel values — unique objects you can use as defaults or markers:

from sentinel import Sentinel

NOT_FOUND = Sentinel("NOT_FOUND")

def find_value(key, data):
    result = data.get(key, NOT_FOUND)
    if result is NOT_FOUND:
        return None
    return result

Unpacking in Comprehensions

PEP 798 allows * and ** unpacking inside comprehensions:

items = [(1, 2), (3, 4)]
results = [x + y for x, *rest in items]  # Works in 3.15

dicts = [{"a": 1}, {"b": 2}]
merged = {**d for d in dicts}  # {'a': 1, 'b': 2}

Type System Improvements

PEP 800 introduces @typing.disjoint_base, a decorator that tells the type checker two types are mutually exclusive:

@typing.disjoint_base(AsyncIterable)
class Iterable: ...

value: Iterable[int] | AsyncIterable[int]  # Properly narrowed

Frame Pointers for Profiling

PEP 831 makes frame pointers the default on supported platforms, making system profilers (like Linux perf) work out of the box with Python without installing debug symbols.

What’s Next

Beta 4 is due July 18, release candidates start August 4, and the final 3.15.0 lands this fall. Now’s the time to test your projects against the betas.

Alexander Pitkin