Python 3.15 changes four defaults, starting with UTF-8 as the text encoding

A default only reaches code that leaves the choice open. Under PEP 686, a path that reads or writes text without naming an encoding now gets UTF-8 ^[1]. Those call sites look the same before and after the upgrade, so a diff review will not find them. I'd search for them before the first 3.15 deploy, starting with file handling that has never had to say what encoding it expected.

PEP 810 works the other way. Taken literally, "explicit" means the startup saving goes only to imports that are marked for it ^[4]. Existing import statements should behave as they did on 3.14. The test work arrives with the edit that marks them, one module at a time.

The JIT figures are the release team's own ^[2][3]. The announcement does not name the benchmark suite behind either geometric mean. A geometric mean can also hold benchmarks that slowed down alongside ones that sped up a lot. For the Linux figure to transfer, a service's hot paths have to look like that suite, and it has to run on x86-64 Linux ^[2]. Its operators also have to accept a compiler that the release notes and Phoronix both still call experimental ^[6][10].

A plain before-and-after benchmark measures more than the JIT. Add the encoding change to the three build and release defaults above and the release highlights list four default changes between 3.14 and 3.15 ^[14]. A team timing a Windows service on 3.14 and again on 3.15 gets one number that covers a different interpreter build ^[8], frame pointers ^[7] and the new encoding ^[1].

The frame-pointer default is the change I'd defend hardest. PEP 799 lands in the same release with a dedicated profiling package and Tachyon, a high-frequency statistical sampling profiler ^[5]. The default gives system-level tools a view of the interpreter ^[7], and the standard library now has a sampler for the Python side ^[5].

Barry Warsaw marked the release with whatsnewt, a text adventure for the terminal ^[11]. "I had this idea to whip up a little TUI adventure game that takes you through a What's New in Python 3.15," he wrote ^[11]. Each of its eighteen puzzles requires a real 3.15 feature, with the code written and run in a 3.15 interpreter ^[12]. It is also a cheap way to put code in front of 3.15 before the CI matrix has one: `uvx --python 3.15 whatsnewt` ^[13].