Pooch - Data File Fetching
Quick Reference
import pooch
# Download single file
file_path = pooch.retrieve(
url="https://example.com/data.csv",
known_hash="sha256:abc123...", # None to skip verification
fname="data.csv",
path=pooch.os_cache("myproject")
)
# Create registry for multiple files
REGISTRY = pooch.create(
path=pooch.os_cache("myproject"),
base_url="https://example.com/data/",
registry={"data.csv": "sha256:abc123...", "model.nc": "sha256:def456..."}
)
data_file = REGISTRY.fetch("data.csv")
# Generate hash for local file
file_hash = pooch.file_hash("/path/to/file.csv")
Key Functions
| Function | Purpose |
|---|
pooch.retrieve() | Download single file with caching |
pooch.create() | Create custom data registry |
pooch.file_hash() | Generate SHA256/MD5 hash of file |
pooch.os_cache() | Get OS-specific cache directory |
Essential Operations
Download Files
# With hash verification
file_path = pooch.retrieve(
url="https://example.com/data.nc",
known_hash="sha256:abc123..."
)
# Without verification (development only)
file_path = pooch.retrieve(url="https://example.com/data.nc", known_hash=None)
# From Zenodo DOI
file_path = pooch.retrieve(
url="doi:10.5281/zenodo.1234567/data.zip",
known_hash="sha256:abc123..."
)
Extract Archives
# ZIP archive
files = pooch.retrieve(
url="https://example.com/data.zip",
known_hash="sha256:abc123...",
processor=pooch.Unzip()
)
# Decompress single gzip file
file_path = pooch.retrieve(
url="https://example.com/data.csv.gz",
known_hash="sha256:abc123...",
processor=pooch.Decompress(name="data.csv")
)
Additional Options
# Progress bar for large downloads
file_path = pooch.retrieve(url=url, known_hash=hash, progressbar=True)
# HTTP authentication
file_path = pooch.retrieve(
url="https://example.com/protected/data.csv",
known_hash=None,
downloader=pooch.HTTPDownloader(auth=("user", "pass"))
)
Processor Options
| Processor | Purpose |
|---|
Unzip() | Extract ZIP archives |
Untar() | Extract TAR/TAR.GZ archives |
Decompress() | Decompress gzip, bz2, lzma, xz |
Cache Locations
| OS | Default Path |
|---|
| Linux | ~/.cache/<project> |
| macOS | ~/Library/Caches/<project> |
| Windows | C:\Users\<user>\AppData\Local\<project>\Cache |
Error Handling
try:
file_path = pooch.retrieve(url=url, known_hash=hash)
except pooch.exceptions.HTTPDownloadError:
print("Download failed - check URL")
except pooch.exceptions.DownloadError:
print("Network issue")
When to Use vs Alternatives
| Tool | Best For | Limitations |
|---|
| pooch | Reproducible data downloads, hash verification, caching | Not a version control system |
| urllib/requests | Simple one-off downloads, custom HTTP logic | No caching, no hash verification |
| DVC | Data version control alongside git | Heavier setup, requires remote storage |
| wget | Quick command-line downloads | No Python integration, no caching logic |
Use pooch when you need reproducible data downloads with automatic caching and integrity verification, especially for scientific data registries.
Consider alternatives when you need full data version control with git integration (use DVC), simple one-off downloads without caching needs (use requests), or command-line batch downloads (use wget).
Common Workflows
Set up reproducible data download with registry
- Identify all required data files and their URLs
- Generate SHA256 hashes with
pooch.file_hash() for each file - Create registry with
pooch.create() specifying base URL and file hashes - Fetch files with
REGISTRY.fetch() in analysis scripts - Add processors for compressed files (
Unzip(), Untar(), Decompress()) - Test registry by clearing cache and re-fetching
- Document registry in project README for collaborators
References
Scripts
