How to Compute Set Union of Tags from Multiple Items in Python

Collect all unique tags from a list of dictionaries using set union with update() in Python.

Easy Python 3.9+ Aug 9, 2026 Dictionaries & sets 14 views 0 copies

set union tags dictionaries sets

Python code

17 lines

Python 3.9+

items = [
    {"id": 1, "tags": {"python", "web"}},
    {"id": 2, "tags": {"web", "api", "sql"}},
    {"id": 3, "tags": {"python", "data"}},
]


def get_union_of_tags(item_list):
    all_tags = set()
    for item in item_list:
        all_tags.update(item["tags"])
    return all_tags


if __name__ == "__main__":
    union_tags = get_union_of_tags(items)
    print(sorted(union_tags))

Output

stdout

['api', 'data', 'python', 'sql', 'web']

How it works

The set.update() method merges each item's tags into a single all_tags set, automatically removing duplicates. Initializing all_tags as an empty set ensures we start with no elements before accumulating. This approach is efficient for any number of items since set operations have average O(1) insert time. Calling sorted() returns the tags in a predictable order for display.

Common mistakes

Using `all_tags = all_tags | item['tags']` inside a loop, which creates a new set each time and is less efficient
Forgetting to initialize `all_tags = set()` before the loop, causing a `NameError`
Assuming tags are already unique and skipping the union, leading to duplicates

Variations

Use `set().union(*[item['tags'] for item in items])` for a one-liner approach
Use a set comprehension: `{tag for item in items for tag in item['tags']}`

Real-world use cases

Aggregating topic tags from multiple blog posts to build a navigation filter.
Collecting all permission scopes from several API clients for unified access control.
Combining feature flags across microservices to determine total enabled functionality.

How to Compute Set Union of Tags from Multiple Items in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Dictionaries & sets

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Dictionaries & sets

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Tutorials

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