A quick reference for the most commonly used Python syntax and concepts.
name = "Phil" # str
age = 25 # int
height = 5.11 # float
is_active = True # bool
type(name) # <class 'str'>
# Type conversion
int("25") # 25
float(10) # 10.0
str(100) # "100"
bool(0) # Falsename = "python"
name.upper() # "PYTHON"
name.lower() # "python"
name.capitalize() # "Python"
name.replace("p", "j") # "jython"
name.strip() # remove whitespace
name.split(",") # split into list
name.startswith("py") # True
name.endswith("on") # True
len(name) # 6
# f-strings
print(f"Hello {name}")
# Slicing
name[0] # "p"
name[-1] # "n"
name[0:3] # "pyt"
name[:3] # "pyt"
name[3:] # "hon"fruits = ["apple", "banana", "cherry"]
fruits[0] # "apple"
fruits[-1] # "cherry"
fruits[0:2] # ["apple", "banana"]
fruits.append("mango") # add to end
fruits.insert(1, "kiwi") # add at index
fruits.remove("apple") # remove by value
fruits.pop() # remove last item
fruits.pop(0) # remove at index
fruits.sort() # sort ascending
fruits.reverse() # reverse list
fruits.clear() # empty the list
len(fruits) # number of items
"apple" in fruits # True / False
# List comprehension
squares = [x ** 2 for x in range(10)]
evens = [x for x in range(10) if x % 2 == 0]coords = (10, 20) # immutable list
coords[0] # 10
x, y = coords # unpacking
len(coords) # 2
10 in coords # Trueuser = {
"name": "Phil",
"age": 25,
"active": True
}
user["name"] # "Phil"
user.get("name") # "Phil" (safe, no error if missing)
user["email"] = "a@b.com" # add key
user.pop("age") # remove key
user.keys() # all keys
user.values() # all values
user.items() # all key/value pairs
"name" in user # True
# Loop through
for key, value in user.items():
print(key, value)tags = {"python", "code", "dev"}
tags.add("learn") # add item
tags.remove("dev") # remove item
tags.discard("x") # remove safely (no error)
# Set operations
a = {1, 2, 3}
b = {2, 3, 4}
a | b # union {1, 2, 3, 4}
a & b # intersection {2, 3}
a - b # difference {1}age = 20
if age >= 18:
print("adult")
elif age >= 13:
print("teen")
else:
print("child")
# One line
status = "adult" if age >= 18 else "minor"
# Operators
== != > < >= <=
and or not# for loop
for i in range(5):
print(i) # 0 1 2 3 4
for item in ["a", "b", "c"]:
print(item)
# while loop
count = 0
while count < 5:
print(count)
count += 1
# Loop controls
break # exit loop
continue # skip to next iteration
pass # do nothing placeholder
# enumerate — index + value
for i, val in enumerate(["a", "b", "c"]):
print(i, val)
# zip — loop two lists together
for a, b in zip([1, 2], ["x", "y"]):
print(a, b)def greet(name):
return f"Hello {name}"
greet("Phil") # "Hello Phil"
# Default parameter
def greet(name="World"):
return f"Hello {name}"
# Multiple return values
def min_max(nums):
return min(nums), max(nums)
low, high = min_max([1, 5, 3])
# *args — variable arguments
def total(*nums):
return sum(nums)
total(1, 2, 3) # 6
# **kwargs — keyword arguments
def display(**info):
for k, v in info.items():
print(k, v)
display(name="Phil", age=25)
# Lambda
square = lambda x: x ** 2
square(4) # 16try:
result = 10 / 0
except ZeroDivisionError:
print("Cannot divide by zero")
except Exception as e:
print(f"Error: {e}")
else:
print("No error")
finally:
print("Always runs")
# Raise an error
raise ValueError("Invalid input")# Read
with open("file.txt", "r") as f:
content = f.read()
# Write
with open("file.txt", "w") as f:
f.write("Hello")
# Append
with open("file.txt", "a") as f:
f.write("\nNew line")
# Read lines into list
with open("file.txt", "r") as f:
lines = f.readlines()class Dog:
def __init__(self, name, age):
self.name = name
self.age = age
def bark(self):
return f"{self.name} says woof!"
def __str__(self):
return f"Dog({self.name}, {self.age})"
# Create instance
dog = Dog("Rex", 3)
dog.bark() # "Rex says woof!"
print(dog) # Dog(Rex, 3)
# Inheritance
class Puppy(Dog):
def __init__(self, name):
super().__init__(name, 0)
def bark(self):
return f"{self.name} says yip!"import math
import random
import os
import json
from datetime import datetime
math.sqrt(16) # 4.0
math.pi # 3.14159
random.randint(1, 10) # random int 1-10
random.choice(["a","b"]) # random item
os.getcwd() # current directory
os.listdir(".") # list files
os.path.exists("file") # check if exists
datetime.now() # current date/timeimport json
# Dict to JSON string
data = {"name": "Phil", "age": 25}
json_str = json.dumps(data, indent=2)
# JSON string to dict
parsed = json.loads(json_str)
# Read JSON file
with open("data.json", "r") as f:
data = json.load(f)
# Write JSON file
with open("data.json", "w") as f:
json.dump(data, f, indent=2)import requests
# GET
response = requests.get("https://api.github.com", timeout=5)
response.status_code # 200
response.json() # parse JSON response
# GET with params
params = {"q": "python"}
response = requests.get(url, params=params)
# POST
data = {"name": "Phil"}
response = requests.post(url, json=data)
# Headers
headers = {"Authorization": "Bearer token"}
response = requests.get(url, headers=headers)
# Error handling
response.raise_for_status() # raises error for 4xx/5xxlen(x) # length
type(x) # data type
print(x) # output
input("msg") # user input
range(n) # sequence of numbers
int/float/str/bool(x) # type conversion
sorted(x) # return sorted copy
reversed(x) # return reversed iterator
enumerate(x) # index + value pairs
zip(a, b) # pair two iterables
map(fn, x) # apply function to each item
filter(fn, x) # filter items by condition
sum(x) # total
min(x) # lowest value
max(x) # highest value
abs(x) # absolute value
round(x, n) # round to n decimal places
isinstance(x, type) # check type.upper() .lower() .capitalize()
.strip() .lstrip() .rstrip()
.replace(a, b) .split(sep) .join(list)
.startswith() .endswith() .find()
.count() .format() .encode()
.isdigit() .isalpha() .isspace().append() .insert() .remove()
.pop() .clear() .copy()
.sort() .reverse() .extend()
.index() .count()Keep this handy — updated as I learn more.