Dictionaries & sets
Key–value maps, uniqueness, counting, grouping, and fast lookups.
Build an OrderedDict insertion order demo in Python 3
Demonstrate how OrderedDict preserves insertion order, how updates keep position, and how re-insertion moves keys to the end.
from collections import OrderedDict
def demo_ordered_dict():
# Create an OrderedDict and insert items in a specific order
ordered = OrderedDict()
ordered['banana'] = 3
ordered['apple'] = 2
ordered['cherry'] = 5
ordered['date'] = 1
print("Insertion order preserved:")
for key, value in …How to Count Words and Find Common Words in Python with Dictionaries and Sets
Build a simple text processor that counts unique words with dictionaries and finds common words across text halves using sets.
def process_text(text):
"""Process text: count unique words with counts, find common words."""
words = text.lower().replace(",", "").replace(".", "").split()
word_counts = {}
for word in words:
word_counts[word] = word_counts.get(word, 0) + 1
total_words = len(words)
unique_wo…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 Filter a List of Dictionaries by Category in Python
Filter a list of dictionaries to include only records whose category is in an allowed set.
def filter_data(records, categories):
"""Return only records whose category is in the allowed set."""
allowed = set(categories)
filtered = []
for record in records:
if record["category"] in allowed:
filtered.append(record)
return filtered
if __name__ == "__main__":
data = …How to Find Symmetric Difference Between Two Python Sets
Compute elements unique to each set and build a flag dictionary showing membership across two Python sets.
def symmetric_difference_with_flags(set_a, set_b):
"""Return elements in either set but not both, grouped by which set they came from."""
only_in_a = set_a - set_b
only_in_b = set_b - set_a
print(f"Only in A: {only_in_a}")
print(f"Only in B: {only_in_b}")
print(f"Symmetric difference: {onl…How to Invert a Dictionary in Python Safely
Swap dictionary keys and values while detecting duplicate values to prevent silent data loss.
def invert_dict_safely(d):
inverted = {}
for key, value in d.items():
if value not in inverted:
inverted[value] = key
else:
raise ValueError(f"Duplicate value '{value}' would cause data loss")
return inverted
if __name__ == "__main__":
sample = {"a": 1, "b": 2,…How to Use Counter for Most Common Elements in Python
This code demonstrates how to find the most frequent elements in a list using Python's Counter class from the collections module.
from collections import Counter
def most_common_elements(items, n=1):
"""Return the n most common elements and their counts."""
counter = Counter(items)
return counter.most_common(n)
if __name__ == "__main__":
data = ["apple", "banana", "apple", "orange", "banana", "apple", "grape"]
print(most_co…How to convert string values to int or float in Python dicts
Recursively convert string values in nested dicts and lists to ints or floats when possible, leaving other strings untouched.
def coerce_str_values(data):
"""Recursively convert string values that look like ints or floats."""
if isinstance(data, dict):
return {key: coerce_str_values(val) for key, val in data.items()}
elif isinstance(data, list):
return [coerce_str_values(item) for item in data]
elif isinstance…Browse by section
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Dictionaries & sets — Python code examples
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