http-language-preference-review

HTTP Language Preference (Review Session) §

Date: 2026-03-25 Category: Parsing Parts Completed: 4/4 (review) Language: Python

Problem Summary §

Parse Accept-Language HTTP headers with exact matching, prefix/generic matching, wildcard support, and q-factor weighting.

Key Bugs Found on Review §

1. Substring matching instead of prefix matching §

# Bug: "fr" in "afr-ZA" → True (false positive)
if region in lang
# Fix: prefix check
if lang.startswith(region + "-")
# Or: if lang.split("-")[0] == region

2. Set for supported languages loses ordering §

# Bug: iteration order non-deterministic
self.supported_languages = set(supported_languages)
# Fix: keep list for order, use set for lookups
self.supported_languages = supported_languages
self.supported_set = set(supported_languages)

3. No handling for missing q-factor §

# Bug: crashes on "en-US" (no ;q=)
lang, q_factor = q.split(';')  # ValueError
# Fix:
if ';' in q:
    lang, q_factor = q.split(';')
    q_val = float(q_factor.split("=")[1])
else:
    lang = q
    q_val = 1.0

4. q=0 languages appended to result §

• HTTP spec says q=0 = “not acceptable”
• Check problem spec: some variants want them listed last, others excluded entirely

Key Learnings §

• Substring vs prefix matching is a classic Stripe interview trap — always use startswith
• Sets lose ordering — use lists when order matters, sets for O(1) lookups
• HTTP spec: missing q-factor = 1.0, q=0 = not acceptable
• Always handle the “missing optional parameter” case