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How to Implement a Stack Class in Python
A complete Stack class implemented with a Python list, featuring push, pop, peek, is_empty, size, and a readable string representation.
class Stack:
def __init__(self):
self._items = []
def push(self, item):
"""Add an item to the top of the stack."""
self._items.append(item)
def pop(self):
"""Remove and return the top item. Raises IndexError if empty."""
if self.is_empty():
raise IndexE…How to Implement the Decorator Pattern in Python to Add Behavior
This Python code demonstrates the decorator pattern by wrapping a function to add logging behavior without modifying the original function.
import functools
def logger(func):
@functools.wraps(func)
def wrapper(*args, **kwargs):
print(f"Calling {func.__name__} with {args} {kwargs}")
result = func(*args, **kwargs)
print(f"{func.__name__} returned {result}")
return result
return wrapper
@logger
def add(a, b):
…How to Make a Python Class Hashable with __eq__ and __hash__
Define __eq__ and __hash__ together on a Python class so equal instances share the same hash and work correctly in sets and dictionary keys.
class Point:
def __init__(self, x, y):
self.x = x
self.y = y
def __eq__(self, other):
if not isinstance(other, Point):
return NotImplemented
return self.x == other.x and self.y == other.y
def __hash__(self):
return hash((self.x, self.y))
def __repr…How to Sort Data in Python with a Class Helper
This beginner-friendly class wraps the built-in sorted() function to sort numbers, strings ignoring case, and dictionaries by a specified key.
class DataSorter:
def __init__(self, data):
self.data = data
def sort_numbers(self, reverse=False):
return sorted(self.data, reverse=reverse)
def sort_strings_ignore_case(self, reverse=False):
return sorted(self.data, key=str.lower, reverse=reverse)
def sort_dicts_by_key(self…How to Use IntEnum Arithmetic for Priority Levels in Python
Demonstrates Python IntEnum arithmetic for priority levels, showing how enum members behave like integers in calculations and comparisons.
from enum import IntEnum
class Priority(IntEnum):
LOW = 1
MEDIUM = 5
HIGH = 10
CRITICAL = 20
if __name__ == "__main__":
current = Priority.MEDIUM
boosted = current + 3
lowered = current - 2
doubled = current * 2
print(f"Current: {current} ({current.value})")
print(f"Boosted (…How to Use NamedTuples for Lightweight Records in Python
Create lightweight, immutable data records with namedtuple that behave like tuples but have named fields for improved readability and access.
from collections import namedtuple
Point = namedtuple("Point", ["x", "y"])
p = Point(3, 4)
print(p)
print(p.x, p.y)
print(p[0], p[1])
x, y = p
print(x, y)
print(p._asdict())
p2 = p._replace(x=10)
print(p2)
if __name__ == "__main__":
print("NamedTuple demo complete")How to Use StrEnum with auto() in Python
Define string-valued enum members automatically by using StrEnum with the auto() helper, making each member's value its own uppercase name.
from enum import StrEnum, auto
class Color(StrEnum):
RED = auto()
GREEN = auto()
BLUE = auto()
class Language(StrEnum):
PYTHON = auto()
JAVASCRIPT = auto()
RUST = auto()
print(list(Color))
print(list(Language))
print(Color.RED == "RED")
print(Language.PYTHON == "PYTHON")
print(f"Color: {Co…How to Validate Data Types in Python with a Class
A beginner-friendly Python class that checks if a value is a string, integer, float, list, or empty, using simple methods and isinstance checks.
class DataValidator:
"""A simple data validation helper for beginners."""
def __init__(self, data):
self.data = data
def is_string(self):
return isinstance(self.data, str)
def is_integer(self):
return isinstance(self.data, int) and not isinstance(self.data, bool)
…How to Validate User Input with a Dataclass in Python
A dataclass stores name, age, and email, and a validator class checks each field, returning a dictionary of boolean results.
from dataclasses import dataclass
@dataclass
class UserInput:
name: str
age: int
email: str
def is_valid_name(self) -> bool:
return bool(self.name.strip()) and len(self.name.strip()) >= 2
def is_valid_age(self) -> bool:
return isinstance(self.age, int) and 0 < self.age < 150
…How to define a custom exception class in Python with an error code attribute
Create a custom exception class with extra attributes like an error code, then raise and catch it in a try/except block.
class UserNotFoundError(Exception):
def __init__(self, user_id, error_code=404):
self.user_id = user_id
self.error_code = error_code
super().__init__(f"User with ID {user_id} was not found (error code: {error_code})")
def find_user(user_id, users_db):
if user_id not in users_db:
…How to merge dictionaries by a key in Python with a class
This code defines a DataMerger class that collects dictionary records and merges them by a specified key, combining fields from multiple records with the same key.
class DataMerger:
def __init__(self):
self.records = []
def add_record(self, record):
if isinstance(record, dict):
self.records.append(record)
else:
raise TypeError("Record must be a dictionary")
def merge_by_key(self, key):
merged = {}
for …Parse CSV Data with a Python Class
Encapsulate CSV file loading and column/row access methods in a reusable DataParser class for beginners.
