What’s new in Python 3.15

Editor:

TBD

This article explains the new features in Python 3.15, compared to 3.14.

For full details, see the changelog.

Note

Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.15 moves towards release, so it’s worth checking back even after reading earlier versions.

Summary — release highlights

New features

High frequency statistical sampling profiler

A new statistical sampling profiler has been added to the new profiling module as profiling.sampling. This profiler enables low-overhead performance analysis of running Python processes without requiring code modification or process restart.

Unlike deterministic profilers (cProfile and profile) that instrument every function call, the sampling profiler periodically captures stack traces from running processes. This approach provides virtually zero overhead while achieving sampling rates of up to 1,000,000 Hz, making it the fastest sampling profiler available for Python (at the time of its contribution) and ideal for debugging performance issues in production environments.

Key features include:

  • Zero-overhead profiling: Attach to any running Python process without affecting its performance

  • No code modification required: Profile existing applications without restart

  • Real-time statistics: Monitor sampling quality during data collection

  • Multiple output formats: Generate both detailed statistics and flamegraph data

  • Thread-aware profiling: Option to profile all threads or just the main thread

Profile process 1234 for 10 seconds with default settings:

python -m profiling.sampling 1234

Profile with custom interval and duration, save to file:

python -m profiling.sampling -i 50 -d 30 -o profile.stats 1234

Generate collapsed stacks for flamegraph:

python -m profiling.sampling --collapsed 1234

Profile all threads and sort by total time:

python -m profiling.sampling -a --sort-tottime 1234

The profiler generates statistical estimates of where time is spent:

Real-time sampling stats: Mean: 100261.5Hz (9.97µs) Min: 86333.4Hz (11.58µs) Max: 118807.2Hz (8.42µs) Samples: 400001
Captured 498841 samples in 5.00 seconds
Sample rate: 99768.04 samples/sec
Error rate: 0.72%
Profile Stats:
      nsamples   sample%   tottime (s)    cumul%   cumtime (s)  filename:lineno(function)
      43/418858       0.0         0.000      87.9         4.189  case.py:667(TestCase.run)
    3293/418812       0.7         0.033      87.9         4.188  case.py:613(TestCase._callTestMethod)
  158562/158562      33.3         1.586      33.3         1.586  test_compile.py:725(TestSpecifics.test_compiler_recursion_limit.<locals>.check_limit)
  129553/129553      27.2         1.296      27.2         1.296  ast.py:46(parse)
      0/128129       0.0         0.000      26.9         1.281  test_ast.py:884(AST_Tests.test_ast_recursion_limit.<locals>.check_limit)
        7/67446       0.0         0.000      14.2         0.674  test_compile.py:729(TestSpecifics.test_compiler_recursion_limit)
        6/60380       0.0         0.000      12.7         0.604  test_ast.py:888(AST_Tests.test_ast_recursion_limit)
        3/50020       0.0         0.000      10.5         0.500  test_compile.py:727(TestSpecifics.test_compiler_recursion_limit)
        1/38011       0.0         0.000       8.0         0.380  test_ast.py:886(AST_Tests.test_ast_recursion_limit)
        1/25076       0.0         0.000       5.3         0.251  test_compile.py:728(TestSpecifics.test_compiler_recursion_limit)
    22361/22362       4.7         0.224       4.7         0.224  test_compile.py:1368(TestSpecifics.test_big_dict_literal)
        4/18008       0.0         0.000       3.8         0.180  test_ast.py:889(AST_Tests.test_ast_recursion_limit)
      11/17696       0.0         0.000       3.7         0.177  subprocess.py:1038(Popen.__init__)
    16968/16968       3.6         0.170       3.6         0.170  subprocess.py:1900(Popen._execute_child)
        2/16941       0.0         0.000       3.6         0.169  test_compile.py:730(TestSpecifics.test_compiler_recursion_limit)

Legend:
  nsamples: Direct/Cumulative samples (direct executing / on call stack)
  sample%: Percentage of total samples this function was directly executing
  tottime: Estimated total time spent directly in this function
  cumul%: Percentage of total samples when this function was on the call stack
  cumtime: Estimated cumulative time (including time in called functions)
  filename:lineno(function): Function location and name

Summary of Interesting Functions:

