blz: a compressed data container

pip install blz==0.6.2


BLZ: A chunked, compressed, data container (for memory and disk)

Note on this repo

This version of the blz code has been deprecated. The project has changed names to bcolz and can be followed at:


BLZ is a chunked container for numerical data. Chunking allows for efficient enlarging/shrinking of data container. In addition, it can also be compressed for reducing memory/disk needs. The compression process is carried out internally by Blosc, a high-performance compressor that is optimized for binary data.

BLZ uses Blosc ( for data compression and numexpr ( transparently so as to accelerate many vector and query operations. Blosc can compress binary data very efficiently, optimizing memory access, while numexpr focus in reducing the memory usage and use several cores for doing the computations. Medium term goal is to leverage the advanced computing capabilities in Blaze ( in addition to numexpr.

Finally, the adoption of the Bloscpack persistent format ( allows the main objects in BLZ (barray / btable, see below) to be persisted, so it can be used for performing out-of-core computations transparently.

btable: a columnar store

The main objects in BLZ are barray and btable. barray is meant for storing multidimensional homogeneous datasets efficiently. barray objects provide the foundations for building btable objects, where each column is made of a single barray. Facilities are provided for iterating, filtering and querying btables in an efficient way. You can find more info about barray and btable in the tutorial:


By using compression, you can deal with more data using the same amount of memory. In case you wonder: which is the price to pay in terms of performance? you should know that nowadays memory access is the most common bottleneck in many computational scenarios, and CPUs spend most of its time waiting for data. Hence having data compressed in memory can reduce the stress of the memory subsystem.

In other words, the ultimate goal for BLZ is not only reducing the memory needs of large arrays, but also making operations to go faster than using a traditional ndarray object from NumPy. That is already the case for some special cases now, but will happen more generally in a short future, when BLZ will be able to take advantage of newer CPUs integrating more cores and wider vector units.


  • Python >= 2.6
  • NumPy >= 1.7
  • Cython >= 0.19
  • numexpr >= 2.2 (optional, if not present, plain NumPy will be used)
  • Blosc >= 1.3.0 (optional, the internal Blosc will be used by default)
  • unittest2 (only in the case you are running Python 2.6)


Assuming that you have the requisites and a C compiler installed, do:

$ python build_ext --inplace

In case you have Blosc installed as an external library (and disregard the included Blosc sources) you can link with it in a couple of ways.

Using an environment variable:

$ BLOSC_DIR=/usr/local     (or "set BLOSC_DIR=\blosc" on Win)
$ export BLOSC_DIR         (not needed on Win)
$ python build_ext --inplace

Using a flag:

$ python build_ext --inplace --blosc=/usr/local


After compiling, you can quickly check that the package is sane by running:

$ PYTHONPATH=.   (or "set PYTHONPATH=." on Windows)
$ export PYTHONPATH    (not needed on Windows)
$ python -c"import blz; blz.test()"  # add `heavy=True` if desired


Install it as a typical Python package:

$ python install


You can find the online manual at:

Also, you may want to look at the bench/ directory for some examples of use.


Visit the main BLZ site repository at:

Home of Blosc compressor:

Home of the numexpr project:

User's mail list:


Please see BLZ.txt in LICENSES/ directory.

Share your experience

Let us know of any bugs, suggestions, gripes, kudos, etc. you may have.


See the AUTHORS.txt file.