vastlint (Python)
High-performance, in-process VAST XML validation for Python backends.
Rule reference: vastlint.org/docs/rules · Web validator: vastlint.org/validate · Test a live tag URL: vastlint.org/tester · Inspect wrapper chains: vastlint.org/inspect
This package wraps the stable vastlint-ffi C API, backed by the same Rust core used by the CLI, Go binding, Ruby gem, Erlang NIF, MCP server, and web validator. The intended use case is a DSP, SSP, ad server, or trafficking backend that needs to validate VAST XML and return structured linting results to a frontend.
Why this shape
- No subprocess management in your Python app
- No network hop to an external validation service
- Stable JSON-compatible result shape for backend-to-frontend responses
- Same rule coverage and behavior as the rest of the vastlint ecosystem
Install
The package bundles a platform-matched libvastlint shared library inside the wheel, so no compiler or Rust toolchain is needed at install time.
pip install vastlint
You can also point the package at an explicit shared library with VASTLINT_LIB_PATH:
export VASTLINT_LIB_PATH=/absolute/path/to/libvastlint.dylib
For development in this monorepo, it automatically falls back to the sibling vastlint/target/debug and vastlint/target/release outputs.
Usage
import vastlint
result = vastlint.validate(vast_xml)
if result.valid:
print("clean tag")
else:
print(result.summary.errors)
print(result.issues[0].message)
print(result.to_json())
Shareable report links
Need a link instead of raw output? The vastlint CLI can upload a validation report (rule IDs, severities, and locations; never the input XML) and print a public URL you can paste into Slack, tickets, or PRs:
vastlint check tag.xml --share
# https://vastlint.org/r/<id>
FastAPI / Flask example
from fastapi import FastAPI
from pydantic import BaseModel
import vastlint
app = FastAPI()
class ValidateRequest(BaseModel):
xml: str
wrapper_depth: int = 0
max_wrapper_depth: int = 5
rule_overrides: dict[str, str] | None = None
@app.post("/validate")
def validate(req: ValidateRequest):
result = vastlint.validate(
req.xml,
wrapper_depth=req.wrapper_depth,
max_wrapper_depth=req.max_wrapper_depth,
rule_overrides=req.rule_overrides,
)
return result.to_dict()
The response shape is stable and frontend-friendly:
{
"version": "4.2",
"issues": [
{
"id": "VAST-2.0-inline-impression",
"severity": "error",
"message": "<InLine> must contain at least one <Impression>",
"path": "/VAST/Ad[0]/InLine",
"spec_ref": "IAB VAST 2.0 §2.2.1",
"line": 4,
"col": 3
}
],
"summary": { "errors": 1, "warnings": 0, "infos": 0, "valid": false }
}
API
vastlint.validate(xml, *, wrapper_depth=0, max_wrapper_depth=5, rule_overrides=None) -> Result
vastlint.version() -> str
xml accepts str or bytes. rule_overrides maps rule IDs to severity levels:
result = vastlint.validate(
vast_xml,
rule_overrides={
"VAST-2.0-mediafile-https": "error",
"VAST-4.1-mezzanine-recommended": "off",
},
)
Result exposes .version, .issues (list of Issue), .summary (Summary), .valid, .to_dict(), and .to_json(). All result types are frozen dataclasses.
Performance
Validation runs in-process against the Rust core, so per-tag cost is dominated by the core, not by Python. Numbers below are single-core, single-threaded, and include the full Python round trip (FFI call, JSON result, parse into dataclasses), so they reflect what a caller actually sees.
| Tag size | Median latency | Throughput (1 core) |
|---|---|---|
| 7 KB | 86 µs | ~11,000 tags/sec |
| 17 KB | 0.36 ms | ~2,800 tags/sec |
| 23 KB | 0.57 ms | ~1,700 tags/sec |
| 44 KB | 2.1 ms | ~470 tags/sec |
| 347 KB | 4.7 ms | ~210 tags/sec |
Measured on an Apple M4, single core, 200 tags per size bucket, warm. Production tags typically run 17 to 44 KB. Each validation is independent and holds no shared state, so throughput scales close to linearly across cores with a process pool. Reproduce with scripts/bench.py.
Native library layout
Vendored release libraries live at:
src/vastlint/native/darwin_arm64/libvastlint.dylibsrc/vastlint/native/darwin_amd64/libvastlint.dylibsrc/vastlint/native/linux_arm64/libvastlint.sosrc/vastlint/native/linux_amd64/libvastlint.so
They come from the vastlint-ffi-* tarballs attached to each vastlint GitHub Release. Refresh them with:
./scripts/fetch-libs.sh v0.4.14
License
Apache 2.0.
1 maintainer
Metadata
1 maintainer
Release files for vastlint 0.14.5
For a detailed explanation of source distributions (sdists) and built distributions (wheels), please see the package formats documentation.
Source distribution (sdist)
| File | Size | Uploaded | Provenance | |
|---|---|---|---|---|
| vastlint-0.14.5.tar.gz | 2.1 MB | With provenance | Details |
Built distribution (wheel)
| File | Interpreter | ABI | Platform | Reset |
|---|---|---|---|---|
| vastlint-0.14.5-py3-none-any.whl | Python 3 | none | any | Details |
Total release size: 4.2 MB
Release files / vastlint-0.14.5.tar.gz
| Download URL | vastlint-0.14.5.tar.gz |
|---|---|
| Size | 2.1 MB |
| Tags | Source |
|
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| Download URL | vastlint-0.14.5-py3-none-any.whl |
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| Size | 2.1 MB |
| Tags | Python 3 |
|
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Yes |
| Uploaded via |
twine/7.0.0 CPython/3.13.14
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Signed by GitHub Actions, verified by PyPI on Oct 13, 2026.
Transparency logSecurity for vastlint 0.14.5
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Signed by GitHub Actions, verified by PyPI on Oct 13, 2026.
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