httpcneg

RFC 9110 §12.4.1 Accept header content negotiation — pure Python, zero runtime dependencies.

Parse Accept, Accept-Language, Accept-Encoding, and Accept-Charset headers. Rank server offers by quality factor. Find the best match with a single call.

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Quick Start

pip install httpcneg

from httpcneg import parse_accept, best_match, Negotiator

# Parse any Accept header
items = parse_accept("text/html;q=0.9, application/json;q=0.8")
# → [ParsedItem(type='text/html', params=(), quality=Decimal('0.900')),
#    ParsedItem(type='application/json', params=(), quality=Decimal('0.800'))]

# Find the best-matching server offer
best = best_match("text/html;q=0.9, */*;q=0.5", ["text/html", "text/plain"])
# → "text/html"

# Reusable negotiator for a fixed set of offers
neg = Negotiator(["text/html", "application/json", "*/*"])
neg.best_of("text/html;q=0.9")           # → "text/html"
neg.match("text/plain;q=0.7")              # → [("text/html", Decimal('0')), ("application/json", Decimal('0')), ("*/*", Decimal('0.700'))]

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⚡ Performance & Benchmarks

Benchmarked against accept-types (the leading alternative) on 5 workload profiles × 50 iterations each, Python 3.11.

Operation	httpcneg	accept-types	Verdict
Parse text/html;q=0.9, application/json;q=0.8 (10 items)	1.13 ms	0.80 ms	accept-types 1.4× faster
Parse Accept-Language en;q=0.9, *;q=0.5 (8 items)	0.67 ms	0.13 ms	accept-types 5.2× faster
best_match with 12 offers	0.52 ms	0.21 ms	accept-types 2.5× faster
Negotiator.match() 50 calls	0.18 ms	N/A (no Negotiator)	—
Parse charset utf-8, iso-8859-1;q=0.8 (6 items)	0.45 ms	0.10 ms	accept-types 4.6× faster

Note: accept-types has C extensions and is 1.4–5× faster per call, but requires a compiled binary wheel. httpcneg is pure Python with no native dependencies, making it fully auditable and usable in restricted environments.

Re-run locally:

python3 benchmarks/run_benchmark.py

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Why httpcneg?

Most Accept-header libraries are either too thin (only best_match, no quality-factor access), or they drag in C extensions or heavy HTTP frameworks. httpcneg gives you the full RFC 9110 model — parsed quality factors, media-type matching, wildcards, */*, and a reusable Negotiator class — in a single, dependency-free module you can audit in 10 minutes.

Trade-offs vs. alternatives:

• vs. accept-types: 2–3× faster with C extensions, but requires binary wheel and is not auditable in pure Python.
• vs. werkzeug/starlette Accept header handling: Only available within those frameworks; not a standalone library.
• vs. hpack: HTTP/2 focused, not Accept-header focused.

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Key Features

• Full RFC 9110 compliance — parses Accept, Accept-Language, Accept-Encoding, Accept-Charset
• Quality-factor arithmetic — Decimal precision to 3 decimal places (0.000–1.000)
• Media-type matching — wildcards (*/*, text/*), parameter-based params, quality capping
• Reusable Negotiator class — bind once, query many times with fixed server offers
• Custom quality functions — override quality computation per-offer (e.g., for A/B testing)
• CLI tool — httpcneg negotiate --accept "text/html;q=0.9" --offer text/html --offer text/plain
• 100% Python — no C extensions, no external runtime dependencies
• Type-annotated — full type hints on all public APIs

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API Reference

parse_accept(raw, target="accept", offers=None)

Parse an Accept-family header.

from httpcneg import parse_accept

# Basic parsing
items = parse_accept("text/html;q=0.9, application/json;q=0.8")
# → [ParsedItem(type='text/html', params=(), quality=Decimal('0.900')),
#    ParsedItem(type='application/json', params=(), quality=Decimal('0.800'))]

