Serc

An experimental CPython bytecode compiler, written in Rust.

Warning

Serc is experimental. All code changes, PR summaries, and additional documentation were authored entirely by GPT-6 Astra in Codex. Use at your own risk.

Highlights

• Compile Python source to CPython .pyc files without a CPython installation.
• Target Python 3.12, 3.13, 3.14, and 3.15 with a single executable.
• Check source files and recursively compile packages from the command line.
• Embed the compiler via the Rust library.

Serc produces byte-for-byte identical output on an extensive corpus of real-world Python code, including Django, Airflow, and Transformers, >40% faster than CPython's built-in compiler:

Project	Serc	CPython 3.14.5 compileall	Speedup
NumPy 2.4.6 (505 files)	1.194 s ± 0.023 s	1.739 s ± 0.060 s	1.46×

But Serc is also experimental! And so may emit incorrect bytecode in some cases.

Installation

Install from crates.io with Rust 1.96 or later:

cargo install serc-cli --locked

Getting started

Given hello.py:

def greet(name):
    return f"Hello, {name}!"

Compile it to bytecode, emitted at __pycache__/hello.cpython-314.pyc:

serc compile hello.py

Pass a directory to compile its Python files recursively, or use --check to compile without writing cache files:

serc compile path/to/package
serc compile --check path/to/package

Compilation uses the available CPUs by default. Use --jobs (or -j) to set a limit, including --jobs 1 for serial compilation. You can also set the default with RAYON_NUM_THREADS:

serc compile --jobs 4 path/to/package

Use --files-from - to read a list of files from standard input. Paths are newline-delimited by default; --null accepts NUL-delimited paths, including filenames that contain newlines:

find path/to/package -type f -name '*.py' -print0 | serc compile --files-from - --null

The list selects individual files without scanning directories. On Unix, paths retain their original bytes; on Windows, the list must use UTF-8.

Current .pyc files are left untouched by default. We compare the target bytecode format and invalidation mode, then the source's timestamp and size or its content hash. Pass -f or --force to rebuild every file, including after changing the compiler or native build profile. --check always compiles the sources without writing bytecode. Existing bytecode destinations must be regular files; compilation rejects symlinks and special files, including with --force.

For integrations, --output-format json reports the numbers of compiled and skipped files:

serc compile --files-from - --null --output-format json

Python version

Serc emits Python 3.14-compatible bytecode by default, but supports target selection with --python-version. For example, to emit Python 3.15 bytecode:

serc compile --python-version 3.15 path/to/package

Serc supports the following CPython minor versions:

Target	Pinned CPython release
3.12	3.12.14
3.13	3.13.15
3.14	3.14.5
3.15	3.15.0rc2

Each target writes its own cache tag and bytecode magic number, so the four versions can coexist under __pycache__.

Optimization

Use -O to remove assertions and set __debug__ to false, matching Python's -O mode. Use -OO to additionally remove module, class, and function docstrings:

serc compile -O hello.py
serc compile -OO hello.py

You can also select the level explicitly with --optimize 0, --optimize 1, or --optimize 2. Level 0 is the default. Optimized caches use Python's naming convention, such as __pycache__/hello.cpython-314.opt-1.pyc and __pycache__/hello.cpython-314.opt-2.pyc, so all three levels can coexist.

Rust library

Add Serc to your Cargo.toml:

[dependencies]
serc = "0.1.0"

Compile Python source and serialize the resulting code object in CPython's marshal format:

let module = serc::compile("print('Hello, world!')\n", "hello.py").unwrap();
let marshaled = module.marshal();

Compilation defaults to Python 3.14. See the library guide for selecting another Python version and generating .pyc files.

License

Serc is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Serc by you, as defined in the Apache-2.0 license, shall be dually licensed as above, without any additional terms or conditions.