Cross-platform gotchas in scripts and installers

Universal set of errors from the AGGG2.0 Windows install bug report
(2026-08-11). Each gotcha — symptom → cause →
fix. Check your code against this checklist if it needs to work
on multiple OSes (especially Windows).

1. platform.system() and sys.platform are DIFFERENT things

Symptom: platform dictionary built on one API, lookup on
another; key not found (None), code silently goes wrong or crashes.

Cause: on Windows platform.system() returns "Windows",
while sys.platform returns "win32". On Linux: "Linux" vs "linux".
Mixing in one expression gives a miss.

Fix: don't mix. One source for all dictionaries:

# bad: keys for sys.platform, lookup by platform.system()
key = {"win32": "win32", "linux": "linux"}.get(platform.system().lower())  # None on Windows!

# good: explicit key for platform.system() OR use sys.platform everywhere
key = {"windows": "win32", "linux": "linux", "darwin": "darwin"} \
    .get(platform.system().lower())
# or: key = sys.platform, if dict is based on it

2. Regex in raw strings: double backslash matches NOT what you want

Symptom: re.search(rf'...{ext}', url) never finds a match,
even though the data contains it.

Cause: in a raw string r"\\.zip" is TWO characters: \\ (literal
backslash) + . (wildcard "any character"), so it searches for \\. in the URL text —
but you need one escaped backslash \\. (dot). In raw strings, backslash
is not doubled for escaping — it's already literal.

Fix: in raw strings, one backslash: