The first full beta of Python 3.15 has arrived, and it’s one of the most feature-packed Python releases in many a moon. Here’s a rundown of the biggest, boldest, and most important innovations, changes, and fixes.

Lazy imports

A long-asked for feature, lazy imports allow imports to be processed only when they’re actually used by the program. Thus for slow-importing modules that impose a large cost on a program’s startup time, you can now easily defer that cost to when the code of that module will actually be executed.

You can use lazy imports explicitly using the new lazy import syntax, but you can also force code with conventional imports to behave lazily, either programmatically or by using an environment variable. This makes it easy to make existing code take advantage of this feature without tons of rewriting. Best of all, there’s no drawback to making imports lazy: they otherwise behave exactly as intended.

The frozendict built-in type

Only rarely does Python add a new data type, but this is a long-debated and long-desired addition: the frozen dictionary. The frozendict behaves like a regular dictionary, except that it’s immutable (you can’t add, remove, or change elements) and it’s hashable (so you can use it as a key in another dictionary, for instance).

The sentinel() built-in type

Another new addition to the language is intended to replace a common and problematic Python pattern: creating a unique sentinel object (as an alternative to None where None could be a valid value, for example) by using object(). The new syntax ,sentinel("NAME"), creates unique objects that compare only to themselves via the is operator. These objects can be type-checked properly, and they have an informative representation instead of just a random object descriptor.

A statistical sampling profiler

The long-standing cProfile module profiles Python code deterministically—that is, it tracks and records every single call. That makes it precise, but it also means a cProfile-tracked program runs far slower than normal. A new profiling module in Python 3.15, profiling.sampling, uses statistical sampling methods to garner useful information about performance at a fraction of the impact on the program’s speed. The existing cProfile profiler is still available—it’s not going away—but has a new alternate name, profiling.tracing.

An upgraded JIT

CPython’s built-in just-in-time (JIT) compiler debuted in Python 3.13. Its long-term goals are to make Python programs run faster without any changes to code, in something of the same way the alternate Python runtime PyPy can speed things up. And it comes without the cost of changing to a totally different interpreter with some of its own limitations.

The first couple of revisions of the JIT didn’t promise, or deliver, a great deal of additional speed, as they were more about laying a foundation for future improvements. With Python 3.15, though, the JIT is now showing an 8% to 13% geometric mean performance improvement over standard CPython, depending on the platform and workload. The biggest changes include a new tracing front end (to enable more speedups on more kinds of code), the use of register allocation for faster and more memory-efficient work, better machine code generated by the JIT, and additional optimizations such as eliminating reference counts for some classes of objects.

Better error messages

Error messages in Python have been made more precise, detailed, and useful over the last couple of versions, and Python 3.15 continues that work. The highlights:

• Suggestions for missing names (“x has no attribute ‘y‘. Did you mean ‘xyz‘?”) now include suggestions from the members of a given object, and not just the object itself.
• Suggestions now also cover checks for deleting attributes, not just accessing them.
• If the interpreter can’t come up with a suggestion for a method based on fuzzy name matching via Levenshtein distance, it consults a list of names commonly used in other languages for such methods. For example, if you attempt to use list.push() (a JavaScript method), the interpreter suggests .append(), the proper method for Python lists.

Type system improvements

The TypedDict class, which lets you create dictionaries with predefined keys and type-hinted keys and values, adds support for two new arguments in its definition. The closed argument lets you specify if only the keys specified can be used at runtime. The extra_items argument lets you specify additional keys at runtime, but only keys with a value of a specified type.

The TypeForm type definition lets you represent the value that results from evaluating a type expression. With this, type annotations can be used in places where the type itself is being used as a value—for instance, variations on operations like typing.cast or even isinstance, or as part of how a third-party type-checking tool works.

Unpacking in comprehensions

This is another long-requested feature. If you wanted to completely unpack or “flatten” a nested object using a comprehension, you used to need a function like itertools.chain() or you would have to write a nested comprehension with an ugly syntax:

x = [[1,2,3],[4,5],[6]]
y = [a for b in x for a in b]
>>> [1, 2, 3, 4, 5, 6] # y

Unpacking in comprehensions using the star operator lets you save yourself a step:

x = [[1,2,3],[4,5],[6]]
y = [*a for a in x]
>>> [1, 2, 3, 4, 5, 6] # y

Unpacking with ** also works, for instance as a way to flatten and combine dictionaries:

dicts = [{'a': 1}, {'b': 2}, {'a': 3}]
y = {**d for d in dicts}
>>> {'a': 3, 'b': 2}

Finally, this kind of unpacking can also be used to form generator expressions:

(*x for x in ["ab","cd","ef"])

The expression above creates a generator that yields:

['a', 'b', 'c', 'd', 'e', 'f']

Reverting the incremental garbage collector

Finally, Python 3.15 takes an important U-turn. Python 3.14 featured a major change to its garbage collection system—an incremental garbage collector intended to reduce the amount of program-stopping time needed to collect garbage. Unfortunately, many users reported the new garbage collector increases process memory usage, sometimes dramatically. Python 3.15 will revert back to the older generational garbage collector used in Python 3.13 and before. The incremental collector may return in a future version, but not without additional work done on it to keep this problem from resurfacing.