Python cheatsheet: the basics by task

How to use this page

Find the job you're trying to do, and copy the pattern. Words in capitals, like FILE or NAME, are yours to replace. The examples come from the website checker you build across the track. Every row links to the lesson that explains it. Use the print button at the top to keep a copy next to you.

Two habits that make everything here easier:

• Try it at the >>> prompt first. Run python3, type the one line you're unsure of, and see what it gives back. exit() leaves.
• Read a traceback from the bottom. The last line names the error; the File ... line just above it says where.

When Python says no

Start here when a script stops. Find the last line of the traceback.

Running Python

Values, types and text

f-strings put values inside text. Anything in braces is worked out, and :.2f means two decimal places:

site = "example.com"
status_code = 200
response_seconds = 0.256
print(f"{site}: {status_code} in {response_seconds:.2f} seconds")

That prints example.com: 200 in 0.26 seconds. Text from the keyboard or a file is always text: "25" * 2 is '2525', so convert first with int().

Lists and dictionaries

A list keeps values in order. A dictionary keeps values with labels, called keys.

Decisions and loops

if status_code < 400:
    print("up")
elif status_code < 500:
    print("client error")
else:
    print("down")

for site in sites:
    print(site["name"])

attempts = 0
while attempts < 3:
    attempts = attempts + 1

The colon and the indented lines after it are the rule: everything indented under if, for or while belongs to it. Use four spaces. for goes through every item; while repeats as long as its condition is true, so make sure something changes it. Lesson 3 covers both.

Functions

def to_ms(seconds):
    return round(seconds * 1000)

def fetch(url, max_attempts=3, timeout=5):
    ...

response_ms = to_ms(0.25)
response = fetch("https://github.com", timeout=10)

Errors you expect: try and except

try:
    status_code = int(site["status_code"])
except ValueError:
    print("status code isn't a number")
    status_code = 0

If the try lines raise that error, Python runs the except lines instead of stopping. Name the exact error you expect, like ValueError or requests.RequestException. A bare except: hides the mistakes you'd want to see. Lessons 4 and 6.

Files: CSV and JSON

import csv
import json

def load_sites(path):
    with open(path, newline="") as file:
        return list(csv.DictReader(file))

def save_results(path, results):
    with open(path, "w") as file:
        json.dump(results, file, indent=2)

• import loads a module from Python's toolbox, at the top of the file.
• with open(...) as file: opens a file and closes it again when the indented lines finish, even if something fails.
• csv.DictReader turns each row into a dictionary, using the header line as the keys. Every value comes back as text.
• json.dump saves lists and dictionaries as JSON; json.load(file) reads them back. "w" means write, replacing what was there.

Lesson 5 covers both.

Talking to APIs

import requests

response = requests.get("https://github.com", timeout=5)
response.status_code
response.elapsed.total_seconds()
data = response.json()

requests.post("https://ntfy.sh/TOPIC", data="2 sites down", timeout=10)

Always set timeout, or a server that never answers makes your script wait forever. A failed connection raises requests.RequestException; catch it, wait with time.sleep(1), and try again a few times. A 404 is not an exception: it's an answer, so check status_code. Lessons 6 and 9.

Scripts you can trust

import sys

def main():
    ...
    if down > 0:
        return 1
    return 0

if __name__ == "__main__":
    sys.exit(main())

• main() holds what the script does, so other files and tests can import its functions without running it.
• The exit code is how cron, CI pipelines and other scripts know it failed: 0 for success, anything else for a problem.
• Tests live in test_checker.py. Each one is a function whose name starts with test_, using assert:

from checker import is_up

def test_400_is_down():
    assert not is_up(400)

Lesson 7 covers all three.

Arguments, logging and schedules

Cron runs with almost nothing set up: no virtual environment and a different folder. That's why the schedule uses full paths, and the .venv's own python.

Settings and secrets

Settings come from the environment, never from the code, so the same script runs anywhere. Secrets especially: a password in the code ends up in Git.

FastAPI

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

app = FastAPI(title="Site checker API")

class SiteResult(BaseModel):
    name: str
    result: str

@app.get("/health")
def health():
    return {"status": "ok"}

@app.get("/results/{name}")
def one_result(name: str) -> SiteResult:
    for site in load_results():
        if site["name"] == name:
            return site
    raise HTTPException(status_code=404, detail="Site not found")

• @app.get("/path") turns the function under it into an endpoint. {name} in the path becomes the function's name.
• class SiteResult(BaseModel) describes the shape of the data, and FastAPI checks every answer against it.
• raise HTTPException(...) answers with an error status, like 404 or 503, instead of crashing.
• /docs on your running API lists every endpoint, and lets you try them.
• Test it with TestClient(app), then client.get("/health"), in a normal pytest test.

Lesson 10 builds it all.