{"id":3582,"date":"2026-10-07T14:27:03","date_gmt":"2026-10-07T14:27:03","guid":{"rendered":"https:\/\/www.ucartz.com\/blog\/python-for-loop\/"},"modified":"2026-10-07T14:27:12","modified_gmt":"2026-10-07T14:27:12","slug":"python-for-loop","status":"publish","type":"post","link":"https:\/\/www.ucartz.com\/blog\/python-for-loop\/","title":{"rendered":"Python for Loop: Syntax and Examples for Server Scripts"},"content":{"rendered":"\n

Short answer:<\/strong> a Python for<\/code> loop runs the same block of code once for each item in a sequence: a list, a string, a dictionary, the lines of a file or a range()<\/code> of numbers. The syntax is for item in sequence:<\/code> followed by an indented block. Use range()<\/code> to repeat something a set number of times, enumerate()<\/code> when you also need a counter, zip()<\/code> to walk two lists side by side, break<\/code> to stop early and continue<\/code> to skip an item. Every example below was run on Python 3.14.7, and the output shown is what we got.<\/p>\n\n\n\n

\"Terminal
The first loops from this post running on Python 3.14.8.<\/figcaption><\/figure>\n\n\n\n

The basic for loop<\/h2>\n\n\n\n
servers = [\"web1\", \"web2\", \"db1\"]\nfor server in servers:\n    print(\"Checking\", server)<\/code><\/pre>\n\n\n\n
Checking web1\nChecking web2\nChecking db1<\/code><\/pre>\n\n\n\n
The name after for<\/code> (here server<\/code>) takes the value of each item in turn. The colon and the indentation matter: everything indented under the for<\/code> line is the loop body. Python has no for (i = 0; i < n; i++)<\/code> counter loop like C or Bash; you loop over things directly.<\/p>\n\n\n\n
Counting with range()<\/h2>\n\n\n\n
range()<\/code> produces numbers on demand. As the Python tutorial<\/a> points out, the end point is never included: range(10)<\/code> gives 0 to 9.<\/p>\n\n\n\n
for i in range(1, 6):\n    print(i, end=\" \")\nprint()\nprint(list(range(0, 10, 3)))<\/code><\/pre>\n\n\n\n
1 2 3 4 5\n[0, 3, 6, 9]<\/code><\/pre>\n\n\n\n
The three arguments are start, stop (not included) and step. A negative step counts down: range(10, 0, -1)<\/code>.<\/p>\n\n\n\n
enumerate(), dictionaries and zip()<\/h2>\n\n\n\n
When you need the position as well as the item, use enumerate()<\/code> instead of range(len(...))<\/code>:<\/p>\n\n\n\n
for number, server in enumerate(servers, start=1):\n    print(number, server)<\/code><\/pre>\n\n\n\n
1 web1\n2 web2\n3 db1<\/code><\/pre>\n\n\n\n
Looping over a dictionary gives you its keys. Use .items()<\/code> to get each key and value together:<\/p>\n\n\n\n
disk_used = {\"\/\": 71, \"\/var\": 93, \"\/home\": 45}\nfor mount, percent in disk_used.items():\n    if percent > 90:\n        print(f\"WARNING: {mount} is {percent}% full\")<\/code><\/pre>\n\n\n\n
WARNING: \/var is 93% full<\/code><\/pre>\n\n\n\n
zip()<\/code> pairs up items from two or more lists, which is handy for building a hosts file or a report:<\/p>\n\n\n\n
names = [\"web1\", \"web2\", \"db1\"]\nips = [\"203.0.113.10\", \"203.0.113.11\", \"203.0.113.20\"]\nfor name, ip in zip(names, ips):\n    print(f\"{ip}\\t{name}\")<\/code><\/pre>\n\n\n\n
Be careful when the lists are not the same length: plain zip()<\/code> silently stops at the shorter one, so a missing IP address simply drops a server from the output. Add strict=True<\/code> and Python raises an error instead:<\/p>\n\n\n\n
ValueError: zip() argument 2 is shorter than argument 1<\/code><\/pre>\n\n\n\n
break, continue and the loop else<\/h2>\n\n\n\n
break<\/code> leaves the loop at once. continue<\/code> skips the rest of the body for the current item and moves to the next. A for<\/code> loop can also have an else<\/code> block, which the Python tutorial describes precisely: it runs when the loop finishes without hitting break<\/code>. That makes “search, and say so if nothing was found” very clean:<\/p>\n\n\n\n
free_gb = {\"web1\": 3, \"web2\": 42, \"db1\": 120}\nfor server, free in free_gb.items():\n    if free >= 40:\n        print(\"Deploy to\", server)\n        break\nelse:\n    print(\"No server has enough space\")<\/code><\/pre>\n\n\n\n
