Python 3.15 makes UTF-8 the default encoding and adds explicit lazy imports

The Python Software Foundation released Python 3.15.0 on October 9, 2026. Two changes matter most for existing code. UTF-8 is now the default text encoding regardless of the operating system's settings, and a new lazy import statement lets a program put off loading a module until it first uses it, which can shorten startup time.

The change that can affect existing code

Under PEP 686, a call such as open("notes.txt") without an encoding argument now uses UTF-8 instead of whatever the system environment says. The Python documentation recommends always passing an explicit encoding. Projects that need the old behavior can turn UTF-8 mode off with PYTHONUTF8=0 or -X utf8=0.

This suggests the change is most visible on Windows, where the system encoding has often not been UTF-8: code that read or wrote files in the local encoding may now handle the same files differently.

Lazy imports and other additions

PEP 810 adds the lazy keyword. With lazy import json, Python does not load the module until the name is first accessed. The keyword works only at module level, and -X lazy_imports=all or PYTHON_LAZY_IMPORTS=all makes every import lazy.

The release adds two built-in types: frozendict, an immutable dictionary that is hashable when its contents are, and sentinel, for unique marker values. Comprehensions now support unpacking, and a new profiling package includes Tachyon, a high-frequency sampling profiler. The old profile module is deprecated and scheduled for removal in Python 3.17.

According to the release page, the experimental JIT compiler is 7 to 8% faster than the standard interpreter on x86-64 Linux and 11 to 12% faster than the tail-calling interpreter on AArch64 macOS, measured as a geometric mean. The official macOS installers now include free-threading support by default, and PEP 803 gives free-threaded builds a stable ABI.

Before upgrading

The release page warns that IDLE and other tkinter applications may hang when opening dialogs on macOS 27.0, and it suggests waiting with that macOS upgrade until a workaround is available. Python 3.15 gets bug-fix releases about every two months for two years and security fixes until about October 2031.