Use a sitemap as a URL-discovery source, not as proof that a page exists or that you may crawl it. Start with the site’s /robots.txt, follow every Sitemap: declaration, identify whether each XML file is a URL set or a sitemap index, recursively collect <loc> values, then normalize, deduplicate, and validate the resulting URLs before making a crawl plan.
What a sitemap can—and cannot—tell you
A sitemap is an XML document published by a site to help systems discover URLs. Google describes it as a discovery aid; listing a URL does not guarantee that a search engine will crawl or index it. The same limitation matters for your own scraper: a listed URL may be stale, redirected, blocked, unavailable, duplicated, or unsuitable for your job.
Finding a URL is therefore a separate step from deciding whether to request it. Treat the extracted list as candidate targets. Check the site’s terms, applicable law, robots rules, authentication requirements, and your request rate before fetching pages. A sitemap is not permission to crawl.
End-to-end workflow
- Request
https://example.com/robots.txtand record every case-insensitiveSitemap:declaration. Google documents this discovery method, and crawler frameworks can use it. - If no declaration appears, try likely paths such as
/sitemap.xmlor/sitemap_index.xmlas a fallback. No universal filename is guaranteed, so treat failures as normal. - Fetch each XML file, including
.gzfiles, and parse it with namespace-aware XML logic. - Inspect the root element. A
urlsetcontains URL records; asitemapindexcontains links to child sitemap files. - For indexes, fetch every child and repeat the test recursively. Collect only
locvalues from URL sets. - Resolve, normalize, filter, and deduplicate URLs. Then validate status codes, redirects, content type, crawl constraints, and pacing before downloading page content.
Discover sitemap files through robots.txt
A robots file can contain one or more declarations, for example:
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User-agent: *
Disallow: /private/
Sitemap: https://example.com/sitemap.xml
Sitemap: https://example.com/news-sitemap.xmlDo not assume there is only one declaration. Preserve absolute sitemap URLs exactly as published, and handle a robots response that is missing, unavailable, or malformed without treating that as evidence that no sitemap exists.
Python: fetch robots.txt and extract declarations
from urllib.parse import urljoin
import requests
def sitemap_locations(site_url: str) -> list[str]:
base = site_url.rstrip("/") + "/"
robots_url = urljoin(base, "robots.txt")
response = requests.get(robots_url, timeout=30,
headers={"User-Agent": "URLDiscovery/1.0"})
if response.status_code >= 400:
return []
locations = []
for line in response.text.splitlines():
name, separator, value = line.partition(":")
if separator and name.strip().lower() == "sitemap":
value = value.strip()
if value:
locations.append(urljoin(base, value))
return list(dict.fromkeys(locations))
print(sitemap_locations("https://example.com"))Use a current crawler library if you need robots policy enforcement; this function only discovers declarations and does not decide which pages you are allowed to fetch.
Parse URL sets and sitemap indexes
The protocol commonly uses the namespace http://www.sitemaps.org/schemas/sitemap/0.9. Namespace-aware lookup is essential because a naive search for an unqualified loc element can return nothing. XML entities must be decoded by the XML parser, not by ad-hoc string replacement.
Complete recursive Python extractor
from collections import deque
from urllib.parse import urldefrag, urljoin, urlsplit, urlunsplit
import gzip
import io
import requests
import xml.etree.ElementTree as ET
NS = {"sm": "http://www.sitemaps.org/schemas/sitemap/0.9"}
def clean_url(value: str, base: str | None = None) -> str | None:
value = value.strip()
if not value:
return None
value = urljoin(base, value) if base else value
value, _fragment = urldefrag(value)
parts = urlsplit(value)
if parts.scheme not in {"http", "https"} or not parts.netloc:
return None
# Keep path and query; remove only the fragment for crawl identity.
return urlunsplit((parts.scheme.lower(), parts.netloc.lower(),
parts.path or "/", parts.query, ""))
def fetch_xml(url: str) -> bytes:
r = requests.get(url, timeout=45,
headers={"User-Agent": "URLDiscovery/1.0"})
r.raise_for_status()
data = r.content
if url.lower().endswith(".gz") or r.headers.get("Content-Type", "").startswith("application/x-gzip"):
data = gzip.decompress(data)
return data
def discover(site_url: str) -> list[str]:
roots = sitemap_locations(site_url)
if not roots:
roots = [urljoin(site_url.rstrip("/") + "/", "sitemap.xml"),
urljoin(site_url.rstrip("/") + "/", "sitemap_index.xml")]
queue, seen_docs, found = deque(roots), set(), set()
while queue:
doc_url = queue.popleft()
if doc_url in seen_docs:
continue
seen_docs.add(doc_url)
try:
root = ET.fromstring(fetch_xml(doc_url))
except (requests.RequestException, ET.ParseError, OSError) as exc:
print(f"Skipping {doc_url}: {exc}")
continue
tag = root.tag.rsplit("}", 1)[-1].lower()
if tag == "sitemapindex":
for node in root.findall("sm:sitemap/sm:loc", NS):
child = clean_url(node.text or "", doc_url)
if child and child not in seen_docs:
queue.append(child)
elif tag == "urlset":
for node in root.findall("sm:url/sm:loc", NS):
target = clean_url(node.text or "", doc_url)
if target:
found.add(target)
else:
print(f"Ignoring unsupported root {tag} in {doc_url}")
return sorted(found)
for target in discover("https://example.com"):
print(target)The queue prevents loops and the sets prevent duplicate documents and URLs. In production, persist progress so a temporary child-sitemap failure can be retried without starting over.