Functions with Highest Direct/Cumulative Ratio (Hot Spots):
  1.000 direct/cumulative ratio, 33.3% direct samples: test_compile.py:(TestSpecifics.test_compiler_recursion_limit.<locals>.check_limit)
  1.000 direct/cumulative ratio, 27.2% direct samples: ast.py:(parse)
  1.000 direct/cumulative ratio, 3.6% direct samples: subprocess.py:(Popen._execute_child)

Functions with Highest Call Frequency (Indirect Calls):
  418815 indirect calls, 87.9% total stack presence: case.py:(TestCase.run)
  415519 indirect calls, 87.9% total stack presence: case.py:(TestCase._callTestMethod)
  159470 indirect calls, 33.5% total stack presence: test_compile.py:(TestSpecifics.test_compiler_recursion_limit)

Functions with Highest Call Magnification (Cumulative/Direct):
  12267.9x call magnification, 159470 indirect calls from 13 direct: test_compile.py:(TestSpecifics.test_compiler_recursion_limit)
  10581.7x call magnification, 116388 indirect calls from 11 direct: test_ast.py:(AST_Tests.test_ast_recursion_limit)
  9740.9x call magnification, 418815 indirect calls from 43 direct: case.py:(TestCase.run)

The profiler automatically identifies performance bottlenecks through statistical analysis, highlighting functions with high CPU usage and call frequency patterns.

This capability is particularly valuable for debugging performance issues in production systems where traditional profiling approaches would be too intrusive.

(Contributed by Pablo Galindo and László Kiss Kollár in gh-135953.)

Improved error messages

  • The interpreter now provides more helpful suggestions in AttributeError exceptions when accessing an attribute on an object that does not exist, but a similar attribute is available through one of its members.

    For example, if the object has an attribute that itself exposes the requested name, the error message will suggest accessing it via that inner attribute:

    @dataclass
class Circle:
   radius: float

   @property
   def area(self) -> float:
      return pi * self.radius**2

class Container:
   def __init__(self, inner: Circle) -> None:
      self.inner = inner

circle = Circle(radius=4.0)
container = Container(circle)
print(container.area)

    Running this code now produces a clearer suggestion:

    Traceback (most recent call last):
File "/home/pablogsal/github/python/main/lel.py", line 42, in <module>
   print(container.area)
         ^^^^^^^^^^^^^^
AttributeError: 'Container' object has no attribute 'area'. Did you mean: 'inner.area'?

Other language changes

  • Python now uses UTF-8 as the default encoding, independent of the system’s environment. This means that I/O operations without an explicit encoding, e.g. open('flying-circus.txt'), will use UTF-8. UTF-8 is a widely-supported Unicode character encoding that has become a de facto standard for representing text, including nearly every webpage on the internet, many common file formats, programming languages, and more.

    This only applies when no encoding argument is given. For best compatibility between versions of Python, ensure that an explicit encoding argument is always provided. The opt-in encoding warning can be used to identify code that may be affected by this change. The special encoding='locale' argument uses the current locale encoding, and has been supported since Python 3.10.

    To retain the previous behaviour, Python’s UTF-8 mode may be disabled with the PYTHONUTF8=0 environment variable or the -X utf8=0 command line option.

    See also

    PEP 686 for further details.

    (Contributed by Adam Turner in gh-133711; PEP 686 written by Inada Naoki.)

  • Several error messages incorrectly using the term “argument” have been corrected. (Contributed by Stan Ulbrych in gh-133382.)

  • The interpreter now tries to provide a suggestion when delattr() fails due to a missing attribute. When an attribute name that closely resembles an existing attribute is used, the interpreter will suggest the correct attribute name in the error message. For example:

    >>> class A:
...     pass
>>> a = A()
>>> a.abcde = 1
>>> del a.abcdf
Traceback (most recent call last):
...
AttributeError: 'A' object has no attribute 'abcdf'. Did you mean: 'abcde'?

    (Contributed by Nikita Sobolev and Pranjal Prajapati in gh-136588.)

  • Unraisable exceptions are now highlighted with color by default. This can be controlled by environment variables. (Contributed by Peter Bierma in gh-134170.)

  • The __repr__() of ImportError and ModuleNotFoundError now shows “name” and “path” as name=<name> and path=<path> if they were given as keyword arguments at construction time. (Contributed by Serhiy Storchaka, Oleg Iarygin, and Yoav Nir in gh-74185.)

