Cheatsheet

Python

Python for scripts and automation: the environment, text and files, data structures, errors and the standard library you use most.

Environment

python3 -m venv .venv && source .venv/bin/activate

A private set of packages for this project

pip freeze > requirements.txt  |  pip install -r requirements.txt

Save and restore the packages

name = "Ann"; f"Hello {name}, {3 * 1.5:.2f}"

f-strings, with number formatting

```
"a,b,c".split(",")  |  ",".join(["a", "b"])
```
Split a string into a list and join it back

s.strip()  s.lower()  s.replace("x", "y")  s.startswith("a")

Common string methods

squares = [n * n for n in range(10) if n % 2 == 0]

A list comprehension

for i, item in enumerate(items, start=1):

Loop with a counter

sorted(users, key=lambda u: u["age"], reverse=True)

Sort by a field

d.get("key", "default")  |  d.items()  |  {**a, **b}

Safe lookup, pairs, and merging dictionaries

```
{k: v for k, v in pairs}  |  set(items)
```
A dict comprehension, and unique items
```
a, *rest = [1, 2, 3, 4]
```
Unpack the first item and the rest

from collections import Counter
Counter(words).most_common(3)

Count things

with open("a.txt", encoding="utf-8") as f:
    for line in f:
        print(line.rstrip())

Read a file line by line (it closes by itself)

```
Path("out").mkdir(exist_ok=True)
```
from pathlib import Path: create a folder safely

Path("a.txt").read_text()  |  Path("a.txt").write_text("hi")

Read or write a small file

import json
data = json.load(open("a.json"))
json.dump(data, open("b.json", "w"), indent=2)

JSON in and out

import csv
for row in csv.DictReader(open("a.csv")):
    print(row["name"])

Read a CSV with column names

def greet(name: str, shout: bool = False) -> str:
    return name.upper() if shout else name

A function with defaults and type hints

try:
    value = int(text)
except ValueError as e:
    print(f"not a number: {e}")
else:
    print(value)

Handle a specific error

raise ValueError("port must be 1-65535")

Signal a problem

```
if __name__ == "__main__":
    main()
```
Run main() only when the file is executed, not imported

assert total >= 0, "total cannot be negative"

A quick check while developing

import argparse
p = argparse.ArgumentParser()
p.add_argument("path")
p.add_argument("--count", type=int, default=1)
args = p.parse_args()

Command-line options

import subprocess
out = subprocess.run(["ls", "-l"], capture_output=True, text=True, check=True).stdout

Run a command and read its output

import os
os.environ.get("HOME")  |  sys.argv  |  sys.exit(1)

Environment variables, arguments, and an exit code

import logging
logging.basicConfig(level=logging.INFO)
logging.info("started")

Logging instead of print

import requests
r = requests.get(url, timeout=10)
r.raise_for_status()
r.json()

Call an HTTP API (always set a timeout)

Cheatsheet for the Python path · Deboistech · All cheatsheets