Reference library

Dictionaries & sets

Key–value maps, uniqueness, counting, grouping, and fast lookups.

6 matches

Build a defaultdict histogram of categories in Python

Count occurrences of each category in a list using collections.defaultdict(int) for automatic initialization.

from collections import defaultdict

def build_category_histogram(items):
    """Count occurrences of each category in a list of items."""
    histogram = defaultdict(int)
    for item in items:
        histogram[item] += 1
    return dict(histogram)

if __name__ == "__main__":
    categories = ["fruit", "vegetable", …

11 0 Open

Dictionaries & sets easy

Filter Dictionary Keys by Prefix in Python

Use a dict comprehension to build a new dictionary containing only keys that start with a given prefix.

dict-comprehension filtering dictionaries

Python

def filter_dict_keys(data, prefix="temp_"):
    """
    Filter a dictionary by keeping only keys that start with a given prefix.
    Uses a dict comprehension to build a new dictionary.
    """
    if not isinstance(data, dict):
        raise ValueError("data must be a dictionary")
    return {key: value for key, valu…

13 0 Open

Dictionaries & sets easy

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.

dictionary filtering lambda

Python

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…

14 0 Open

Dictionaries & sets easy

How to Find the Intersection of Permission Sets in Python

This code defines a function that takes a list of permission sets and returns a set containing only the permissions common to all sets, with a short-circuit for empty results.

sets intersection permissions

Python

from typing import Set

def intersect_permissions(permission_sets: list[Set[str]]) -> Set[str]:
    """
    Given a list of permission sets, return the common permissions
    present in every set.
    """
    if not permission_sets:
        return set()
    
    common = permission_sets[0]
    for perm_set in permissi…

12 0 Open

Dictionaries & sets easy

How to Use defaultdict(list) to Group Words by First Letter in Python

This code groups a list of words by their first letter using a defaultdict with a list factory, then prints each group sorted by initial.

defaultdict grouping dictionaries

Python

from collections import defaultdict

def group_by_initial(words):
    groups = defaultdict(list)
    for word in words:
        groups[word[0].upper()].append(word)
    return dict(groups)

if __name__ == "__main__":
    words = ["apple", "banana", "apricot", "blueberry", "cherry"]
    result = group_by_initial(words)…

13 0 Open

Dictionaries & sets easy

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.

json datetime serialization

Python

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…

15 0 Open

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