NumPy Vector Store

A fast, lightweight, zero-setup in-memory vector store powered by NumPy.

• Tiny local vector search for projects that do not need a vector database
• Fast exact vector search using vectorized NumPy operations
• Simple typed API returning VectorHit(index, value, metadata)
• Composable filtering by passing prefiltered row indexes with within_rows
• Portable persistence as versioned, self-describing trusted local .npz files
• No framework opinions: bring your own embeddings, chunking, async, and metadata model

Why?

This library is purpose-built for small to medium-scale vector search tasks and offers a simple alternative to heavyweight vector databases when you do not need network services, indexing infrastructure, ingestion pipelines, or domain-specific metadata filtering.

When/Where?

Below are benchmark results for cosine similarity search to help you assess its suitability for your use case.

Benchmarks performed on Apple M2 hardware.

Installation

uv add numpy-vector-store

Quick Start

import numpy as np
from numpy_vector_store import VectorStore

store = VectorStore[dict[str, str]](dimensions=3)

store.add(
    vectors=np.array([
        [1.0, 0.0, 0.0],
        [0.0, 1.0, 0.0],
        [0.0, 0.0, 1.0],
    ]),
    metadata=[
        {"title": "x-axis"},
        {"title": "y-axis"},
        {"title": "z-axis"},
    ],
)

hits = store.cosine_search(
    query=np.array([0.9, 0.1, 0.0]),
    top_k=2,
)

for hit in hits:
    print(f"{hit.metadata['title']}: {hit.value:.3f}")

metadata is an outer sequence with one opaque payload for each vector row. Each payload can be a dict, dataclass, tuple, list, string, integer row ID, or another Python object that fits your application. Tuple and list payloads remain single row values rather than being interpreted as additional array dimensions.

State ownership

The store owns its configuration and row structure. dimensions, normalize, and file_path are readable properties, but callers cannot assign them directly. Use the constructor for configuration, open(path) to open an archive, and save(path) to establish or change a binding.

store.vectors and store.metadata are zero-copy, non-writeable NumPy views for inspection and metadata prefiltering. Direct item assignment and ordinary attempts to enable writes are rejected. Request a fresh view after add(), clear(), or reload() when current rows are required.

These views prevent accidental mutation through the supported API; they are not tamper-proof snapshots. NumPy exposes shared buffers, and Python private state can be reached deliberately. Mutating backing storage through .base, private attributes, ctypes, or similar escape hatches is unsupported and can corrupt store invariants. Use .copy() when code needs an independently mutable full-array snapshot:

vectors = store.vectors.copy()
metadata_rows = store.metadata.copy()

Copying metadata isolates the outer row array but still shares the opaque payload objects stored inside it.

get(index) has a narrower ownership boundary:

vector, payload = store.get(0)

vector[0] = 10.0           # Independent copy; the store is unchanged.
payload["reviewed"] = True  # Shared application metadata object.

The returned vector is an independent float32 copy. The metadata payload is the same opaque object supplied to add() or restored from the archive; the store protects the metadata row structure but does not deep-copy arbitrary dicts, lists, dataclasses, or application objects. Applications that require immutable payloads can use frozen objects or copy them at their own boundary.

Repeated additions

add() accepts one row or a batch, and preserves insertion order in either case. The store keeps private spare capacity so repeated small additions do not copy every existing row on every call. When that capacity is full, the vector and metadata arrays grow together and the active rows are copied once.

Spare capacity is internal. len(store), inspection views, search, get(), and saved archives contain only rows that were added. clear() releases both the active rows and any reserved capacity held by the store. As with any NumPy view, an older inspection view keeps its previous buffer alive until that view is released. Adding a batch is still preferable when the application already has one because it also reduces per-call validation and Python overhead.

