Python basics for IT work

This lesson covers the small set of ideas that make up most Python you will read: values, text, lists, dictionaries, decisions and loops. The examples come from IT work, because that is where you will use them.

Values and variables

A variable is a name for a value. Python figures out the type itself.

server = "web-01"        # str: text
cpu_percent = 73.5       # float: decimal number
open_tickets = 12        # int: whole number
is_production = True     # bool: True or False
nothing_yet = None       # None: "no value"

print(server, type(server))
print(cpu_percent > 70, open_tickets + 3)

Names are lowercase with underscores by convention. Lines starting with # are comments, which Python ignores.

Text

host = "WEB-01.example.com"
print(host.lower())
print(host.split("."))            # ['WEB-01', 'example', 'com']
print(host.startswith("WEB"))     # True
print(host.replace("WEB", "DB"))

user = "ada"
count = 3
print(f"{user} has {count} failed logins")

Strings are written in single or double quotes. Use f-strings, as above, to build messages.

Lists

A list is an ordered collection you can change.

servers = ["web-01", "web-02", "db-01"]
servers.append("cache-01")
print(servers[0])        # first item: counting starts at 0
print(servers[-1])       # last item
print(len(servers))      # 4
print("db-01" in servers)

Dictionaries

A dictionary stores values under keys, like a small record. It is the shape of almost all JSON and API data you will meet.

server = {"name": "web-01", "cpu": 73.5, "tags": ["prod", "eu"]}
print(server["name"])
print(server.get("memory", "unknown"))   # a safe default if the key is missing
server["cpu"] = 80.2
server["owner"] = "platform-team"
print(server)

Use .get(key, default) when a key might be missing, so your script does not crash.

Decisions

cpu = 91

if cpu >= 90:
    status = "critical"
elif cpu >= 70:
    status = "warning"
else:
    status = "ok"

print(status)

Indentation matters. The indented lines belong to the if. Use four spaces. Comparison operators are ==, !=, <, >, <= and >=, and you combine conditions with and, or and not.

Loops

log_lines = [
    "2026-03-01 10:02:11 ERROR disk /dev/sda1 is 93% full",
    "2026-03-01 10:05:40 INFO backup completed",
    "2026-03-01 10:09:02 ERROR login failed for user admin",
]

error_count = 0
for line in log_lines:
    if "ERROR" in line:
        error_count += 1
        print("Found:", line)

print(f"{error_count} errors out of {len(log_lines)} lines")

for repeats for each item. while repeats while a condition holds, and break leaves a loop early. Loop over a dictionary's pairs with .items():

limits = {"cpu": 90, "memory": 85, "disk": 95}
for metric, limit in limits.items():
    print(f"alert when {metric} passes {limit}%")

List comprehensions

A compact way to build a list from another list. You will see this a lot, so learn to read it:

log_lines = [
    "10:02 ERROR disk full",
    "10:05 INFO backup completed",
    "10:09 ERROR login failed",
]
errors = [line for line in log_lines if "ERROR" in line]
times = [line.split(" ")[0] for line in log_lines]
print(errors)
print(times)

Read it as "give me line for each line in the list, where it contains ERROR". If it confuses you, write it as an ordinary loop first.

Putting it together

servers = [
    {"name": "web-01", "cpu": 91},
    {"name": "web-02", "cpu": 45},
    {"name": "db-01", "cpu": 78},
]

for s in servers:
    if s["cpu"] >= 90:
        level = "critical"
    elif s["cpu"] >= 70:
        level = "warning"
    else:
        level = "ok"
    print(f'{s["name"]}: {level}')

busiest = max(servers, key=lambda s: s["cpu"])
print("Busiest:", busiest["name"])

A list of dictionaries is the most common shape of real data. The lambda here is a tiny inline function: "for each server s, use s["cpu"]". Tools like pandas (lesson 5) turn this shape into tables.

Common beginner mistakes

• Indentation errors. Mixed tabs and spaces, or the wrong number.
• Off-by-one with lists. The first item is [0], not [1].
• Using = instead of ==. One assigns, the other compares.
• Changing a list while looping over it. Build a new list instead.
• Forgetting the quotes. name is a variable, "name" is text.

Use a visualizer such as Python Tutor to watch code run one line at a time. It makes loops and variables concrete.

Go deeper (optional)

Python Full Course for BeginnersProgramming with Mosh · 374 min

Resources

Check yourself

1. Write a script with a list of five servers (dictionaries with a name and a disk percentage). Print the names of those over 80 percent.
2. Rewrite your answer using a list comprehension. Which version is easier to read for you?
3. What is the difference between server["owner"] and server.get("owner") when the key is missing?