Python Quickstart Reference - Quick Reference

Quick reference guide for Python. Essential syntax, control flow, data structures, and common patterns.

Hello World

print("Hello, World!")  # The universal programmer's rite of passage.

Variables

x = 42          # integer
name = "Alice"  # string
flag = True     # boolean
pi = 3.14       # float

Control Flow

If/Else

if x > 10:               # Colon is mandatory; indentation rules supreme
    print("x is big")    
else:
    print("x is small")

if x > 10:
    print("big")
elif x > 5:
    print("medium")
else:
    print("small")

For Loops

for i in range(5):       # i goes from 0 to 4
    print(i)

for item in [1, 2, 3]:   # Iterate over list
    print(item)

for i in range(0, 10, 2):  # Start, stop, step
    print(i)                # Prints 0, 2, 4, 6, 8

While Loops

while x > 0:             # Beware infinite loops
    x -= 1
    print(x)

Functions

def add(a, b):           # 'def' starts the function
    return a + b

print(add(2, 3))         # Call a function with parentheses

def greet(name="World"): # Default parameter
    return f"Hello, {name}!"

greet()                  # Uses default
greet("Alice")           # Overrides default

Lists / Dictionaries

Lists

nums = [1, 2, 3]         # Lists are ordered, mutable
for n in nums:           
    print(n)

nums.append(4)           # Add to end
nums.insert(0, 0)        # Insert at index
nums.pop()               # Remove and return last item
nums[0]                  # Access by index
nums[-1]                 # Last item (negative indexing)
nums[1:3]                # Slice: items 1 to 2 (not 3)

Dictionaries

person = {"name": "Alice", "age": 30}  # Key-value mapping
print(person["name"])                    # Access via keys
person["city"] = "NYC"                   # Add new key-value
person.get("name")                       # Safe access (returns None if missing)
person.get("phone", "N/A")               # With default value

for key, value in person.items():        # Iterate over key-value pairs
    print(f"{key}: {value}")

Tuples

point = (3, 4)           # Immutable ordered collection
x, y = point             # Unpacking

Sets

unique = {1, 2, 3}       # Unordered collection of unique items
unique.add(4)            # Add item
unique.remove(2)         # Remove item

String Operations

text = "Hello, World!"
text.upper()             # "HELLO, WORLD!"
text.lower()             # "hello, world!"
text.split(",")          # ["Hello", " World!"]
text.replace("World", "Python")  # "Hello, Python!"
f"Value: {x}"            # f-string formatting
"Value: {}".format(x)    # .format() method

List Comprehensions

squares = [x**2 for x in range(10)]           # [0, 1, 4, 9, 16, ...]
evens = [x for x in range(10) if x % 2 == 0] # [0, 2, 4, 6, 8]

Error Handling

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Can't divide by zero!")
except Exception as e:
    print(f"Error: {e}")
finally:
    print("This always runs")

File Operations

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

# Writing
with open("file.txt", "w") as f:
    f.write("Hello, World!")

# Appending
with open("file.txt", "a") as f:
    f.write("New line\n")

Classes

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    
    def greet(self):
        return f"Hello, I'm {self.name}"

person = Person("Alice", 30)
print(person.greet())

Tips

Indentation *is* syntax — Python hates lazy spacing.

input() always returns a string; convert to int/float if needed:
```
age = int(input("How old are you? "))
```

Python is dynamically typed: flexible, but watch out for sneaky type errors.

Everything is an object, even functions and classes.

range(5) goes 0..4, not 5 — the Pythonic gotcha.

Use with statements for file operations — automatic cleanup.

List comprehensions are faster and more Pythonic than loops for simple transformations.

Use enumerate() when you need both index and value:

for i, item in enumerate(items):
    print(f"{i}: {item}")

Use zip() to iterate over multiple lists simultaneously:

for name, age in zip(names, ages):
    print(f"{name} is {age}")

Import modules with import module or specific functions with from module import function.