x = 10 → Integer
x = 3.14 → Float
x = "hello" → String
x = True / False → Boolean
x = None → NoneType
x = [1, 2, 3] → List (mutable, ordered)
x = (1, 2, 3) → Tuple (immutable, ordered)
x = {1, 2, 3} → Set (unique, unordered)
x = {"k": "v"} → Dictionary (key-value pairs)
x = 42 → Assign variable
type(x) → Return type of variable
isinstance(x, int) → Check if x is an int (returns bool)
int("5") / str(5) → Type casting
f"{variable}" → f-string (preferred string formatting)
s = "hello world"
s.upper() → "HELLO WORLD"
s.lower() → "hello world"
s.strip() → Remove leading/trailing whitespace
s.split(" ") → Split into list by delimiter
s.replace("hello", "hi") → Replace substring
s.startswith("hello") → Returns True/False
s.find("world") → Returns index of substring (-1 if not found)
len(s) → Length of string
s[0:5] → Slice string (index 0 to 4)
" ".join(["a", "b", "c"]) → Join list into string → "a b c"
lst = [1, 2, 3]
lst.append(4) → Add item to end
lst.insert(1, 99) → Insert 99 at index 1
lst.remove(2) → Remove first occurrence of value
lst.pop() → Remove & return last item
lst.pop(0) → Remove & return item at index
lst.sort() → Sort in place (ascending)
lst.reverse() → Reverse list in place
lst.index(3) → Return index of value
len(lst) → Length of list
lst[0] → Access by index
lst[-1] → Access last item
lst[1:3] → Slice list
[x for x in lst if x > 1] → List comprehension with filter
d = {"name": "Alice", "age": 30}
d["name"] → Access value by key
d.get("name", "default") → Safe access (no KeyError)
d["city"] = "NYC" → Add/update key
d.pop("age") → Remove key & return value
d.keys() → Return all keys
d.values() → Return all values
d.items() → Return all key-value pairs
"name" in d → Check if key exists (True/False)
if x > 10:
pass
elif x == 10:
pass
else:
pass
for i in range(5): → Loop 0 to 4
pass
for i, v in enumerate(lst): → Loop with index
pass
while condition:
break → Exit loop
continue → Skip to next iteration
def greet(name, greeting="Hello"): → Default parameter
return f"{greeting}, {name}!"
lambda x: x * 2 → Anonymous function
*args → Accept variable positional args
**kwargs → Accept variable keyword args
try:
risky_code()
except ValueError as e:
print(e)
except (TypeError, KeyError):
pass
finally:
pass → Always runs
raise ValueError("msg") → Manually raise exception
with open("file.txt", "r") as f: → Read
content = f.read()
lines = f.readlines()
with open("file.txt", "w") as f: → Write (overwrites)
f.write("hello\n")
with open("file.txt", "a") as f: → Append
f.write("more\n")
class Dog:
def init(self, name): → Constructor
self.name = name
def bark(self):
return f"{self.name} says woof!"
class Poodle(Dog): → Inheritance
pass
isinstance(obj, Dog) → Check class membership
range(start, stop, step) → Generate number sequence
zip(lst1, lst2) → Pair up two iterables
map(func, lst) → Apply function to each item
filter(func, lst) → Filter items by function
sorted(lst, reverse=True) → Return sorted copy
min(lst) / max(lst) → Min/max of iterable
sum(lst) → Sum of iterable
abs(x) → Absolute value
round(x, 2) → Round to decimal places
print(x, end="", sep=",") → Print with custom end/separator
import os
import sys
from pathlib import Path
import json
import re → Regex
from datetime import datetime
import requests → HTTP (pip install)
with open("data.json") as f:
data = json.load(f)
with open("data.json", "w") as f:
json.dump(data, f, indent=2)
files = os.listdir(".")
Path("file.txt").exists()
datetime.now().strftime("%Y-%m-%d %H:%M:%S")
re.findall(r"\d+", "abc123") → ['123']