DataCache

Download, verify, transform, and cache datasets for Python applications, including OpenVax libraries such as pyensembl. DataCache provides streaming downloads, gzip/ZIP decompression, reusable local paths, offline inspection, and SQLite caches built from pandas DataFrames.

Install

Python 3.9 or newer is required.

python -m pip install datacache
# Optional progress bars and HTML-table conversion:
python -m pip install "datacache[progress,html]"

Progress is opt-in with show_progress=True. Normal use does not import tqdm or configure your application's logging.

The existing pandas dependency range is unchanged; upgrading DataCache does not introduce a pandas 1.5 requirement. CI covers pandas 1.4.4, 1.5.3, and current releases on supported Python versions.

Quickstart

Download a file once, then reuse its local path on later calls:

from datacache import Cache

cache = Cache("my-project")
url = "https://raw.githubusercontent.com/openvax/datacache/master/LICENSE"
path = cache.fetch(url, filename="LICENSE", timeout=30)
print(path)

# Reuses the cached file without a network request, even in a later process.
assert cache.fetch(url, filename="LICENSE") == path

Replace the URL and filename with your dataset. Existing files are reused until you explicitly refresh them with force=True; DataCache does not check whether the remote file has changed. Add show_progress=True to display a download bar after installing datacache[progress].

Find, inspect, or clear your cache

Cache("my-project") selects a platform cache directory without creating it. Cache() and top-level helpers without a subdir use the name datacache.

A cache name selects its own application directory; it is not nested inside the datacache directory. To choose the exact root instead, use Cache("my-project", cache_root="/data/my-project"). A relative root stays relative and is re-resolved against the working directory on every call, so use an absolute root if your program may change directories.

Using the cache from the quickstart:

print(cache.cache_directory_path)  # Directory containing cached files.
print(cache.local_path(filename="LICENSE"))  # Computes a path without creating it.
print(cache.inspect(filename="LICENSE").status)  # available, missing, corrupt, or inaccessible

# Explicit cleanup; uncomment only when you want to remove these cached files:
# cache.delete_url(url)  # Removes this root's downloads for this URL.
# cache.delete_all()  # Removes ALL contents, keeping the root directory.

Inspection works offline and never repairs files. Without an expected SHA-256, available means readable and regular; it does not prove the bytes are correct. delete_url also finds the URL-derived filenames under this root, whichever instance created them, but explicit filenames from earlier instances are not recorded persistently. Keep a dedicated cache directory: delete_all() removes every file and subdirectory in it and raises FileNotFoundError if the root does not exist. For the default platform location, clear_cache("my-project") removes the root too. See the cleanup API for details.

Offline example with integrity checking

This example runs entirely offline and cleans up after itself:

import gzip
import hashlib
from pathlib import Path
from tempfile import TemporaryDirectory

from datacache import Cache

with TemporaryDirectory() as directory:
    root = Path(directory)
    contents = b">reference\nACGT\n"
    source = root / "reference.fa.gz"
    source.write_bytes(gzip.compress(contents))

    cache = Cache("references", cache_root=root / "cache")
    path = cache.fetch(
        source.as_uri(),  # HTTP, HTTPS, and FTP URLs also work.
        filename="reference.fa",
        expected_sha256=hashlib.sha256(contents).hexdigest(),
    )
    assert Path(path).read_bytes() == contents
    source.unlink()
    assert cache.fetch(source.as_uri(), filename="reference.fa") == path
    print(cache.inspect(filename="reference.fa").status)  # available

For real releases, get the expected hash from trusted release metadata. Hashes describe the installed bytes after decompression or conversion. A hash computed from an untrusted download does not establish authenticity.

To install at an exact path instead of using a cache key:

from datacache import fetch_file

path = fetch_file(
    "https://example.org/releases/v1/records.tsv.gz",
    destination="references/v1/records.tsv",
    decompress=True,
    timeout=30,
    show_progress=True,  # Requires datacache[progress].
)

Replace the example URL with your dataset URL. Existing files are reused; force=True explicitly replaces them. When integrity expectations are supplied, an invalid cache hit raises FileValidationError instead of silently replacing the file. Failed downloads leave the previous file intact.

Choose the right API

The complete API reference lists every public signature, default, return value, exception, and example.

The library does not export fetch_fasta_dict or fetch_fasta_db. Download FASTA files with fetch_file, then parse them in the consuming library.

Guides

• API reference: all public functions, Cache methods, inspection results, and exceptions.
• Downloads and cache inspection: destinations, naming, decompression, integrity, retries, concurrency, and downstream compatibility.
• Progress and logging: tqdm, callbacks, retries, and independent download options for CSV helpers.
• SQLite and transformations: numeric fidelity, column names, versioning, rollback, connection ownership, and custom transformations.
• Shared caches: permissions, read-only use, and troubleshooting existing installations.
• Downstream integration: contracts for consuming libraries.
• Release notes and release procedure.

Guarantees and limits

Downloads are staged privately and published atomically after validation. New files respect the process umask; replacements preserve existing access permissions. This includes pyensembl's private download helpers.

Custom single-file transformations publish only successful output. Existing SQLite caches rebuild in a transaction: failure rolls back both schema and rows. New databases are built privately before publication. Cached data is reused by path or database version; DataCache does not automatically discover remote changes or repair previously corrupted caches.

Upgrades keep existing cache names and database metadata compatible. Valid cache hits do not rewrite files, change permissions, or apply new schema constraints. See upgrading existing caches and sharing old private files.

File publication requires local filesystem support for atomic replacement; new SQLite database publication also requires hard links. SQLite locking and transactions govern database rebuilds. These are single-file guarantees, not a multi-file release installer or a distributed lock service.

Development

python -m pip install -e ".[test]"
./lint-and-test.sh
python -m examples.basic_usage

Tests use local files, mocked responses, and local HTTP servers. They do not depend on external dataset servers. See CI for the Python, dependency, and operating-system combinations exercised.