Build a Partial Index Mock in Python for Database Filtering

Simulate a partial database index by filtering keys with a predicate, then return a limited mock lookup dictionary.

Easy Python 3.9+ Aug 9, 2026 Database scaling & optimization 12 views 0 copies

partial-index database mock filtering dictionary

Python code

20 lines

Python 3.9+

data = [
    "alpha", "beta", "gamma", "delta", "epsilon",
    "zeta", "eta", "theta", "iota", "kappa"
]

filtered_keys = [item for item in data if len(item) >= 5]

def mock_partial_index(keys, filter_func, limit=3):
    result = {}
    for key in keys:
        if not filter_func(key):
            continue
        result[key] = f"mock://{key}"
        if len(result) >= limit:
            break
    return result

if __name__ == "__main__":
    index = mock_partial_index(filtered_keys, lambda k: k.startswith(("a", "b", "d", "e")))
    print(index)

Output

stdout

{'alpha': 'mock://alpha', 'beta': 'mock://beta', 'delta': 'mock://delta'}

How it works

This function mimics a partial index by applying a filter_func to each key before adding it to the result. It limits entries to limit (default 3) to simulate practical index size constraints. The lambda checks if keys start with common prefixes, and only filtered keys are stored. The mock URL format gives a placeholder for actual indexed lookups.

Common mistakes

Forgetting that the limit applies after filtering, not before
Assuming the input order is alphabetical when it's the original list order
Using `filter()` built-in which returns an iterator, not a dict
Not handling empty filtered results gracefully

Variations

Use `itertools.islice` to lazy-limit the filtered sequence
Return a `dict.fromkeys` with the filter applied for one-liner mock

Real-world use cases

Simulating a partial index in a test suite before implementing the real database query
Prototyping index-aware query behavior in a mock service for unit tests
Comparing filtered vs full index lookup performance in a profiling script

Build a Partial Index Mock in Python for Database Filtering

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 Database scaling & optimization

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