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Limits, compression, and XML details
- Google documents a limit of 50 MB uncompressed or 50,000 URLs per sitemap.
- A sitemap index may list up to 50,000 sitemap locations.
- Large sites commonly split URL sets and use gzip compression. Check both the filename and the response
Content-Type; do not decompress ordinary XML twice. - Prefer fully qualified absolute URLs. Resolve relative values defensively, but record that the publisher supplied an unusual value.
lastmodcan help prioritize work when it is consistently accurate. Google says it ignorespriorityandchangefreq; do not build scheduling logic around them.
These are documented protocol/search guidance limits, not a promise that every server enforces them correctly. Reject oversized or deeply nested inputs safely and cap recursion and response sizes to protect your parser.
Normalize and validate candidate targets
Normalization decisions
- Remove fragments because browsers do not send them in HTTP requests.
- Lowercase the scheme and host, but preserve path and query semantics unless the target site documents a canonicalization rule.
- Deduplicate exact normalized strings. Decide separately whether tracking parameters such as
utm_*should be removed; that is a project policy, not a sitemap rule. - Keep a record of the original
locand the normalized value for auditing.
Validation pass
Use a bounded HEAD request only when the server supports it; otherwise make a small GET and stop reading after headers or a short byte limit. Record status, final URL, content type, and redirect chain. A 200 response still does not prove that the content is the page you want, while a 403, 429, timeout, or challenge requires a policy decision rather than aggressive retries.
Apply an explicit allowlist of hosts and schemes, a concurrency limit, exponential backoff for transient failures, and a durable checkpoint. Do not follow redirects to unrelated hosts without reviewing them. Never place credentials or sensitive query strings in logs.
Custom parser versus a crawler framework
| Concern | Custom parser | Crawler framework |
|---|---|---|
| Robots discovery | You implement fetching and policy handling. | Framework components can discover sitemap declarations from robots.txt. |
| Nested indexes | Recursive queue and cycle protection are yours to maintain. | Sitemap-aware spiders generally support nested sitemaps. |
| Filtering and output | Exact control over normalization, storage, and schemas. | Built-in scheduling, pipelines, retries, and exports. |
| Operational controls | You add throttling, retries, limits, and observability. | Use mature settings, but verify their current names and defaults. |
Scrapy’s documented SitemapSpider supports sitemap discovery, nested indexes, and robots.txt declarations, but the readily available reference is for release 0.24.6. Check the current Scrapy documentation before copying APIs or settings into a new project.
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Troubleshooting
Robots.txt has no Sitemap line
Try the fallback filenames, inspect the site’s HTML or webmaster documentation, and stop if discovery is unsuccessful. Do not brute-force thousands of filenames.
XML parser reports no loc elements
Check the namespace and root type. Sitemaps can use a default namespace, so query with the namespace URI shown above.
Child sitemap returns HTML
It may be a login page, bot challenge, error document, or proxy response. Check status and Content-Type, save a bounded diagnostic sample, and do not feed arbitrary HTML to the XML parser.
Compressed sitemap fails
Inspect magic bytes and response headers, then decompress once. Some servers label gzip incorrectly; handle that case only with a safe size limit.
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Too many duplicates
Normalize fragments, host casing, default ports, and redirect targets consistently. Keep query parameters unless you have a documented reason to discard them.
Requests are blocked or throttled
Reduce concurrency, honor Retry-After, add backoff, and review the site’s rules. A sitemap does not override access controls.
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Cost, performance, and reliability planning
- Parse sitemap XML once and cache it with a timestamp; refetch according to the site’s publishing cadence.
- Use streaming or bounded downloads for untrusted files, and enforce maximum compressed and uncompressed sizes.
- Separate discovery from page crawling so a failed page request does not lose the URL inventory.
- Store source sitemap, discovery time, HTTP metadata, normalized URL, and validation result.
- For recurring jobs, compare URL sets to identify additions and removals instead of recrawling everything.
- Measure request rate and error classes, not just total URLs. A fast run that triggers blocking is not reliable.
FAQ
Does every website have a sitemap?
No. A site may omit one, publish it under an unadvertised path, or expose only part of its URL inventory.
Can I assume sitemap URLs are canonical?
No. Validate redirects and canonical signals during your later page-processing stage.
Should I crawl every URL immediately?
No. Treat the list as candidates, apply scope and authorization checks, then schedule controlled validation and crawling.
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