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Python's robots.txt parser misinterprets standard, allowing disallowed endpoints

By PulseAugur Editorial · [1 sources] ·

A recent analysis of robots.txt files across 2,347 hosts revealed significant discrepancies in how Python's `urllib.robotparser` interprets the protocol compared to the RFC 9309 standard. The standard dictates that the most specific rule wins, with special handling for '*' and '$' characters, and line order being irrelevant. However, Python's older parser incorrectly prioritizes the first matching rule and treats '*' and '$' as literal characters. This led to 250 endpoints being incorrectly allowed by the old parser, while only 2 were incorrectly disallowed. The issue has been addressed in newer Python versions, with a fix merged in May 2026 and backported. AI

IMPACT Incorrect parsing of robots.txt can impact web scraping and data collection for AI training.

RANK_REASON Analysis of a software component's adherence to a technical standard. [lever_c_demoted from research: ic=1 ai=0.4]

Read on dev.to — MCP tag →

AI-generated summary · Google Gemini · from 1 sources. How we write summaries →

Python's robots.txt parser misinterprets standard, allowing disallowed endpoints

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Analysis of a software component's adherence to a technical standard. [lever_c_demoted from research: ic=1 ai=0.4]
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COVERAGE [1]

  1. dev.to — MCP tag TIER_1 English(EN) · Alex ·

    We read the robots.txt of 2,347 MCP hosts: Python 3.12's parser gets 252 endpoints wrong

    <p>Two crawlers that log MCP servers compared notes on 1 November, and one server didn't add up. <a href="http://www.apier.no" rel="noopener noreferrer">www.apier.no</a> had changed its tool definitions twice in a day. Our log, HISTOR, recorded both changes. ScraperWatch, which h…

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