shelvez 0.7.0

A lightweight Python package providing shelve-like persistent storage with zstd compression, SQLite transactions, multithreading support, and flexible serialization options.

Introduction

This package functions similarly to Python's built-in shelve but offers additional features such as:

• Zstandard (zstd) compression for efficient storage ✅ DONE
• SQLite-backed transactions — batch writes with with shelve.open(...) as db:; the underlying db.dict also exposes begin / commit / transaction() for advanced use ✅ DONE
• Multiple serialization formats support: JSON, Pickle, and Pydantic models ✅ DONE
• SQLite-based function caching with TTL and LRU strategies ✅ DONE

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📊 Benchmark

Benchmark functions are defined in tests/benchmarks/test_shelve_bench.py.

🔥 Performance Comparison

Test Case	Min (ms)	Max (ms)	Mean (ms)	StdDev (ms)	Median (ms)	OPS (ops/sec)	Rounds
shelvez_pickle_speed	147.78	154.81	149.30	2.29	148.49	6.70	8
shelvez_pydantic_speed	151.57	155.64	153.13	1.53	152.90	6.53	7
shelvez_json_speed	154.66	157.41	155.99	1.00	156.28	6.41	7
stdlib_shelve_speed	264.41	277.42	268.82	5.14	266.58	3.72	5

OPS = Operations Per Second (calculated as 1 / Mean). Timings are from one environment and will differ on other CPUs / interpreters.

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🗂️ Database Size Comparison

Same workload as the speed benchmarks: 10,000 random key–value pairs (_make_payload in tests/benchmarks/test_shelve_bench.py). For shelvez, sizes are taken after db.dict.optimize_database() (custom zstd dictionary + recompression). Reproduce the printed sizes with pytest tests/benchmarks -m benchmark -s.

Backend	File Size
shelve	380.00 kB
shelvez (Pickle)	312.00 kB
shelvez (JSON)	312.00 kB
shelvez (Pydantic)	308.00 kB

The kB values above were checked with pytest tests/benchmarks -m benchmark -s (stdout from _report_size); minor drift is normal because the benchmark uses random keys/values and trains a zstd dictionary. For this workload, shelvez produces smaller files and faster writes than stdlib shelve in the same test harness.

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Installation

pip install shelvez

Optional: Pydantic support

pydantic is not installed by default — shelvez itself does not import it, so users who don't need PydanticSerializer can keep their dependency tree small. If you want to use shelvez.serializer.PydanticSerializer, install pydantic explicitly:

pip install shelvez[pydantic]
# or equivalently:
pip install pydantic

Instantiating PydanticSerializer without pydantic installed will raise a clear ImportError telling you to run the command above.

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Base Usage

import shelvez as shelve

db = shelve.open("any_db_path/your_db.db")
db["key"] = "value"
print(db["key"])
db.close()

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Serialization (default is Pickle)

The default serialization method uses Pickle, with the Pickle data further compressed by zstd. For specific data types, you can also choose other serialization methods to achieve better version compatibility and reduce storage size.

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with JSON-serializable dicts

import shelvez as shelve

# Use Json serializer
serializer = shelve.serializer.JsonSerializer()
db = shelve.open("any_db_path/your_db.db", serializer=serializer)

db["key"] = {"key":"value"}

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with Pydantic model

from pydantic import BaseModel
import shelvez as shelve

class MyModel(BaseModel):
    value: str

# use Pydantic serializer
serializer = shelve.serializer.PydanticSerializer(MyModel)
db = shelve.open("any_db_path/your_db.db", serializer=serializer)

db["key"] = MyModel(value="value")

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with Self Custom Serialization

To implement your own serialization method, you need to create a subclass of serializer.BaseSerializer and override the following two methods:

1. serialize(self, obj) -> bytes: This method takes a Python object (obj) and returns its serialized form as bytes. Implement this method with your custom serialization logic.
2. unserialize(self, data: bytes): This method takes the serialized bytes (data) and returns the original Python object by deserializing it.

Here is a template example:

from shelvez import serializer

class CustomSerializer(serializer.BaseSerializer):
    def serialize(self, obj) -> bytes:
        # Implement custom serialization logic here
        # Convert `obj` to bytes
        pass

    def unserialize(self, data: bytes):
        # Implement custom deserialization logic here
        # Convert bytes back to original object
        pass

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Using zstd Compression Dictionary

After accumulating a sufficient amount of data, you can generate a custom zstd compression dictionary for your database by calling db.dict.optimize_database(). This function will also recompress the existing data using the newly created dictionary. When stored data shares similar structures or formats, a personalized zstd dictionary can greatly enhance compression efficiency, particularly for relatively small datasets. Typically, generating the dictionary after storing a few thousand samples yields good results.

⚠️ Warning: During the optimization process, do not perform any other read or write operations on the database to prevent data corruption or inconsistent states.

import shelvez as shelve

db = shelve.open("any_db_path/your_db.db")
db["key"] = "value"
# ... store more data as needed

# Generate and apply a custom zstd compression dictionary
db.dict.optimize_database()

db.close()

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Function Caching with SqlCache

Shelvez now includes a powerful SQLite-based caching system that allows you to cache function results to SQLite with compression. This is particularly useful for expensive computations that you want to persist across application restarts.

Basic Usage

import shelvez.sqlcache as sqlcache
import time

# TTL Cache - cache results for 1 hour
@sqlcache.ttl_cache(cache_path="cache.db", max_size=1000, ttl=3600)
def expensive_function(x):
    time.sleep(1)  # Simulate expensive computation
    return x * x

# First call - computes and caches
result = expensive_function(5)  # Takes 1 second
print(result)  # 25

# Second call - retrieves from cache
result = expensive_function(5)  # Instant!
print(result)  # 25

LRU Cache

# LRU Cache - keeps only the 100 most recently used results
@sqlcache.lru_cache(cache_path="cache.db", max_size=100)
def fibonacci(n):
    if n <= 1:
        return n
    return fibonacci(n-1) + fibonacci(n-2)

# Results are cached and persist across application restarts
result = fibonacci(30)  # Computed once, cached forever

Custom Cache Configuration

# Custom cache with specific settings
@sqlcache.sqlcache(
    cache_path="my_cache.db",
    max_size=500,
    ttl=1800,  # 30 minutes
    cache_type="ttl"  # or "lru"
)
def my_function(x, y):
    return x + y

Cache Management

# Create a cache instance for advanced management
cache = sqlcache.SqlCache(
    cache_path="advanced_cache.db",
    max_size=1000,
    ttl=3600,
    cache_type="ttl"
)

@cache
def my_function(x):
    return x * 2

# Get cache statistics
stats = cache.get_stats()
print(f"Cache items: {stats['disk_cache']['total_items']}")
print(f"Memory cache size: {stats['memory_cache_size']}")

# Clear all cached data
cache.clear()

# Close the cache
cache.close()

Features

• Dual-layer caching: Memory cache for speed + disk cache for persistence
• Compression: All cached data is compressed using zstd for efficient storage
• Thread-safe: Safe to use in multi-threaded applications
• Automatic cleanup: TTL caches automatically expire old entries, LRU caches remove least recently used items
• Flexible serialization: Supports any Python object that can be pickled
• Statistics: Get detailed information about cache usage and performance

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