  • The __dict__ and __weakref__ descriptors now use a single descriptor instance per interpreter, shared across all types that need them. This speeds up class creation, and helps avoid reference cycles. (Contributed by Petr Viktorin in gh-135228.)

New modules

  • None yet.

Improved modules

dbm

  • Added new reorganize() methods to dbm.dumb and dbm.sqlite3 which allow to recover unused free space previously occupied by deleted entries. (Contributed by Andrea Oliveri in gh-134004.)

  • Add the 'm' flag for dbm.gnu.open() which allows to disable the use of mmap(2). This may harm performance, but improve crash tolerance. (Contributed by Serhiy Storchaka in gh-66234.)

difflib

hashlib

  • Ensure that hash functions guaranteed to be always available exist as attributes of hashlib even if they will not work at runtime due to missing backend implementations. For instance, hashlib.md5 will no longer raise AttributeError if OpenSSL is not available and Python has been built without MD5 support. (Contributed by Bénédikt Tran in gh-136929.)

http.client

  • A new max_response_headers keyword-only parameter has been added to HTTPConnection and HTTPSConnection constructors. This parameter overrides the default maximum number of allowed response headers. (Contributed by Alexander Enrique Urieles Nieto in gh-131724.)

http.cookies

  • Allow ‘"’ double quotes in cookie values. (Contributed by Nick Burns and Senthil Kumaran in gh-92936.)

locale

  • setlocale() now supports language codes with @-modifiers. @-modifier are no longer silently removed in getlocale(), but included in the language code. (Contributed by Serhiy Storchaka in gh-137729.)

math

mmap

  • mmap.mmap now has a trackfd parameter on Windows; if it is False, the file handle corresponding to fileno will not be duplicated. (Contributed by Serhiy Storchaka in gh-78502.)

os.path

resource

shelve

  • Added new reorganize() method to shelve used to recover unused free space previously occupied by deleted entries. (Contributed by Andrea Oliveri in gh-134004.)

sqlite3

  • The command-line interface has several new features:

    • SQL keyword completion on <tab>. (Contributed by Long Tan in gh-133393.)

    • Prompts, error messages, and help text are now colored. This is enabled by default, see Controlling color for details. (Contributed by Stan Ulbrych and Łukasz Langa in gh-133461.)

ssl

  • Indicate through ssl.HAS_PSK_TLS13 whether the ssl module supports “External PSKs” in TLSv1.3, as described in RFC 9258. (Contributed by Will Childs-Klein in gh-133624.)

  • Added new methods for managing groups used for SSL key agreement

    • ssl.SSLContext.set_groups() sets the groups allowed for doing key agreement, extending the previous ssl.SSLContext.set_ecdh_curve() method. This new API provides the ability to list multiple groups and supports fixed-field and post-quantum groups in addition to ECDH curves. This method can also be used to control what key shares are sent in the TLS handshake.

    • ssl.SSLSocket.group() returns the group selected for doing key agreement on the current connection after the TLS handshake completes. This call requires OpenSSL 3.2 or later.

    • ssl.SSLContext.get_groups() returns a list of all available key agreement groups compatible with the minimum and maximum TLS versions currently set in the context. This call requires OpenSSL 3.5 or later.

    (Contributed by Ron Frederick in gh-136306.)

  • Added a new method ssl.SSLContext.set_ciphersuites() for setting TLS 1.3 ciphers. For TLS 1.2 or earlier, ssl.SSLContext.set_ciphers() should continue to be used. Both calls can be made on the same context and the selected cipher suite will depend on the TLS version negotiated when a connection is made. (Contributed by Ron Frederick in gh-137197.)

  • Added new methods for managing signature algorithms:

    (Contributed by Ron Frederick in gh-138252.)

sys

  • Add sys.abi_info namespace to improve access to ABI information. (Contributed by Klaus Zimmermann in gh-137476.)

tarfile

types

unittest

zlib

Optimizations

module_name

  • TODO

Deprecated

CLI

  • Deprecate -b and -bb command line options and schedule them to become no-op in Python 3.17. These were primarily helpers for the Python 2 -> 3 transition. Starting with Python 3.17, no BytesWarning will be raised for these cases; use a type checker instead.

