Cheat sheets

Python at a glance 📄

The syntax you look up most, on one page. Print it (it is styled for paper) and keep it by your keyboard for the first few weeks. Muscle memory does the rest.

Print and f-strings

print("hello")
print("a", "b", sep="-")   # a-b
name, n = "Sam", 5
print(f"{name} has {n}")
print(f"{3.14159:.2f}")    # 3.14
print(f"{1000000:,}")      # 1,000,000
print(f"{n=}")             # n=5  (debug)

Numbers

7 / 2     # 3.5   float divide
7 // 2    # 3     floor divide
7 % 2     # 1     remainder
2 ** 10   # 1024  power
int("42") # 42    text to int
round(3.14159, 2)  # 3.14
abs(-5)   # 5

Strings

s = "Monkey Island"
s.upper()  s.lower()  s.strip()
s.replace("Monkey", "Rubber")
s.split()          # list of words
"-".join(["a", "b"])   # "a-b"
s[0]   s[-1]   s[0:6]   s[::-1]
len(s)   "Monkey" in s

Lists

nums = [3, 1, 2]
nums.append(4)     nums.insert(0, 9)
nums.remove(3)     nums.pop()
nums.sort()        sorted(nums)
nums[0]  nums[-1]  nums[1:3]
len(nums)  sum(nums)  max(nums)
copy = nums.copy()   # not = nums!

Dictionaries

d = {"a": 1, "b": 2}
d["c"] = 3         # add
d.get("z", 0)      # 0 if missing
"a" in d           # True
for k, v in d.items(): ...
d.keys()  d.values()
from collections import Counter

Sets

s = {1, 2, 3}
s.add(4)   s.discard(2)
"x" in s           # instant
a | b   union
a & b   intersection
a - b   difference
list(set(items))   # dedupe

If / elif / else

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "F"

x = "adult" if age >= 18 else "minor"

Loops

for item in things:
    print(item)

for i, x in enumerate(things, start=1):
    print(i, x)

for a, b in zip(names, ages):
    ...

for n in range(1, 6):   # 1..5
    ...

while condition:
    ...
    break     continue

Functions

def greet(name, greeting="Hi"):
    """Say hello."""
    return f"{greeting}, {name}"

def total(*nums):        # any count
    return sum(nums)

def config(**options):   # named
    ...

greet("Sam", greeting="Yo")

Comprehensions

[n*n for n in range(5)]
[n for n in nums if n > 0]
{w: len(w) for w in words}
{c for c in text}
(n*n for n in nums)   # generator
any(x > 0 for x in nums)
all(x > 0 for x in nums)

Files

from pathlib import Path

Path("f.txt").write_text(s, encoding="utf-8")
text = Path("f.txt").read_text(encoding="utf-8")

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

Path("dir").mkdir(exist_ok=True)
Path(".").rglob("*.py")

Exceptions

try:
    n = int(raw)
except ValueError:
    print("not a number")
except (TypeError, KeyError) as err:
    print(err)
else:
    print("no error")
finally:
    print("always runs")

raise ValueError("message")

JSON

import json

text = json.dumps(data, indent=2)
data = json.loads(text)

with open("f.json") as f:
    data = json.load(f)
with open("f.json", "w") as f:
    json.dump(data, f)

Classes

from dataclasses import dataclass

@dataclass
class Point:
    x: int
    y: int

class Dog:
    def __init__(self, name):
        self.name = name
    def speak(self):
        return f"{self.name}: woof"
    def __repr__(self):
        return f"Dog({self.name!r})"

Type hints

def f(name: str, n: int = 1) -> str:
    ...

x: list[str] = []
y: dict[str, int] = {}
z: int | None = None

# checked by mypy, not at runtime

The venv + pip ritual

python3 -m venv .venv
source .venv/bin/activate   # mac/linux
.venv\Scripts\activate      # windows

pip install requests
pip freeze > requirements.txt
pip install -r requirements.txt
deactivate

Calling a model (Level 6)

import anthropic
client = anthropic.Anthropic()   # key from env

r = client.messages.create(
    model="claude-sonnet-5",
    max_tokens=1024,
    system="You are Jarvis.",
    messages=[{"role": "user",
               "content": "Hello"}],
)
print(r.content[0].text)

Common dunder methods

__init__     build the object
__repr__     for programmers (debug)
__str__      for humans (print)
__len__      len(x)
__eq__       x == y
__getitem__  x[i]  and iteration
__enter__/__exit__   with x:

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