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Internal Data Ingestion

Dask contains internal tools for extensible data ingestion in the dask.bytes package. These functions are developer-focused rather than for direct consumption by users. These functions power user facing functions like dd.read_csv and db.read_text which are probably more useful for most users.

.. currentmodule:: dask.bytes

.. autosummary::
   read_bytes
   open_files

These functions are extensible in their output formats (bytes, file objects), their input locations (file system, S3, HDFS), line delimiters, and compression formats.

These functions provide data as dask.delayed objects. These objects either point to blocks of bytes (read_bytes) or open file objects (open_files). They can handle different compression formats by prepending protocols like s3:// or hdfs://. They handle compression formats listed in the dask.bytes.compression module.

These functions are not used for all data sources. Some data sources like HDF5 are quite particular and receive custom treatment.

Delimiters

The read_bytes function takes a path (or globstring of paths) and produces a sample of the first file and a list of delayed objects for each of the other files. If passed a delimiter such as delimiter=b'\n', it will ensure that the blocks of bytes start directly after a delimiter and end directly before a delimiter. This allows other functions, like pd.read_csv, to operate on these delayed values with expected behavior.

These delimiters are useful both for typical line-based formats (log files, CSV, JSON) as well as other delimited formats like Avro, which may separate logical chunks by a complex sentinel string.

Locations

These functions dispatch to other functions that handle different storage backends, like S3 and HDFS. These storage backends register themselves with protocols, and so are called whenever the path is prepended with a string like the following:

s3://bucket/keys-*.csv

The various back-ends accept optional extra keywords, detailing authentication and other parameters, see :doc:`remote data services <remote-data-services>`.

Compression

These functions support widely available compression technologies like gzip, bz2, xz, snappy, and lz4. More compressions can be easily added by inserting functions into dictionaries available in the dask.bytes.compression module. This can be done at runtime and need not be added directly to the codebase.

However, not all compression technologies are available for all functions. In particular, compression technologies like gzip do not support efficient random access, and so are useful for streaming open_files but not useful for read_bytes which splits files at various points.

Functions

.. autofunction:: read_bytes
.. autofunction:: open_files