How to merge dictionaries and sets in Python

Merges multiple dictionaries with the ** unpacking operator and combines sets using union operations into a single structure.

Easy Python 3.9+ Aug 9, 2026 Dictionaries & sets 12 views 0 copies

dict-merge set-union unpacking deduplication

Python code

42 lines

Python 3.9+

def merge_dictionaries_and_sets(school_dict, teacher_dict, course_dict, student_sets):
    """
    Merges multiple dictionaries and sets into a single combined structure.
    Demonstrates dict unpacking and set union operations.
    """
    # Merge all dictionaries using the unpacking operator (Python 3.9+)
    merged_schedule = {
        **school_dict,
        **teacher_dict,
        **course_dict
    }
    
    # Merge all sets using the union operator (Python 3.9+)
    merged_students = set()
    for student_set in student_sets:
        merged_students |= student_set
    
    return merged_schedule, merged_students


if __name__ == "__main__":
    # Sample data: school, teacher, and course information
    school_info = {"name": "Green Valley High", "founded": 1965, "principal": "Dr. Smith"}
    teacher_info = {"math": "Mrs. Jones", "science": "Mr. Chen", "name": "Staff Directory"}
    course_info = {"algebra": "Math 101", "biology": "Life Science"}
    
    # Sample student sets from different grade levels
    freshmen = {"Alice", "Bob", "Charlie"}
    sophomores = {"Charlie", "David", "Emma"}
    juniors = {"Emma", "Frank", "Grace"}
    
    combined_info, all_students = merge_dictionaries_and_sets(
        school_info, teacher_info, course_info,
        [freshmen, sophomores, juniors]
    )
    
    print("Combined schedule:")
    for key, value in combined_info.items():
        print(f"  {key}: {value}")
    
    print(f"\nAll unique students ({len(all_students)} total):")
    print("  " + ", ".join(sorted(all_students)))

Output

stdout

Combined schedule:
  name: Staff Directory
  founded: 1965
  principal: Dr. Smith
  math: Mrs. Jones
  science: Mr. Chen
  algebra: Math 101
  biology: Life Science

All unique students (6 total):
  Alice, Bob, Charlie, David, Emma, Frank, Grace

How it works

The ** unpacking operator merges dictionaries by inserting key-value pairs from left to right, so later dictionaries override earlier ones with the same key — that's why name becomes "Staff Directory" from teacher_info. The |= operator performs an in-place set union, adding all unique elements from each set into merged_students. This avoids duplicates automatically since sets only store unique values. The function returns a tuple containing the combined dictionary and set, which Python unpacks into two variables at the call site. Sorting the final set in the print statement gives deterministic, readable output.

Common mistakes

Forgetting that later dict keys overwrite earlier ones during unpacking
Using `student_set | merged_students` instead of `merged_students |= student_set` (same result but less efficient)
Assuming set order is preserved when printing without sorted()
Passing a set instead of a list of sets to the function

Variations

Use `merged = {**a, **b, **c}` or `merged = a | b | c` (Python 3.9+) for quick one-line merges
Use `merged_students = set().union(*student_sets)` to combine all sets in one call

Real-world use cases

Consolidating configuration dictionaries from multiple services into one settings object at app startup.
Aggregating user IDs from several access groups into a single deduplicated roster for permission checks.
Combining class rosters scraped from different sources into one master student list for a school database.

How to merge dictionaries and sets in Python

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