This prints Deploy to web2<\/code>. Change the threshold to 500 and nothing matches, so the else<\/code> block prints No server has enough space<\/code>.<\/p>\n\n\n\n
continue<\/code> is the natural way to skip blank lines and comments when reading a config file:<\/p>\n\n\n\n
with open(\"app.conf\") as config:\n    for line in config:\n        line = line.strip()\n        if not line or line.startswith(\"#\"):\n            continue\n        key, value = line.split(maxsplit=1)\n        print(key, \"=\", value)<\/code><\/pre>\n\n\n\n
Three loops you can use on a server<\/h2>\n\n\n\n
Count HTTP status codes in an access log<\/h3>\n\n\n\n
Looping over an open file reads it one line at a time, so this works on a log of any size without loading it all into memory:<\/p>\n\n\n\n
from collections import Counter\n\nstatuses = Counter()\nwith open(\"\/var\/log\/nginx\/access.log\") as log:\n    for line in log:\n        status = line.split('\"')[2].split()[0]\n        statuses[status] += 1\n\nfor status, count in sorted(statuses.items()):\n    print(status, count)<\/code><\/pre>\n\n\n\n
On our five-line sample log in the standard combined format, it printed 200 2<\/code>, 404 2<\/code> and 500 1<\/code>. Adjust the split()<\/code> if your log format is different.<\/p>\n\n\n\n
List log files and their sizes<\/h3>\n\n\n\n
from pathlib import Path\n\nfor path in sorted(Path(\"\/var\/log\/myapp\").glob(\"*.log\")):\n    size_mb = path.stat().st_size \/ 1_000_000\n    print(f\"{path.name}: {size_mb:.1f} MB\")<\/code><\/pre>\n\n\n\n
Check that your sites answer<\/h3>\n\n\n\n
import urllib.request, urllib.error\n\nurls = [\"https:\/\/example.com\/\", \"https:\/\/example.com\/missing.html\"]\nfor url in urls:\n    try:\n        with urllib.request.urlopen(url, timeout=5) as response:\n            print(response.status, url)\n    except urllib.error.HTTPError as e:\n        print(e.code, url)\n    except urllib.error.URLError as e:\n        print(\"DOWN\", url, e.reason)<\/code><\/pre>\n\n\n\n
Against a local test server this printed 200<\/code> for the home page, 404<\/code> for the missing file and, for a port with nothing listening, DOWN ... [Errno 111] Connection refused<\/code>. The try<\/code> block inside the loop matters: without it, the first failure would stop the whole loop and the remaining sites would never be checked. To run a script like this every few minutes, schedule it with a cron job<\/a>.<\/p>\n\n\n\n
Common mistakes and the errors they cause<\/h2>\n\n\n\n
Mistake<\/th>What Python 3.14 said<\/th>Fix<\/th><\/tr><\/thead>
for i in 10:<\/code><\/td>TypeError: 'int' object is not iterable<\/code><\/td>Use range(10)<\/code><\/td><\/tr>
Loop body not indented<\/td>IndentationError: expected an indented block after 'for' statement on line 1<\/code><\/td>Indent the body, four spaces is the convention<\/td><\/tr>
range(3)<\/code> over a list with two items<\/td>IndexError: list index out of range<\/code><\/td>Loop over the list itself, or use enumerate()<\/code><\/td><\/tr>
Removing items from the list you are looping over<\/td>No error, but items are skipped<\/td>Build a new list instead<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
The last one is the sneakiest because nothing crashes. Here we try to remove every port from 8000 upwards:<\/p>\n\n\n\n
ports = [22, 80, 8080, 8081, 443]\nfor port in ports:\n    if port >= 8000:\n        ports.remove(port)\nprint(ports)<\/code><\/pre>\n\n\n\n
[22, 80, 8081, 443]<\/code><\/pre>\n\n\n\n
Port 8081 survived. Removing 8080 shifted every later item one place to the left, so the loop stepped straight past 8081. The Python tutorial’s advice is to loop over a copy or create a new collection. A list comprehension does it in one line and gives the right answer, [22, 80, 443]<\/code>:<\/p>\n\n\n\n
ports = [port for port in ports if port < 8000]<\/code><\/pre>\n\n\n\n
for or while?<\/h2>\n\n\n\n