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Serialize Python dict to JSON with custom default for datetime
Convert a Python dict containing datetime and set objects into JSON by providing a custom default serializer.
import json
from datetime import datetime
def custom_serializer(obj):
if isinstance(obj, datetime):
return obj.isoformat()
if isinstance(obj, set):
return list(obj)
return str(obj)
data = {
"name": "Alice",
"created_at": datetime(2024, 3, 15, 10, 30, 45),
"tags": {"python", "j…Text Processor with Dictionaries and Sets in Python
Build a simple text processor that counts word frequencies with a dictionary and tracks unique words with a set.
def analyze_text(text):
words = text.lower().split()
word_freq = {}
unique_words = set()
for word in words:
clean_word = word.strip('.,!?;:')
if clean_word:
word_freq[clean_word] = word_freq.get(clean_word, 0) + 1
unique_words.add(clean_word)
return…Traverse Nested Dict Paths Depth-First in Python
Recursively walk a nested dictionary depth-first and yield each full path from root to leaf as lists.
def depth_first_paths(node, path=None):
if path is None:
path = []
if not isinstance(node, dict):
yield path + [node]
return
for key, value in node.items():
new_path = path + [key]
if isinstance(value, dict):
yield from depth_first_paths(value, …Unflatten Dot Keys to Nested Dict in Python
Convert a flat dictionary with dot-separated keys into a nested dictionary structure using recursive setdefault loops.
def unflatten_dot_keys(flat_dict):
result = {}
for flat_key, value in flat_dict.items():
parts = flat_key.split(".")
current = result
for part in parts[:-1]:
current = current.setdefault(part, {})
current[parts[-1]] = value
return result
if __name__ == "__main_…Validate dictionary data with sets in Python
Validate a dictionary against required keys and allowed value sets, returning a list of validation errors.
def validate_data(data, required_keys, allowed_values=None):
"""
Validate a dictionary against required keys and optional allowed value sets.
Returns a list of validation errors (empty list if valid).
"""
errors = []
# Check for missing required keys
missing = set(required_keys) - set(…Borg pattern shared state in Python
Implement the Borg pattern to share state across class instances by assigning a class-level dictionary to each instance's __dict__.
class Borg:
_shared_state = {}
def __init__(self):
self.__dict__ = Borg._shared_state
class ConfigManager(Borg):
def __init__(self):
super().__init__()
if not hasattr(self, "settings"):
self.settings = {}
def set(self, key, value):
self.settings[key] = va…Filtering data with a Python class helper
A beginner-friendly DataFilter class that filters lists of dictionaries by exact match, greater-than, and substring conditions.
class DataFilter:
"""A beginner-friendly helper to filter lists of dictionaries."""
def __init__(self, data):
self.data = data
def filter_by(self, key, value):
"""Return items where data[key] == value."""
return [item for item in self.data if item.get(key) == value]
…Graph Class with Adjacency Dict in Python
Build an undirected graph class using a dictionary of adjacency lists with methods to add vertices, edges, remove edges, and query neighbors.
class Graph:
def __init__(self):
self.adjacency = {}
def add_vertex(self, vertex):
if vertex not in self.adjacency:
self.adjacency[vertex] = []
def add_edge(self, u, v):
self.add_vertex(u)
self.add_vertex(v)
self.adjacency[u].append(v)
self.adja…How to Build a Class Method Alternative Constructor from Dict in Python
Use a classmethod alternative constructor to build a Book instance from a dictionary with sensible defaults.
class Book:
def __init__(self, title, author, pages):
self.title = title
self.author = author
self.pages = pages
@classmethod
def from_dict(cls, data):
"""Alternative constructor that builds a Book from a dictionary."""
return cls(
title=data["title"],
…How to Build an In-Memory CRUD Repository Class in Python
Define a Python Repository class that stores objects in a dictionary and supports create, read, update, delete, and list operations.
class Repository:
def __init__(self):
self._data = {}
def create(self, key, value):
self._data[key] = value
return key
def read(self, key):
return self._data.get(key)
def update(self, key, value):
if key not in self._data:
raise KeyError(f"Key '{ke…How to Convert Data Types in Python with a Helper Class
This code defines a beginner-friendly OOP helper class for common data conversions like string to list, list to dict, JSON string, and CSV row, with an advanced subclass for numeric casting.
