`.pyc` files are larger than they need to be

[brandtbucher]

Python 3.11 made .pyc files almost twice as large. There are two main reasons for this:

• PEP 659 made the bytecode stream ~3x as large as 3.10.
• PEP 657 made the location tables ~9x as large as 3.10.

(Note that these effects compound each other, since longer bytecode means more location entries.)

However, there is low-hanging fruit for improving this situation in 3.12:

• Bytecode can be compressed using a fairly simple scheme (one byte for instructions without an oparg, two bytes for instructions with an oparg, and zero bytes for CACHE entries). This results in serialized bytecode that is ~66% smaller than 3.11.
• The location table format already has a mechanism for compressing multiple code units into a single entry. Currently it's only used for EXTENDED_ARGs and CACHEs corresponding to a single instruction, but with slight changes the compiler can use the same mechanism to share location table entries between adjacent instructions. This is a double-win, since it not only makes .pyc files smaller, but also shrinks the memory footprint of all code objects in the process. Experiments show that this makes location tables ~33% smaller than 3.11.

When both of these optimizations are applied, .pyc files become ~33% smaller than 3.11. This is only ~33% larger than 3.10, despite all of the rich new debugging information present.

Linked PRs

• GH-99554: marshal bytecode more efficiently #99555
• GH-99554: Pack location tables more effectively #99556
• GH-99554: Trim trailing whitespace #100435

[stonebig]

as a remark, packing a directory of distros of about 600 same packages on windows:

• with .7z compaction, python-3.11 one is 5% bigger than the 3.10 one (754 mo vs 713 Mo)
• with no compaction it's 9% bigger (3.9 Go vs 3.57 Go)
• sympy-1.11.1 is the most inflated package by about 90% (117 Mo vs 61 Mo), and only shrinks back to 101 Mo with 3.12a7

[hugovk]

Triage: can this issue be closed or is there more to do?