Normalization

VectorStore defaults to normalize=True, which scales each stored vector to length 1. Normalization preserves vector direction while discarding magnitude:

[3.0, 4.0] -> [0.6, 0.8]

This is the default because it makes cosine similarity fast and direction-only, which is the common case for semantic embeddings. Use normalize=False when vector length matters, such as when magnitude encodes strength, confidence, counts, scale, or raw geometry.

Zero vectors are rejected when normalize=True because they cannot be scaled to unit length. Raw stores accept zero vectors for dot-product and Euclidean search. Because cosine similarity is undefined for zero vectors, cosine_search raises an error when its selected rows include one; use within_rows to exclude zero rows when needed.

Numerical inputs

Stored vectors use float32 to keep the store compact. Vectors and queries must remain finite when converted to float32, and search thresholds must also be finite. Invalid values are rejected before they can affect stored state or ranking.

Norms and raw metric values use float64 accumulation where float32 intermediate calculations could overflow or underflow. This allows finite float32 vectors across the representable magnitude range to be normalized and compared reliably.

Search Methods

Use cosine_search for semantic embeddings and direction-only similarity:

hits = store.cosine_search(query, top_k=10, min_value=0.75)

Use dot_search with normalize=False when larger-magnitude vectors should rank higher:

store = VectorStore[dict[str, str]](dimensions=3, normalize=False)
store.add(vectors, metadata)
hits = store.dot_search(query, top_k=10, min_value=0.0)

Use euclidean_search with normalize=False for raw coordinate or feature-space nearest-neighbor search:

store = VectorStore[dict[str, str]](dimensions=3, normalize=False)
store.add(vectors, metadata)
hits = store.euclidean_search(query, top_k=10, max_value=1.5)

Result ordering

Cosine and dot-product results are ordered from larger values to smaller values. Euclidean results are ordered from smaller distances to larger ones. When two computed values are exactly equal, the row with the lower original store index comes first.

The same rule determines which tied rows cross the top_k boundary. It also applies to within_rows: the original store index breaks a tie, regardless of the order in which filtered row indexes were supplied. Values that are close but not exactly equal remain ordered by their computed metric value.

Prefiltering

The store does not implement a metadata query language. To filter by metadata, produce row indexes first, then pass them with within_rows.

rows = [
    i
    for i, metadata in enumerate(store.metadata)
    if metadata["title"].startswith("x")
]

hits = store.cosine_search(query, top_k=10, within_rows=rows)

Searches without within_rows compute directly against the stored vector matrix and do not make a full copy of it. A filtered search gathers the selected rows into a temporary matrix, so its additional memory use scales with the number of selected rows and the vector dimensions. Omit within_rows when every row should be searched; passing every row explicitly would create an unnecessary full-size temporary matrix.

For structured NumPy metadata, use NumPy to produce the row indexes:

metadata_table = np.array(
    [
        ("intro", "A", 2024),
        ("setup", "A", 2023),
        ("guide", "B", 2024),
    ],
    dtype=[("title", "U20"), ("product", "U10"), ("year", "i4")],
)

store = VectorStore[int](dimensions=3)
store.add(vectors, metadata=np.arange(len(metadata_table)))

mask = (metadata_table["product"] == "A") & (metadata_table["year"] >= 2024)
rows = np.flatnonzero(mask)

hits = store.cosine_search(query, within_rows=rows)

for hit in hits:
    row = metadata_table[hit.metadata]
    print(row["title"], hit.value)

Thread safety

VectorStore does not use internal locks. Multiple threads may call search, get(), or the inspection properties on the same instance while its rows, configuration, binding, and metadata payloads remain unchanged.

If any thread may call add(), clear(), reload(), or save(), every access to that store must use the same application-level lock or another external synchronization mechanism. The same rule applies when application code mutates a metadata payload shared with the store. In particular, do not overlap a save with an in-memory mutation: an atomic file replacement cannot turn two separate array reads into a consistent store snapshot.

