learning-website-framework1-2  Exercise 2: site_for() — Which Site Does This Page Belong To?
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Write a function that takes a content-relative path and returns the site name,
including pages under sidebar/<subject>/ (routed by subject).

SITES = {
    "languages": ["france", "germany", "hungary", "japan", "culture"],
    "webdevelopment": ["web-development", "web-platforms", "web-servers"],
    "systems": ["linux", "operating-systems"],
    "humanities": ["history"],
}
SIDEBAR_ROUTES = {"linux": "systems", "history": "humanities",
                   "web-development": "webdevelopment"}

FOLDER_TO_SITE = {f: s for s, fs in SITES.items() for f in fs}

def site_for(path):
    parts = path.strip("/").split("/")
    if parts[0] == "sidebar":
        if len(parts) < 2:
            raise ValueError("sidebar page without a subject folder: " + path)
        return SIDEBAR_ROUTES[parts[1]]
    return FOLDER_TO_SITE[parts[0]]

tests = [
    "hungary/hungarian-basic-3/hungarian_basic_conversation_3_1.html",
    "japan/hiragana-1/hiragana_1_1.html",
    "sidebar/linux/example_cheat_sheet.html",
    "sidebar/web-development/example_lesson.html",
    "web-servers/apache-in-depth/example_chapter.html",
]
for t in tests:
    print(f"{site_for(t):<16} {t}")

Output (checked by running it):

languages        hungary/hungarian-basic-3/hungarian_basic_conversation_3_1.html
languages        japan/hiragana-1/hiragana_1_1.html
systems          sidebar/linux/example_cheat_sheet.html
webdevelopment   sidebar/web-development/example_lesson.html
webdevelopment   web-servers/apache-in-depth/example_chapter.html

Design points
-------------
- A KeyError for an unknown folder is deliberate: a path that belongs nowhere
  should stop the build, not silently land on the wrong site.
- Sidebar pages are routed by the subject folder under sidebar/, so a Linux
  cheat sheet moves with the systems site instead of staying behind.
- The same function can be used by the build script, the navigation and the
  redirect generator, so all three agree about where a page lives.

WHY THIS WORKS AS AN ANSWER
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One function that answers "which site owns this page?" is the single source of
truth for everything later in the course: building each site, drawing its
navigation and generating redirects for the old URLs. Keeping the mapping in
data (the two dictionaries) rather than in if-statements means a change of mind
is a one-line edit, not a code change.