Step 3: Status codes and the error rate

Status codes group by their first digit: 2xx success, 3xx redirect, 4xx the client made a mistake (not found, not allowed), 5xx the server failed. A 5xx is your problem, so it matters most. A Counter groups codes in one line, and the error rate is errors divided by total requests.

Examplepython

import re
from collections import Counter, defaultdict

LOG = """2026-11-05 09:00:01 10.0.0.5 GET /home 200 120
2026-11-05 09:00:03 10.0.0.9 GET /login 200 80
2026-11-05 09:00:07 10.0.0.9 POST /login 401 95
2026-11-05 09:00:09 10.0.0.9 POST /login 401 91
2026-11-05 09:00:12 10.0.0.9 POST /login 401 99
2026-11-05 09:00:15 10.0.0.9 POST /login 401 97
2026-11-05 09:01:02 10.0.0.5 GET /courses 200 340
2026-11-05 09:01:20 10.0.0.7 GET /courses/python 200 410
2026-11-05 09:01:44 10.0.0.7 GET /missing 404 30
2026-11-05 09:02:05 10.0.0.5 GET /api/progress 500 1200
2026-11-05 09:02:09 10.0.0.5 GET /api/progress 500 1180
2026-11-05 09:02:30 10.0.0.8 GET /home 200 110
2026-11-05 09:03:10 10.0.0.8 GET /courses 200 360
2026-11-05 09:03:12 10.0.0.7 GET /home 200 105
2026-11-05 09:03:50 10.0.0.6 GET /api/progress 200 220
this line is not a valid log entry
2026-11-05 09:04:01 10.0.0.6 GET /home 200 98
2026-11-05 09:04:33 10.0.0.7 GET /courses/sql 200 395
2026-11-05 09:05:02 10.0.0.8 GET /missing 404 28"""

PATTERN = re.compile(
    r"^(?P<date>\d{4}-\d{2}-\d{2}) (?P<time>\d{2}:\d{2}:\d{2}) "
    r"(?P<ip>\S+) (?P<method>[A-Z]+) (?P<path>\S+) (?P<status>\d{3}) (?P<ms>\d+)$"
)

def parse(text):
    entries = []
    for line in text.splitlines():
        m = PATTERN.match(line)
        if m:
            e = m.groupdict()
            e["status"], e["ms"] = int(e["status"]), int(e["ms"])
            entries.append(e)
    return entries

entries = parse(LOG)

by_class = Counter(f"{e['status'] // 100}xx" for e in entries)
print(dict(sorted(by_class.items())))

errors = [e for e in entries if e["status"] >= 400]
print("errors:", len(errors), "of", len(entries))
print(f"error rate: {len(errors) / len(entries):.1%}")

server_errors = [e for e in entries if e["status"] >= 500]
print("server errors:", [(e["path"], e["status"]) for e in server_errors])

Open in editor »

The 500s on /api/progress stand out. A healthy site keeps 5xx close to zero, and rising 5xx is what wakes people up at night.

Your turn: write the code

Write error_rate(statuses) that returns the fraction of status codes that are 400 or higher, rounded to 2 decimals (0 for an empty list). Print error_rate([200, 200, 404, 500]) and error_rate([]).

Quick check

Which status class means the server itself failed?

5xx4xx2xx

Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.

Tip: use the left and right arrow keys to move between lessons.