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How to Sort a List of Dictionaries by a Key in Python
Sort a list of dictionaries by a specified key field, optionally in descending order, using Python's built-in sorted() function.
def sort_dicts_by_key(data, key, reverse=False):
return sorted(data, key=lambda item: item.get(key), reverse=reverse)
if __name__ == "__main__":
people = [
{"name": "Alice", "age": 30},
{"name": "Bob", "age": 25},
{"name": "Charlie", "age": 35},
]
sorted_by_age = sort_dicts_b…How to Sort a List of Tuples by the Second Element in Python
Sorts a list of tuples by the second element using the sorted() function with a lambda key, preserving the original list.
def sort_tuples_by_second(tuples_list):
"""Sort a list of tuples by the second element."""
return sorted(tuples_list, key=lambda x: x[1])
if __name__ == "__main__":
data = [(1, 5), (3, 2), (2, 8), (4, 1)]
sorted_data = sort_tuples_by_second(data)
print("Original list:", data)
print("Sorted by…How to Compose Two Functions into a Single Callable in Python
Combine two Python functions into a single callable using a compose helper, then apply the chained call.
def add_one(x):
return x + 1
def double(x):
return x * 2
def compose(f, g):
return lambda x: f(g(x))
add_then_double = compose(double, add_one)
double_then_add = compose(add_one, double)
result1 = add_then_double(5)
result2 = double_then_add(5)
print(f"add_one then double(5) = {result1}")
print(f"doub…How to Sort a List of Dictionaries by Key with a Lambda in Python
Sort a list of dictionaries ascending or descending by one of their keys using sorted() with a lambda as the key function — a beginner-friendly pattern.
def get_students():
return [
{"name": "alice", "score": 85},
{"name": "bob", "score": 92},
{"name": "carol", "score": 78},
{"name": "dave", "score": 92},
]
students = get_students()
sorted_by_score = sorted(students, key=lambda s: s["score"])
print("Sorted by score (ascending)…How to Use Lambda Sorting Keys in Python
Learn to sort lists of dictionaries using lambda functions as key arguments in Python's sorted() method.
# Demonstrate lambda as a sorting key function
students = [
{"name": "Alice", "grade": 88},
{"name": "Bob", "grade": 92},
{"name": "Charlie", "grade": 75},
{"name": "Diana", "grade": 95}
]
# Sort by grade (ascending) using a lambda key
sorted_by_grade = sorted(students, key=lambda student: student["g…How to Use a Lambda Sort Key in Python
Sort a list of strings by length, then alphabetically, using a lambda function as the sorting key in Python.
def sort_words(words):
"""Sort words by length, then alphabetically using a lambda key."""
return sorted(words, key=lambda word: (len(word), word))
if __name__ == "__main__":
sample_words = ["apple", "kiwi", "banana", "fig", "cherry"]
result = sort_words(sample_words)
print("Original:", sampl…How to Use a Lambda Sorting Key in Python
Sort a list of strings by their last letter using a lambda function as the sorting key.
def get_last_letter(word):
return word[-1]
words = ["banana", "apple", "cherry", "date", "elderberry"]
if __name__ == "__main__":
sorted_words = sorted(words, key=get_last_letter)
print(sorted_words)How to Use functools.reduce in Python
Apply functools.reduce with operator functions and lambda expressions to aggregate lists into sums, products, maximums, and concatenated strings.
from functools import reduce
import operator
# Sum all numbers in a list using reduce
numbers = [1, 2, 3, 4, 5]
sum_result = reduce(operator.add, numbers)
# Find the maximum value using reduce
max_result = reduce(lambda a, b: a if a > b else b, numbers)
# Multiply all numbers using reduce
product_result = reduce(la…Sort a List of Dictionaries by Key in Python
Uses a lambda function with sorted() to order a list of dictionaries by a specified key, like price.
def get_items():
return [
{"name": "apple", "price": 3},
{"name": "banana", "price": 1},
{"name": "cherry", "price": 2},
]
if __name__ == "__main__":
items = get_items()
sorted_items = sorted(items, key=lambda item: item["price"])
for item in sorted_items:
print(f"{…How to Filter CSV Rows by Column Value in Python
Filter CSV rows based on a column value condition using the standard csv module and a lambda function.
import csv
def filter_csv(input_file, output_file, column, condition):
with open(input_file, newline='', encoding='utf-8') as infile, \
open(output_file, 'w', newline='', encoding='utf-8') as outfile:
reader = csv.DictReader(infile)
fieldnames = reader.fieldnames
writer = csv.Dict…How to Sort Files by Name and Size in Python
Sort a list of file dictionaries by name then size using Python's sorted() with a lambda key.
from pathlib import Path
def sort_files_data(files):
"""Sort a list of file dictionaries by name, then by size."""
return sorted(files, key=lambda f: (f["name"], f["size"]))
if __name__ == "__main__":
files_data = [
{"name": "report.pdf", "size": 2048},
{"name": "data.csv", "size": 1024},…How to Filter a Dictionary by Predicate on Values in Python
This code defines a reusable function that builds a new dictionary containing only the items whose values satisfy a given predicate function.
def filter_dict_by_predicate(d, predicate):
"""Return a new dict with only items whose value passes the predicate."""
return {k: v for k, v in d.items() if predicate(v)}
if __name__ == "__main__":
scores = {"Alice": 85, "Bob": 42, "Charlie": 91, "Diana": 60}
# Keep only values greater than or equal t…How to Sort a List of Dictionaries by Key in Python
Sort a list of dictionaries by various keys (grade, age, name) using lambda, itemgetter, and extract unique sorted names into a set.
from operator import itemgetter
# Sample data: a list of dictionaries representing students
students = [
{"name": "Alice", "grade": 88, "age": 23},
{"name": "Bob", "grade": 95, "age": 22},
{"name": "Charlie", "grade": 78, "age": 24},
{"name": "Diana", "grade": 92, "age": 21}
]
# Sort by grade (descen…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(…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__"…How to Sort a List of Dictionaries by Key in Python
A reusable helper function that sorts a list of dictionaries by a specified key, with optional descending order support.
from typing import List
def sort_records(records: List[dict], key: str, descending: bool = False) -> List[dict]:
"""Sort a list of dictionaries by a specified key."""
return sorted(records, key=lambda record: record[key], reverse=descending)
def demonstrate_sorting() -> None:
users = [
{"name": …How to Parse an AWS API Gateway Proxy Event in Python
Extract and parse common fields from a mock API Gateway proxy event, turning the JSON body into a native Python dict.
import json
from typing import Any, Dict, Optional
def parse_proxy_event(event: Dict[str, Any]) -> Dict[str, Any]:
"""Extract and parse common fields from an API Gateway proxy event."""
body = event.get("body", "")
if isinstance(body, str):
body = json.loads(body) if body else {}
elif body is…Mock Lambda handler event context dict in Python
Simulates an AWS Lambda invocation by passing a mock event dict and context object to a handler, then prints the response.
import json
def lambda_handler(event, context):
"""
A mock AWS Lambda handler that processes an event dict and context object.
Demonstrates the typical Lambda function signature and basic event/context usage.
"""
print("Received event:", json.dumps(event, indent=2))
print("Function name:", co…How to Build a Data Validation Schema in Python
Create a lightweight validation schema using dataclasses and lambda validators to check fields in a dictionary.
import re
from dataclasses import dataclass, field
from typing import Any, Callable
@dataclass
class Field:
name: str
validator: Callable[[Any], bool]
required: bool = True
def validate(self, value: Any) -> bool:
if not self.required and value is None:
return True
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