class DataParser:
def __init__(self, file_path):
self.file_path = file_path
self.data = []
def load_data(self):
with open(self.file_path, 'r') as file:
for line in file:
row = line.strip().split(',')
self.data.append(row)
return self.…Python Adapter Class: Wrap Legacy Interface
Convert a legacy system's interface into a modern one using the Adapter pattern in Python, translating method calls and data formats.
class LegacySystem:
"""Legacy interface - old method names and parameter format."""
def query_employee_info(self, emp_id, emp_name):
return f"Legacy: {emp_id} - {emp_name}"
def update_employee_department(self, emp_id, department_code):
return f"Legacy: Updated {emp_id} to dept {department_…Python Factory Method: Create Shapes by Type String
A factory method that maps a type string to a concrete shape class and returns an instance, with runtime error handling.
class Shape:
def draw(self):
raise NotImplementedError
class Circle(Shape):
def draw(self):
return "Drawing a circle"
class Square(Shape):
def draw(self):
return "Drawing a square"
class Triangle(Shape):
def draw(self):
return "Drawing a triangle"
class ShapeFact…Python object equality: id vs value comparison
Demonstrates the difference between default identity comparison and custom equality, with a value-based class implementing __eq__ and __hash__.
import copy
class IdOnly:
def __init__(self, name):
self.name = name
class ValueId:
def __init__(self, name):
self.name = name
def __eq__(self, other):
return isinstance(other, ValueId) and self.name == other.name
def __hash__(self):
return hash(self.name)
def…Slots Class: How to Reduce Memory Usage in Python
Use __slots__ to prevent dynamic attribute creation and reduce per-instance memory overhead, while keeping methods intact.
class SlotsDemo:
__slots__ = ("name", "age", "email")
def __init__(self, name, age, email):
self.name = name
self.age = age
self.email = email
def describe(self):
return f"{self.name}, {self.age}, {self.email}"
if __name__ == "__main__":
instance = SlotsDemo("Alice", …Validate dataclass fields with __post_init__ in Python
Add custom validation to a Python dataclass inside __post_init__, raising ValueError or TypeError for invalid field values.
from dataclasses import dataclass, field
from typing import Optional
@dataclass
class Product:
name: str
price: float
quantity: int = 1
category: Optional[str] = None
def __post_init__(self):
if not self.name or not isinstance(self.name, str):
raise ValueError("name must be a…Bucket Numbers into Histogram Bin Counts in Python
Partition a list of numbers into equal-width histogram bins and count how many fall into each bin using only the Python standard library.
from collections import Counter
def histogram_bins(numbers, num_bins):
"""Bucket numbers into histogram bin counts."""
if not numbers:
return []
min_val = min(numbers)
max_val = max(numbers)
bin_width = (max_val - min_val) / num_bins
# Handle edge case where all values are id…Count Smaller Elements to the Right in Python
Return a list where each index counts how many elements to its right are smaller than that element using a clean O(n²) nested-loop approach.
def count_smaller_elements(arr):
"""
Return a list where result[i] is the number of elements
to the right of arr[i] that are smaller than arr[i].
"""
result = []
for i in range(len(arr)):
count = 0
for j in range(i + 1, len(arr)):
if arr[j] < arr[i]:
…Depth First Search Traversal Order in Python
Recursive depth-first search that returns the visit order of nodes in an adjacency list graph starting from a given node.
def dfs_order(adj, start):
visited = set()
order = []
def dfs(node):
visited.add(node)
order.append(node)
for neighbor in adj.get(node, []):
if neighbor not in visited:
dfs(neighbor)
dfs(start)
return order
if __name__ == "__main__":
# Dem…Drop Elements From Start While Condition Is True in Python
This generator function drops elements from the beginning of an iterable while a predicate returns true, then yields the rest.
def drop_while(predicate, iterable):
"""Drop elements from the start while predicate is true."""
it = iter(iterable)
for item in it:
if not predicate(item):
yield item
break
yield from it
if __name__ == "__main__":
numbers = [1, 2, 3, 4, 1, 2, 5]
result = list(d…Extract n largest elements from a large list using heapq
Uses heapq.nlargest to efficiently extract the top n largest numbers from a large list, even with millions of elements.
import heapq
import random
def n_largest(numbers, n):
"""Return the n largest numbers from a list using heapq."""
if n <= 0:
return []
return heapq.nlargest(n, numbers)
if __name__ == "__main__":
# Create a large list with 1,000,000 random numbers
large_list = [random.randint(1, 1_000_000…Filter List to Keep Only Whitelist Values in Python
Filter a list of values to keep only those present in a predefined whitelist set using a list comprehension.
def filter_whitelist(values, whitelist):
"""Return only values that are present in the whitelist set."""
return [value for value in values if value in whitelist]
if __name__ == "__main__":
raw_values = ["apple", "banana", "cherry", "date", "apple", "elderberry"]
allowed = {"apple", "banana", "date"}
…Find All Indices of a Target Value in a Python List
Returns a list of all indices where a given target value appears in a Python list using a list comprehension with enumerate.
def find_all_indices(arr, target):
return [i for i, value in enumerate(arr) if value == target]
if __name__ == "__main__":
sample_list = [4, 2, 7, 2, 9, 2, 1, 2]
target = 2
result = find_all_indices(sample_list, target)
print(result)Browse by section
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