    (Contributed by Nikita Sobolev in gh-136355.)

hashlib

  • In hash function constructors such as new() or the direct hash-named constructors such as md5() and sha256(), their optional initial data parameter could also be passed a keyword argument named data= or string= in various hashlib implementations.

    Support for the string keyword argument name is now deprecated and is slated for removal in Python 3.19. Prefer passing the initial data as a positional argument for maximum backwards compatibility.

    (Contributed by Bénédikt Tran in gh-134978.)

Removed

ctypes

  • Removed the undocumented function ctypes.SetPointerType(), which has been deprecated since Python 3.13. (Contributed by Bénédikt Tran in gh-133866.)

glob

  • Removed the undocumented glob.glob0() and glob.glob1() functions, which have been deprecated since Python 3.13. Use glob.glob() and pass a directory to its root_dir argument instead. (Contributed by Barney Gale in gh-137466.)

http.server

  • Removed the CGIHTTPRequestHandler class and the --cgi flag from the python -m http.server command-line interface. They were deprecated in Python 3.13. (Contributed by Bénédikt Tran in gh-133810.)

pathlib

  • Removed deprecated pathlib.PurePath.is_reserved(). Use os.path.isreserved() to detect reserved paths on Windows. (Contributed by Nikita Sobolev in gh-133875.)

platform

  • Removed the platform.java_ver() function, which was deprecated since Python 3.13. (Contributed by Alexey Makridenko in gh-133604.)

sre_*

  • Removed sre_compile, sre_constants and sre_parse modules. (Contributed by Stan Ulbrych in gh-135994.)

sysconfig

threading

  • Remove support for arbitrary positional or keyword arguments in the C implementation of RLock objects. This was deprecated in Python 3.14. (Contributed by Bénédikt Tran in gh-134087.)

typing

  • The undocumented keyword argument syntax for creating NamedTuple classes (for example, Point = NamedTuple("Point", x=int, y=int)) is no longer supported. Use the class-based syntax or the functional syntax instead. (Contributed by Bénédikt Tran in gh-133817.)

  • Using TD = TypedDict("TD") or TD = TypedDict("TD", None) to construct a TypedDict type with zero field is no longer supported. Use class TD(TypedDict): pass or TD = TypedDict("TD", {}) instead. (Contributed by Bénédikt Tran in gh-133823.)

  • Code like class ExtraTypeVars(P1[S], Protocol[T, T2]): ... now raises a TypeError, because S is not listed in Protocol parameters. (Contributed by Nikita Sobolev in gh-137191.)

  • Code like class B2(A[T2], Protocol[T1, T2]): ... now correctly handles type parameters order: it is (T1, T2), not (T2, T1) as it was incorrectly infered in runtime before. (Contributed by Nikita Sobolev in gh-137191.)

wave

  • Removed the getmark(), setmark() and getmarkers() methods of the Wave_read and Wave_write classes, which were deprecated since Python 3.13. (Contributed by Bénédikt Tran in gh-133873.)

zipimport

Porting to Python 3.15

This section lists previously described changes and other bugfixes that may require changes to your code.

Build changes

  • Removed implicit fallback to the bundled copy of the libmpdec library. Now this should be explicitly enabled with --with-system-libmpdec set to no or with --without-system-libmpdec. (Contributed by Sergey B Kirpichev in gh-115119.)

C API changes

New features

Porting to Python 3.15

Deprecated C APIs

Removed C APIs

  • Remove deprecated PyUnicode functions:

    • PyUnicode_AsDecodedObject(): Use PyCodec_Decode() instead.

    • PyUnicode_AsDecodedUnicode(): Use PyCodec_Decode() instead; Note that some codecs (for example, “base64”) may return a type other than str, such as bytes.

    • PyUnicode_AsEncodedObject(): Use PyCodec_Encode() instead.

    • PyUnicode_AsEncodedUnicode(): Use PyCodec_Encode() instead; Note that some codecs (for example, “base64”) may return a type other than bytes, such as str.

    (Contributed by Stan Ulbrych in gh-133612.)

  • PyImport_ImportModuleNoBlock(): deprecated alias of PyImport_ImportModule(). (Contributed by Bénédikt Tran in gh-133644.)

The following functions are removed in favor of PyConfig_Get(). The pythoncapi-compat project can be used to get PyConfig_Get() on Python 3.13 and older.