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Cheatsheet Python

Aide-mémoire syntaxe et patterns Python.

Types de Base

# Nombres
x = 42              # int
x = 3.14            # float
x = 1_000_000       # Séparateur lisibilité

# Chaînes
s = "hello"
s = 'hello'
s = """multiline"""
s = f"Hello {name}"  # f-string

# Booléens
b = True
b = False

# None
x = None

Structures de Données

# Liste (mutable, ordonné)
l = [1, 2, 3]
l.append(4)
l[0]                # Premier élément
l[-1]               # Dernier élément
l[1:3]              # Slice

# Tuple (immutable, ordonné)
t = (1, 2, 3)
x, y, z = t         # Unpacking

# Dictionnaire
d = {"key": "value"}
d["key"]            # Accès
d.get("key", "default")
d.keys()
d.values()
d.items()

# Set (unique, non ordonné)
s = {1, 2, 3}
s.add(4)
s1 | s2             # Union
s1 & s2             # Intersection

Compréhensions

# Liste
[x*2 for x in range(10)]
[x for x in items if x > 0]

# Dict
{k: v*2 for k, v in d.items()}

# Set
{x.lower() for x in names}

# Generator
(x*2 for x in range(10))

Contrôle de Flux

# Conditions
if x > 0:
    pass
elif x < 0:
    pass
else:
    pass

# Ternaire
value = "yes" if condition else "no"

# Boucle for
for item in items:
    pass

for i, item in enumerate(items):
    pass

for key, value in dict.items():
    pass

# Boucle while
while condition:
    pass

Fonctions

# Définition
def func(arg1, arg2="default"):
    """Docstring."""
    return result

# Args variables
def func(*args, **kwargs):
    pass

# Lambda
f = lambda x: x * 2

# Décorateur
@decorator
def func():
    pass

Classes

class Server:
    """Un serveur."""

    def __init__(self, hostname, ip):
        self.hostname = hostname
        self.ip = ip

    def __str__(self):
        return f"{self.hostname} ({self.ip})"

    def ping(self):
        return True

# Héritage
class WebServer(Server):
    def __init__(self, hostname, ip, port=80):
        super().__init__(hostname, ip)
        self.port = port

Fichiers

# Lecture
with open("file.txt") as f:
    content = f.read()
    lines = f.readlines()

# Écriture
with open("file.txt", "w") as f:
    f.write("content")

# pathlib
from pathlib import Path
p = Path("/var/log")
p.exists()
p.is_file()
p.read_text()
p.write_text("content")
list(p.glob("*.log"))

Exceptions

try:
    result = risky_operation()
except ValueError as e:
    print(f"Error: {e}")
except (TypeError, KeyError):
    pass
except Exception as e:
    raise
else:
    print("Success")
finally:
    cleanup()

# Lever une exception
raise ValueError("Invalid input")

Modules Utiles

import os
import sys
import json
import yaml
import re
import subprocess
import logging
from pathlib import Path
from datetime import datetime
from collections import Counter, defaultdict

# os
os.getenv("HOME")
os.getcwd()
os.listdir(".")

# sys
sys.argv          # Arguments CLI
sys.exit(1)       # Quitter

# datetime
now = datetime.now()
now.strftime("%Y-%m-%d %H:%M:%S")
datetime.fromisoformat("2024-01-15")

# subprocess
result = subprocess.run(["ls", "-la"], capture_output=True, text=True)
result.stdout
result.returncode

Patterns Courants

# Valeur par défaut
value = x or "default"

# Swap
a, b = b, a

# Aplatir une liste
flat = [item for sublist in nested for item in sublist]

# Grouper par clé
from collections import defaultdict
groups = defaultdict(list)
for item in items:
    groups[item["key"]].append(item)

# Compteur
from collections import Counter
counts = Counter(items)
counts.most_common(10)

# Context manager
from contextlib import contextmanager

@contextmanager
def timer():
    start = time.time()
    yield
    print(f"Elapsed: {time.time() - start}")

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