larzqr

QR codes from scratch — real Reed-Solomon error correction, all eight masks, zero dependencies.

Most "pure Python" QR libraries stop at drawing squares. larzqr implements the actual standard: Reed-Solomon ECC over GF(256), the finder / timing / alignment patterns, BCH-coded format and version information, and penalty-based mask selection — so the codes it emits are genuinely scannable. No numpy, no pillow, no C extension. Just the standard library.

It covers QR versions 1 through 10 in byte mode (up to ~270 bytes at EC level L), which comfortably fits URLs, Wi-Fi configs, and TOTP provisioning URIs from larztotp.

Install

pip install larzqr

Use

from larzqr import QR

qr = QR("https://example.com")
print(qr.to_text())              # scannable straight from a terminal

qr = QR("otpauth://totp/App:alice?secret=JBSWY3DPEHPK3PXP&issuer=App",
        error_correction="M")    # L, M, Q, or H
qr.to_svg("login.svg", scale=10) # crisp vector output
qr.matrix                        # the raw list-of-lists 0/1 module grid
qr.version, qr.mask              # picked automatically

Why it's correct, not just plausible

QR generators are easy to get almost right and impossible to eyeball. larzqr is anchored to published constants at every layer:

• Reed-Solomon is checked against the standard's worked v1-M example — data codewords in, the exact ECC bytes [196, 35, 39, 119, 235, 215, 231, 226, 93, 23] out.
• Format information matches the spec table (M/mask0 = 0x5412, L/mask0 = 0x77C4); version information matches the version-7 constant.
• GF(256) products are verified against hand-computed values (2 · 128 = 0x1d).
• Finder patterns, timing patterns, the dark module, and mask selection are all tested as structural invariants.

Tests

python -m unittest discover -s tests -v   # 29 tests

The Larz stack

One of 60+ pure-Python, zero-dependency libraries at github.com/larz-scripter. Pairs naturally with larztotp (turn a provisioning URI into a scannable enrollment code) and larzhtml (drop the SVG straight into a page).

License

MIT (c) larz-scripter