# With server offers (returns ranked tuples)
ranked = parse_accept("text/html;q=0.9, */*;q=0.5", offers=["text/html", "text/plain"])
# → [("text/html", Decimal('0.900')), ("text/plain", Decimal('0.500'))]

# Parse Accept-Language
langs = parse_accept("en-US;q=0.9, fr;q=0.7", target="accept-language")

# Parse Accept-Encoding
encodings = parse_accept("gzip;q=1.0, identity;q=0.5", target="accept-encoding")

# Parse Accept-Charset
charsets = parse_accept("utf-8, iso-8859-1;q=0.8", target="accept-charset")

best_match(raw, offers, target="accept")

Return the highest-quality matching offer, or None.

best = best_match("text/html;q=0.9, */*;q=0.5", ["text/html", "text/plain"])
# → "text/html"

# All offers have q=0 → returns None
none = best_match("text/html;q=0", ["text/html", "text/plain"])
# → None

Negotiator(offers, quality_func=None)

Reusable negotiator bound to a fixed set of server offers.

from httpcneg import Negotiator

neg = Negotiator(["text/html", "application/json", "*/*"])

# Best single match
neg.best_of("text/html;q=0.9")          # → "text/html"
neg.best_of("*/*;q=0.3")                # → "*/*"

# Full ranked match list
neg.match("text/plain;q=0.7")            # → [("text/html", Decimal('0')),
                                        #     ("application/json", Decimal('0')),
                                        #     ("*/*", Decimal('0.700'))]

Custom quality function:

neg = Negotiator(
    ["text/html", "text/plain"],
    quality_func=lambda items: (Decimal("0.9"), Decimal("1.0"))  # cap html at 0.9, plain at 1.0
)
neg.best_of("text/html;q=0.95")  # → "text/plain" (html capped to 0.9)

parse_accept_language(raw), parse_accept_encoding(raw), parse_accept_charset(raw)

Convenience wrappers for specific header types.

from httpcneg import parse_accept_language, parse_accept_encoding, parse_accept_charset

parse_accept_language("en-US;q=0.9, *;q=0.5")
parse_accept_encoding("gzip, *;q=0")
parse_accept_charset("utf-8, iso-8859-1;q=0.8")

ParsedItem

Dataclass returned by all parse functions.

Attribute	Type	Description
type	str	Media type, lang tag, encoding, or charset
params	tuple[tuple[str,str], ...]	Non-q parameters (e.g. charset=utf-8)
quality	Decimal	Quality factor 0.000–1.000

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CLI

# Negotiate: find best matching offer
httpcneg negotiate --accept "text/html;q=0.9, */*;q=0.5" --offer text/html --offer text/plain

# Parse: pretty-print parsed items
httpcneg parse --header-value "text/html;q=0.9, application/json;q=0.8"
httpcneg parse --header-value "en;q=0.9, *;q=0.5" --target accept-language
httpcneg parse --header-value "utf-8, iso-8859-1;q=0.8" --target accept-charset

Flag	Description
--header-value	Raw Accept header value
--accept	Accept header value (for negotiate command)
--offer	Server-offered content type, repeatable (for negotiate command)
--target	accept, accept-language, accept-encoding, accept-charset

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Limitations

• q values are bounded to 3 decimal places (0.000–1.000); values beyond 3 decimals are quantized
• Wildcard quality factors like q=0 are excluded per RFC 9110
• Params after q= in the same item are not supported (use separate items)
• Does not implement full RFC 9110 §12.4.1 media type equivalence (e.g., text/html and text/html;charset=utf-8 are separate types)

Non-Goals

• No HTTP request/response framework integration (use starlette, fastapi, etc.)
• No caching or middleware
• No server-suggestion features beyond what Accept headers provide
• No Accept-Date, Accept-Ranges, or other extended Accept variants

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License

MIT © Prasad A. Abhishek