Simon Willison’s Weblog

How uv got so fast. Andrew Nesbitt provides an insightful teardown of why uv is so much faster than pip. It's not nearly as simple as just "they rewrote it in Rust" - uv gets to skip a huge amount of Python packaging history (which pip needs to implement for backwards compatibility) and benefits enormously from work over recent years that makes it possible to resolve dependencies across most packages without having to execute the code in setup.py using a Python interpreter.

Two notes that caught my eye that I hadn't understood before:

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. None of this requires Rust.

[...]

Compact version representation. uv packs versions into u64 integers where possible, making comparison and hashing fast. Over 90% of versions fit in one u64. This is micro-optimization that compounds across millions of comparisons.

I wanted to learn more about these tricks, so I fired up an asynchronous research task and told it to checkout the astral-sh/uv repo, find the Rust code for both of those features and try porting it to Python to help me understand how it works.

Here's the report that it wrote for me, the prompts I used and the Claude Code transcript.

You can try the script it wrote for extracting metadata from a wheel using HTTP range requests like this:

uv run --with httpx https://raw.githubusercontent.com/simonw/research/refs/heads/main/http-range-wheel-metadata/wheel_metadata.py https://files.pythonhosted.org/packages/8b/04/ef95b67e1ff59c080b2effd1a9a96984d6953f667c91dfe9d77c838fc956/playwright-1.57.0-py3-none-macosx_11_0_arm64.whl -v

The Playwright wheel there is ~40MB. Adding -v at the end causes the script to spit out verbose details of how it fetched the data - which looks like this.

Key extract from that output:

[1] HEAD request to get file size...
    File size: 40,775,575 bytes
[2] Fetching last 16,384 bytes (EOCD + central directory)...
    Received 16,384 bytes
[3] Parsed EOCD:
    Central directory offset: 40,731,572
    Central directory size: 43,981
    Total entries: 453
[4] Fetching complete central directory...
    ...
[6] Found METADATA: playwright-1.57.0.dist-info/METADATA
    Offset: 40,706,744
    Compressed size: 1,286
    Compression method: 8
[7] Fetching METADATA content (2,376 bytes)...
[8] Decompressed METADATA: 3,453 bytes

Total bytes fetched: 18,760 / 40,775,575 (100.0% savings)

The section of the report on compact version representation is interesting too. Here's how it illustrates sorting version numbers correctly based on their custom u64 representation:

Sorted order (by integer comparison of packed u64):
  1.0.0a1 (repr=0x0001000000200001)
  1.0.0b1 (repr=0x0001000000300001)
  1.0.0rc1 (repr=0x0001000000400001)
  1.0.0 (repr=0x0001000000500000)
  1.0.0.post1 (repr=0x0001000000700001)
  1.0.1 (repr=0x0001000100500000)
  2.0.0.dev1 (repr=0x0002000000100001)
  2.0.0 (repr=0x0002000000500000)