rBloom

A fast, simple and lightweight Bloom filter library for Python, fully implemented in Rust. It's designed to be as pythonic as possible, mimicking the built-in set type where it can. While it's a new kid on the block (this project was started in 2023), it's also currently the fastest kid on the block by a long shot (see the section Benchmarks). Releases are published on PyPI.

Quickstart

This library defines only one class, which can be used as follows:

>>> from rbloom import Bloom
>>> bf = Bloom(200, 0.01)  # 200 items max, false positive rate of 1%
>>> bf.add("hello")
>>> "hello" in bf
True
>>> "world" in bf
False
>>> bf.update(["hello", "world"])  # "hello" and "world" now in bf
>>> other_bf = Bloom(200, 0.01)

### add some items to other_bf

>>> third_bf = bf | other_bf  # third_bf now contains all items in
                              # bf and other_bf
>>> third_bf = bf.copy()
... third_bf.update(other_bf)  # same as above

Unlike BF's in real life, you can have as many of these as you want, and they all work well together!

For the full API, see the section Documentation.

Installation

On almost all platforms, simply run:

pip install rbloom

If you're on an uncommon platform, this may cause pip to build the library from source, which requires the Rust toolchain. You can also build rbloom by cloning this repository and running maturin:

maturin build --release

This will create a wheel in the target/wheels/ directory, which can subsequently also be passed to pip.

Why rBloom?

Why should you use this library instead of one of the other Bloom filter libraries on PyPI?

• Simple: Almost all important methods work exactly like their counterparts in the built-in set type.
• Fast: rbloom is implemented in Rust, which makes it blazingly fast. See section Benchmarks for more information.
• Lightweight: rbloom has no dependencies of its own.
• Maintainable: The entire library fits comfortably in a few hundred lines of code, and it's written in idiomatic Rust. Even if I were to stop maintaining rbloom (which I don't intend to), it would be trivially easy for you to fork it and keep it working for you.

I started rbloom because I was looking for a simple Bloom filter dependency for a project, but the pure Python implementations were too slow. The only maintained fast alternative I could find, pybloomfiltermmap3 (which is written in C and is a great library), failed to work on recent versions of Python (see below), so I felt very uncomfortable using it as a dependency. I also felt like the thousands of lines of code in that library were a bit hard to handle should it stop being maintained (which is what happened to the original pybloomfiltermmap). However, please note that pybloomfiltermmap3 implements persistent filters, while rbloom currently does not, so if that's something you require, you should definitely give that library a try.

Benchmarks

I implemented the following simple benchmark in the respective API of each library:

bf = Bloom(10_000_000, 0.01)

for i in range(10_000_000):
    bf.add(i + 0.5)  # floats because ints are hashed as themselves

for i in range(10_000_000):
    assert i + 0.5 in bf

This resulted in the following runtimes:

[1] It refused to install on Python 3.11 and kept segfaulting on 3.10, so I installed 3.7 on my machine for this benchmark.
[2] I tested both converting to bytes and pickling, and chose the faster time.

The benchmark was run on a 2019 Dell XPS 15 7590 with an Intel Core i5-9300H. It was run 5 times for each library, and the average time was used.

Also note that rbloom is compiled against a stable ABI for portability, and that you can get a small but measurable speedup by removing the "abi3-py37" flag from Cargo.toml and building it yourself.

Documentation

This library defines only one class, the signature of which should be thought of as:

class Bloom:

    # expected_items: max number of items to be added to the filter
    # false_positive_rate: max false positive rate of the filter
    # hash_func: optional argument, see section "Cryptographic security"
    def __init__(self, expected_items: int, false_positive_rate: float,
                 hash_func=__builtins__.hash)

    @property
    def size_in_bits(self) -> int  # number of buckets in the filter

    @property
    def hash_func(self) -> Callable[[Any], int]  # retrieve the hash_func
                                                 # given to __init__

    @property
    def approx_items(self) -> float  # estimated number of items in
                                     # the filter

    #                all subsequent methods are
    # -------- equivalent to the corresponding methods ---------
    #                 of the built-in set type

    def add(self, object)

    def __contains__(self, object) -> bool    # object in self

    def __bool__(self) -> bool                # False if empty

    def __repr__(self) -> str                 # Basic info

    def __or__(self, other: Bloom) -> Bloom   # self | other

    def __ior__(self, other: Bloom)           # self |= other

    def __and__(self, other: Bloom) -> Bloom  # self & other

    def __iand__(self, other: Bloom)          # self &= other

    def update(self, *others: Union[Iterable, Bloom])

    def intersection_update(self, *others: Union[Iterable, Bloom])

    def clear(self)                           # remove all items

    def copy(self) -> Bloom

To prevent death and destruction, the bitwise set operations only work on filters where all parameters are equal (including the hash functions being the exact same object). Because this is a Bloom filter, the __contains__ and approx_items methods are probabilistic.

Cryptographic security

Python's built-in hash function is designed to be fast, not maximally collision-resistant, so if your program depends on the false positive rate being perfectly correct, you may want to supply your own hash function. This is especially the case when working with very large filters (more than a few tens of millions of items) or when false positives are very costly and could be exploited by an adversary. Just make sure that your hash function returns an integer between -2^127 and 2^127 - 1. Feel free to use the following example in your own code:

from rbloom import Bloom
from hashlib import sha256
from pickle import dumps

def hash_func(obj):
    h = sha256(dumps(obj)).digest()
    return int.from_bytes(h[:16], "big") - 2**127

bf = Bloom(100_000_000, 0.01, hash_func=hash_func)

When you throw away Python's built-in hash function and start hashing serialized representations of objects, however, you open up a breach into the scary realm of the unpythonic:

• Numbers like 2, 2.0 and 2 + 0j will suddenly no longer be equal.
• Instances of classes with custom hashing logic (e.g. to stop caches inside instances from affecting their hashes) will suddenly display undefined behavior.
• Objects that can't be serialized simply won't be hashable at all.

Making you supply your own hash function in this case is a deliberate design decision intended to show you what you're doing and prevent you from shooting yourself in the foot.

This implementation of a Bloom filter doesn't use multiple hash functions, but instead works by redistributing the entropy of a single hash over multiple integers by using the single hash as the seed of a simple linear congruential generator (LCG). Those integers are then used as indexes for the bit array that makes up the filter. The constant used for the LCG is one proposed by (L'Ecuyer, 1999).