Not an island: bringing compression to the tabular ecosystem

Francesc Alted — Fri, 17 Jul 2026 12:00:00 GMT

A compression library can go one of two ways: try to be everything (its own dataframe, its own query engine, its own format that nothing else reads), or be a fast, compact layer that slots underneath the tools you already use. Python-Blosc2 has initially leaned toward the first path, but this is changing quite dramatically. During the past year, we put a lot of effort in making Python-Blosc2 as much compatible as possible with the array API initiative.

And today's 4.9.1 release pushes compatibility with the tabular library ecosystem further than ever: CTable, our compressed table container, now plugs straight into Arrow, pandas, Polars and DuckDB.

Speaking Arrow

The star of this release is support for the Arrow PyCapsule protocol — the lingua franca of modern tabular tools. In plain terms: any Arrow-aware tool can now read a CTable directly, streaming it in small batches, without you converting anything first. Here is the same query — average fare per taxi company — asked five different ways, all against the same compressed file:

import blosc2

t = blosc2.CTable.open("trips.b2z")

# With Blosc2 itself...
t.group_by("company").mean("fare")

# ...or in SQL, with DuckDB reading the compressed table directly
import duckdb
duckdb.sql("SELECT company, avg(fare) FROM t GROUP BY company").show()

# ...or with Polars
import polars as pl
pl.DataFrame(t).group_by("company").agg(pl.col("fare").mean())

# ...or with pandas >= 3.0
import pandas as pd
pd.DataFrame.from_arrow(t).groupby("company")["fare"].mean()

# ...or with plain pyarrow
import pyarrow as pa
pa.table(t).group_by("company").aggregate([("fare", "mean")])

Pick whichever syntax you like best — the data stays compressed on disk either way.

And it works the other way around too — hand CTable.from_arrow() a Polars dataframe, a pyarrow table or a Parquet reader, and you get a compressed table back:

import polars as pl

pdf = pl.DataFrame({"ticker": ["AAPL", "GOOG"], "close": [184.2, 140.8]})
t2 = blosc2.CTable.from_arrow(pdf)

Blosc2 isn't inventing anything here — it just implements a protocol the whole ecosystem already agreed on.

Strings that everyone understands

Text columns used to force an awkward choice in Blosc2: fixed-width strings (fast, but every value pays for the longest one) or variable-length strings (flexible, but slow to query). The new blosc2.utf8() column type ends that dilemma: it stores text in the exact same layout Arrow uses, so every row costs only what it needs — up to 13x less memory than fixed-width on free-form text — and data flows to and from Arrow tools with no translation.

from dataclasses import dataclass
import blosc2

@dataclass
class Place:
    name: str = blosc2.field(blosc2.utf8())
    visitors: int = blosc2.field(blosc2.int64())

t = blosc2.CTable(Place, new_data={
    "name": ["café", "O'Hare", "日本語のテキスト", "zürich"],
    "visitors": [120, 85_000, 42, 300],
})

And yes, it is fast: reading a full 10-million-row column requires just a fraction of a second; filtering is fast too, and utf8() even beats fixed-width strings as a group_by() key. Filters, sorting and grouping all work on it out of the box. However, sometimes you may still want to use string() / vlstring() for specific use cases; see the "Choosing a string column type" guide if you want the details.

Feeling at home for pandas users

CTable also picked up more of pandas' vocabulary this cycle, so familiar idioms now just work — chaining with assign() and col(), proper missing-data handling with fillna() and dropna(), and custom aggregation functions in group_by():

from blosc2 import col

(t.assign(profit=col("revenue") - col("cost"))
  [col("profit") > 0]
  .sort_by("profit", ascending=False)
  .head(10))

And if you'd rather stay in pandas entirely, you can still bring Blosc2's compute engine with you: df.apply(func, engine=blosc2.jit) now works correctly inside pandas 3, and Series.map() supports it too. See the "Using Blosc2 as a pandas engine" guide.

Cooperation, not completeness

Although CTable object comes with powerful machinery, it doesn't need to replace DuckDB, Polars or pandas to be useful; it just needs to hand them data in the shape they already expect — while doing the compression and fast querying underneath. If interoperating with the tools you already love sounds like the right job for a compression library, this is the release that makes the case.

Install it with:

pip install blosc2 --upgrade

Full release notes: https://github.com/Blosc/python-blosc2/releases

Enjoy data!

Blosc Home Page (Posts about ctable arrow pandas interop compression)

Not an island: bringing compression to the tabular ecosystem

Speaking Arrow

Strings that everyone understands

Feeling at home for pandas users

Cooperation, not completeness