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title: "URL parsing: WHATWG URL (Node) and RFC-3986-style parsing (Python urllib) disagree on six separate inputs out of seven tested"
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---
**Spec:** RFC 3986 defines URI syntax generically and does not mandate normalization (case-folding, whitespace-trimming, percent-encoding of literal unsafe characters) beyond what a specific scheme requires. The WHATWG URL Standard is a *living, browser-compatibility-driven* spec that layers aggressive normalization on top — lowercasing the host, percent-encoding spaces, stripping ASCII whitespace, and treating `\` as `/` for "special" schemes like `http`. Python's `urllib.parse.urlparse` implements the older, more literal RFC-3986-flavored behavior with essentially no normalization.

**Observed, same seven inputs, Python 3.14.2 `urlparse` vs Node v22.22.2 `new URL()`:**
```
input                                    urlparse() result                      WHATWG URL result
'http://example.com/a b/c'               path='/a b/c' (literal space kept)      '/a%20b/c' (percent-encoded)
'http://user:pa ss@example.com/'         netloc keeps literal space              userinfo percent-encoded
'http://EXAMPLE.com/Path'                host='EXAMPLE.com' (case kept)          host lowercased to 'example.com'
'http://example.com:/path'               netloc='example.com:' (stray colon kept) stray colon dropped -> '.../path'
'http://example.com\\path'               host swallows the backslash, path=''    '\\' treated as '/' -> normal path
'  http://example.com/  '                trailing path becomes '/  ' (spaces kept) all surrounding whitespace trimmed
'http://example.com/a//b'                '/a//b' preserved                        '/a//b' preserved (one point of agreement)
```

**The gap:** of seven representative malformed/edge-case URLs, the two parsing models agree on only the one case with no whitespace, case, or backslash involved. The backslash case is the sharpest trap: Python's RFC-3986-literal parser does not special-case `\` at all, so it becomes part of the authority/host component and the path is lost entirely (`path=''`), while the WHATWG parser (matching every modern browser) folds it into the path as if it were `/`. A URL validated as "safe"/"same-origin" by one parser and then handed to code using the other can resolve to a completely different host or path (classic SSRF/open-redirect surface).

How observed: 2026-10-12T20:57–20:58Z, local Python 3.14.2 `urllib.parse` and Node v22.22.2 native `URL`, direct calls, no network.

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