Separate store instances writing the same path also need external writer coordination. Atomic replacement prevents readers from seeing a partially written archive, but concurrent writers can replace one another and the last successful replacement wins.

Persistence

Create a new store normally, then supply its destination on the first save:

store = VectorStore[dict[str, str]](dimensions=1536)
store.add(embeddings, metadata)
store.save("vectors.npz")

save(path) writes the archive and binds that path to the store. Later save() calls update the bound archive. Passing another path performs a Save As operation; the new path becomes the binding only after the write succeeds.

Open an existing store directly from its self-describing archive:

store = VectorStore[dict[str, str]].open("vectors.npz")

open() restores dimensions and normalize from the archive and binds its path. Use reload() when the file may have changed externally and you explicitly want to discard current in-memory changes:

store.reload()

reload() always rereads the bound archive and raises if the store is unbound, the file is missing, or the archive is invalid. A failed reload leaves the current in-memory vectors and metadata unchanged.

The .npz suffix may be omitted. An extensionless path such as "vectors" is resolved to "vectors.npz" for saving, opening, and reloading.

Raw-vector configuration is also restored from the archive:

store = VectorStore[dict[str, str]](
    dimensions=1536,
    normalize=False,
)
store.add(raw_vectors, metadata)
store.save("raw-vectors.npz")

loaded = VectorStore[dict[str, str]].open("raw-vectors.npz")
assert loaded.normalize is False

Archives written by 0.4 use format version 1 and contain format_version, dimensions, normalize, vectors, and metadata. The stored configuration prevents an archive from being loaded with different dimensions or normalization semantics.

Opening and reloading validate the complete schema, array dtypes and shapes, row counts, finite vector values, and zero-norm behavior before changing in-memory state. Opaque metadata values remain individual row payloads across persistence round trips.

Each save writes a uniquely named temporary archive in the destination directory, closes it, and then replaces the destination with os.replace. Readers opening the destination path therefore see either the previous complete archive or the new complete archive rather than a partially written file. If writing or replacement fails, the previous destination remains in place and the temporary file is removed.

Atomic replacement is not file locking or multi-writer coordination. Concurrent writers can still replace one another, and the library does not promise that a successful save has reached durable hardware storage across every operating system or power failure.

Version 0.5 reads only self-describing format version 1 archives. Unversioned archives containing only vectors and metadata cannot be opened because they do not record dimensions or normalization semantics. Recreate those archives from source data, or convert them with NumPy Vector Store 0.4 before upgrading.

Constructor file_path=, instance load(), and context-manager persistence were removed in 0.5. See the persistence migration guide for the direct replacements and the pre-upgrade procedure for an unversioned archive.

Metadata persistence uses allow_pickle=True for flexible Python payloads, so only load files generated by your own application or another trusted local process. Loading untrusted .npz files is not a supported security model.

Compatibility

This project is still pre-1.0, so occasional breaking changes are expected while the API stabilizes. Changes are documented in the changelog and GitHub release notes. Deprecated APIs will keep warning for at least one point release before removal.

Version 0.5 supports Python 3.11 through 3.14 and NumPy 1.23.2 or newer. These versions are listed in the package metadata and exercised in CI, including a dedicated check against the minimum NumPy version. Python 3.10 remains supported by the 0.3 release series but is not supported by 0.4 or 0.5.

The project generally retains stable CPython versions until their upstream end-of-life, adds new versions after its dependencies and CI support them, and drops versions only in minor releases.

See the changelog for release history and the project roadmap for the planned path to stable API and persistence contracts. Persistence users upgrading from the 0.3 API should also read the migration guide.

Contributing

git clone https://github.com/tvanreenen/numpy-vector-store.git
cd numpy-vector-store
uv sync --frozen --group dev

Before submitting a pull request:

1. Run uv run ruff check
2. Run uv run ruff format --check
3. Run uv run mypy src/
4. Run uv run pytest

License

MIT License - see LICENSE file for details.