Build a defaultdict histogram of categories in Python

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

Easy Python 3.9+ Aug 9, 2026 Dictionaries & sets 10 views 0 copies

defaultdict histogram collections dictionary counting

Python code

13 lines

Python 3.9+

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", "fruit", "dairy", "vegetable", "fruit", "meat"]
    result = build_category_histogram(categories)
    print(result)

Output

stdout

{'fruit': 3, 'vegetable': 2, 'dairy': 1, 'meat': 1}

How it works

defaultdict(int) creates a dictionary where missing keys are automatically assigned a default value of 0 when first accessed or incremented. In the loop, histogram[item] += 1 works without checking if the key exists, because the defaultdict handles initialization. Converting to a plain dict with dict(histogram) is optional but gives a cleaner output without the defaultdict repr. This approach avoids verbose if statements and makes the counting logic concise and readable.

Common mistakes

Forgetting that `defaultdict(int)` allows direct increment without explicit key check, but it also silently creates keys for any new category.
Confusing `defaultdict(int)` with `defaultdict(list)` — each has a different default factory.
Mixing up `defaultdict` and `Counter` — both count, but Counter has extra methods like `most_common`.

Variations

Use `collections.Counter` for a more feature-rich histogram with `most_common`.
Use a plain dict and check `if item in histogram` to manually set default.

Real-world use cases

Counting log entries by severity level in a monitoring system.
Aggregating product sales by category for a dashboard report.
Tallying user actions by type in an analytics pipeline.

Build a defaultdict histogram of categories in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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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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