v26.3.1

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Stable style

• Prevent Jupyter notebook magic masking collisions from corrupting cells by using
  exact-length placeholders for short magics and aborting if a placeholder can no longer
  be unmasked safely (#​5038)

Configuration

• Always hash cache filename components derived from --python-cell-magics so custom
  magic names cannot affect cache paths (#​5038)

Blackd

• Disable browser-originated requests by default, add configurable origin allowlisting
  and request body limits, and bound executor submissions to improve backpressure
  (#​5039)

v26.3.0

Compare Source

Stable style

• Don't double-decode input, causing non-UTF-8 files to be corrupted (#​4964)
• Fix crash on standalone comment in lambda default arguments (#​4993)
• Preserve parentheses when # type: ignore comments would be merged with other
  comments on the same line, preventing AST equivalence failures (#​4888)

Preview style

• Fix bug where if guards in case blocks were incorrectly split when the pattern had
  a trailing comma (#​4884)
• Fix string_processing crashing on unassigned long string literals with trailing
  commas (one-item tuples) (#​4929)
• Simplify implementation of the power operator "hugging" logic (#​4918)

Packaging

• Fix shutdown errors in PyInstaller builds on macOS by disabling multiprocessing in
  frozen environments (#​4930)

Performance

• Introduce winloop for windows as an alternative to uvloop (#​4996)
• Remove deprecated function uvloop.install() in favor of uvloop.new_event_loop()
  (#​4996)
• Rename maybe_install_uvloop function to maybe_use_uvloop to simplify loop
  installation and creation of either a uvloop/winloop evenloop or default eventloop
  (#​4996)

Output

• Emit a clear warning when the target Python version is newer than the running Python
  version, since AST safety checks cannot parse newer syntax. Also replace the
  misleading "INTERNAL ERROR" message with an actionable error explaining the version
  mismatch (#​4983)

Blackd

• Introduce winloop to be used when windows in use which enables blackd to run faster on
  windows when winloop is installed. (#​4996)

Integrations

• Remove unused gallery script (#​5030)
• Harden parsing of black requirements in the GitHub Action when use_pyproject is
  enabled so that only version specifiers are accepted and direct references such as
  black @​ https://... are rejected. Users should upgrade to the latest version of the
  action as soon as possible. This update is received automatically when using
  psf/black@stable, and is independent of the version of Black installed by the
  action. (#​5031)

Documentation

• Expand preview style documentation with detailed examples for wrap_comprehension_in,
  simplify_power_operator_hugging, and wrap_long_dict_values_in_parens features
  (#​4987)
• Add detailed documentation for formatting Jupyter Notebooks (#​5009)