class DataConverter:
"""A beginner-friendly helper class for common data conversions."""
def __init__(self, data):
self.data = data
def to_list(self):
"""Convert string data (comma-separated) to a list."""
if isinstance(self.data, str):
return [item.strip() for…How to Count Items in a Python Class
A beginner-friendly Inventory class that stores item quantities in a dictionary and provides add, remove, count, and summary methods.
class Inventory:
def __init__(self):
self.items = {}
def add(self, item, quantity=1):
self.items[item] = self.items.get(item, 0) + quantity
def remove(self, item, quantity=1):
if item not in self.items:
raise ValueError(f"{item} not in inventory")
self.items[it…How to Create a Data Formatter Class in Python
A beginner-friendly helper class to format lists, dictionaries, and stored records into readable strings.
class DataFormatter:
"""Helper class for beginners to format common data types."""
def __init__(self, name="data"):
self.name = name
self.records = []
def add_record(self, key, value):
"""Add a key-value record to the formatter."""
self.records.append({"key": key, …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 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 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 …Find Elements Appearing More Than n/3 Times in Python
Return all elements that occur more than len(array)/3 times using a simple dictionary counter.
def majority_third(arr):
"""Return elements appearing more than len(arr)/3 times."""
cutoff = len(arr) / 3
counts = {}
for x in arr:
counts[x] = counts.get(x, 0) + 1
return [x for x, c in counts.items() if c > cutoff]
if __name__ == "__main__":
test1 = [3, 2, 3]
test2 = [1, 1, 1, …Find First Duplicate Index in Python
Return the index of the first element that appears more than once in a list, using a dictionary for O(n) time.
def find_first_duplicate(arr):
seen = {}
for index, value in enumerate(arr):
if value in seen:
return index
seen[value] = index
return -1
if __name__ == "__main__":
test_array = [3, 5, 2, 8, 5, 1, 2]
result = find_first_duplicate(test_array)
print(f"Array: {test_arr…Find Maximum Distance Between Identical Elements in Python
Compute the maximum index distance between any two identical elements in a list using a dictionary to track first occurrences.
from collections import defaultdict
def max_distance_between_identical(nums):
first_occurrence = {}
max_dist = 0
for i, num in enumerate(nums):
if num in first_occurrence:
dist = i - first_occurrence[num]
max_dist = max(max_dist, dist)
else:
first_occur…How to Evaluate RPN Expressions in Python
Use a stack to evaluate Reverse Polish Notation token lists with a dictionary of operator lambdas, truncating division toward zero.
def eval_rpn(tokens):
stack = []
ops = {
'+': lambda a, b: a + b,
'-': lambda a, b: a - b,
'*': lambda a, b: a * b,
'/': lambda a, b: int(a / b) # truncate toward zero
}
for token in tokens:
if token in ops:
b = stack.pop()
a = stack.pop(…How to Flatten List of Dict Values in Python
This code flattens the values of a list of dictionaries into a single list, handling both list values and scalar values.
def flatten_dict_values(dicts):
flattened = []
for d in dicts:
for value in d.values():
if isinstance(value, list):
flattened.extend(value)
else:
flattened.append(value)
return flattened
if __name__ == "__main__":
data = [
{"a": …Implement Insert Delete GetRandom O(1) in Python
Build a RandomizedSet class that supports insert, delete, and get_random in average O(1) time using a list and a dictionary mapping values to indices.
import random
class RandomizedSet:
def __init__(self):
self.values = []
self.index_map = {}
def insert(self, val):
if val in self.index_map:
return False
self.index_map[val] = len(self.values)
self.values.append(val)
return True
def delete(self…Sort list by multiple keys with tuple ordering in Python
Sort a list of dictionaries by multiple criteria — surname, age, then score descending — using a tuple key and negation.
def sort_multi_key(data):
# Sorts by surname, then age, then score descending
return sorted(
data,
key=lambda person: (
person['surname'].lower(),
person['age'],
-person['score'] # negative to reverse sort by score
)
)
if __name__ == "__main__"…Browse by section
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