How uv got so fast

Optimizations that don’t need Rust Some of uv’s speed comes from Rust. But not as much as you’d think. Several key optimizations could be implemented in pip today: HTTP range requests for metadata. Wheel files are zip archives, and zip archives put their file listing at the end. uv tries PEP 658 metadata first, falls back to HTTP range requests for the zip central directory, then full wheel download, then building from source. Each step is slower and riskier. The design makes the fast path cover 99% of cases. This is HTTP protocol work, not Rust. Parallel downloads. pip downloads packages one at a time. uv downloads many at once. This is concurrency, not language magic. Global cache with hardlinks. pip copies packages into each virtual environment. uv keeps one copy globally and uses hardlinks (or copy-on-write on filesystems that support it). Installing the same package into ten venvs takes the same disk space as one. This is filesystem ops, not language-dependent. Python-free resolution. pip needs Python running to do anything, and invokes build backends as subprocesses to get metadata from legacy packages. uv parses TOML and wheel metadata natively, only spawning Python when it hits a setup.py-only package that has no other option. PubGrub resolver. uv uses the PubGrub algorithm, originally from Dart’s pub package manager. pip uses a backtracking resolver. PubGrub is faster at finding solutions and better at explaining failures. It’s an algorithm choice, not a language choice

No .egg support. Eggs were the pre-wheel binary format. pip still handles them; uv doesn’t even try. The format has been obsolete for over a decade. No pip.conf. uv ignores pip’s configuration files entirely. No parsing, no environment variable lookups, no inheritance from system-wide and per-user locations. No bytecode compilation by default. pip compiles .py files to .pyc during installation. uv skips this step, shaving time off every install. You can opt in if you want it. Virtual environments required. pip lets you install into system Python by default. uv inverts this, refusing to touch system Python without explicit flags. This removes a whole category of permission checks and safety code. Stricter spec enforcement. pip accepts malformed packages that technically violate packaging specs. uv rejects them. Less tolerance means less fallback logic. Ignoring requires-python upper bounds. When a package says it requires python<4.0, uv ignores the upper bound and only checks the lower. This reduces resolver backtracking dramatically since upper bounds are almost always wrong. Packages declare python<4.0 because they haven’t tested on Python 4, not because they’ll actually break. The constraint is defensive, not predictive. First-index wins by default. When multiple package indexes are configured, pip checks all of them. uv picks from the first index that has the package, stopping there. This prevents dependency confusion attacks and avoids extra network requests. Each of these is a code path pip has to execute and uv doesn’t.

PEP 658 went live on PyPI in May 2023. uv launched in February 2024. The timing isn’t coincidental. uv could be fast because the ecosystem finally had the infrastructure to support it. A tool like uv couldn’t have shipped in 2020. The standards weren’t there yet.

I’m writing this on October 15th, 2024. Last week I would’ve said you probably shouldn’t be using uv’s Python in production, because you wouldn’t be getting security updates to OpenSSL. This week, I would tentatively say that it’s fine. This makes me a little uncomfortable, because there may well be other issues I haven’t thought of, and uv is still very new.

The uv-provided Python executable is slower than the one shipped by Ubuntu 24.04 LTS, but it’s faster than the “official” Docker image.

The ability to install Python with uv adds interesting possibilities for production packaging. For example, you can use an Ubuntu 24.04 base Docker image, download uv, and rely on uv to trivially install any Python version. Which is to say, you won’t be limited to the versions Ubuntu packages for you.

Unlike most Python packaging tools, uv doesn’t require Python to be installed to use it.

Despite lively discussion on the role that UV-based technologies can play in reducing SARS-CoV-2 transmission [[27], [28], [29], [30]], available data on its use and impact are still scant, setting-specific and heterogeneous in terms of study design and assessed outcomes

an UV-C dose of just 3.7 mJ/cm2 was sufficient to achieve a more than 3-log inactivation

Researchers offer first proof that Ultraviolet C light with a 222 nm wavelength — which is safer to use around humans — effectively kills the SARS-CoV-2 virus.