.. include:: references.txt

.. _fast_ascii_io:

Fast ASCII I/O
**************

While :mod:`astropy.io.ascii` was designed with flexibility and extensibility
in mind, there is also a less flexible but significantly faster Cython/C engine
for reading and writing ASCII files. By default, |read| and |write| will
attempt to use this engine when dealing with compatible formats. The following
formats are currently compatible with the fast engine:

 * ``basic``
 * ``commented_header``
 * ``csv``
 * ``no_header``
 * ``rdb``
 * ``tab``

The fast engine can also be enabled through the format parameter by prefixing
a compatible format with "fast" and then an underscore. In this case, or
when enforcing the fast engine by either setting ``fast_reader='force'``
or explicitly setting any of the :ref:`fast_conversion_opts`, |read|
will not fall back on an ordinary reader if fast reading fails.

Examples
--------

..
  EXAMPLE START
  Read and Write a CSV File Using Fast ASCII

To open a CSV file and write it back out::

   >>> from astropy.table import Table
   >>> t = ascii.read('file.csv', format='fast_csv')  # doctest: +SKIP
   >>> t.write('output.csv', format='ascii.fast_csv')  # doctest: +SKIP

To disable the fast engine, specify ``fast_reader=False`` or
``fast_writer=False``. For example::

   >>> t = ascii.read('file.csv', format='csv', fast_reader=False) # doctest: +SKIP
   >>> t.write('file.csv', format='csv', fast_writer=False) # doctest: +SKIP

.. Note:: Guessing and Fast reading

   By default |read| will try to guess the format of the input data by
   successively trying different formats until one succeeds
   (see the section on :ref:`guess_formats`). For each supported
   format it will first try the fast, then the slow version of that
   reader. Without any additional options this means that both some pure
   Python readers with no fast implementation and the Python versions
   of some readers will be tried before getting to some of the fast
   readers. To bypass them entirely, a fast reader should be explicitly
   requested as above.

   **For optimum performance** however, it is recommended to turn off
   guessing entirely (``guess=False``) or narrow down the format options
   as much as possible by specifying the format (e.g., ``format='csv'``)
   and/or other options such as the delimiter.

..
  EXAMPLE END

Reading
=======

Since the fast engine is not part of the ordinary :mod:`astropy.io.ascii`
infrastructure, fast readers raise an error when passed certain
parameters which are not implemented in the fast reader infrastructure.
In this case |read| will fall back on the ordinary reader, unless the
fast reader has been explicitly requested (see above).
These parameters are:

 * Negative ``header_start`` (except for commented-header format)
 * Negative ``data_start``
 * ``data_start=None``
 * ``comment`` string not of length 1
 * ``delimiter`` string not of length 1
 * ``quotechar`` string not of length 1
 * ``converters``
 * ``outputter_cls``
 * ``inputter_cls``
 * ``data_splitter_cls``
 * ``header_splitter_cls``

.. _fast_conversion_opts:

Fast Conversion Options
-----------------------

In addition to ``True`` and ``False``, the parameter ``fast_reader`` can also
be a ``dict`` specifying any of two additional parameters,
``use_fast_converter`` and ``exponent_style``.

Example
=======

..
  EXAMPLE START
  Fast Conversion Options for Faster Table Reading

To specify additional parameters using ``fast_reader``::

   >>> ascii.read('data.txt', format='basic',
   ...            fast_reader={'use_fast_converter': True}) # doctest: +SKIP

..
  EXAMPLE END

These options allow for even faster table reading when enabled, but both are
disabled by default because they come with some caveats.

Setting ``use_fast_converter`` to be ``True`` enables a faster but
slightly imprecise conversion method for floating-point values, as described
below.

The ``exponent_style`` parameter allows to define a different character
from the default ``'e'`` for exponential formats in the input file.
The special setting ``'fortran'`` enables auto-detection of any valid
exponent character under Fortran notation. For details see the section on
:ref:`fortran_style_exponents`.

Fast Converter
--------------

Input floating-point values should ideally be converted to the
nearest possible floating-point approximation; that is, the conversion
should be correct within half of the distance between the two closest
representable values, or 0.5 `ULP
<https://en.wikipedia.org/wiki/Unit_in_the_last_place>`__. The ordinary readers,
as well as the default fast reader, are guaranteed to convert floating-point
values within 0.5 ULP, but there is also a faster and less accurate
conversion method accessible via ``use_fast_converter``. If the input
data has less than about fifteen significant figures, or if accuracy is
relatively unimportant, this converter might be the best option in
performance-critical scenarios.

For values with a reasonably small number of
significant figures, the fast converter is guaranteed to produce an optimal
conversion (within 0.5 ULP). Once the number of significant figures exceeds
the precision of 64-bit floating-point values, the fast converter is no
longer guaranteed to be within 0.5 ULP, but about 60% of values end up
within 0.5 ULP and about 90% within 1.0 ULP.

Reading Large Tables
--------------------

For reading very large tables using the fast reader, see the section on
:ref:`chunk_reading`.

Writing
=======

The fast engine supports the same functionality as the ordinary writing engine
and is generally about two to four times faster than the ordinary engine.
The speed advantage of the faster engine is greatest for integer data and least
for floating-point data; the fast engine is around 3.6 times faster for a
sample file including a mixture of floating-point, integer, and text data.
Also note that stripping string values slows down the writing process, so
specifying ``strip_whitespace=False`` can improve performance.

Speed Gains
===========

The fast ASCII engine was designed based on the general parsing strategy
used in the `Pandas <https://pandas.pydata.org/>`__ data analysis library, so
its performance is generally comparable (although slightly slower by
default) to the Pandas ``read_csv`` method.

The ``genfromtxt`` and the ordinary :mod:`astropy.io.ascii` reader
are very similar in terms of speed, while ``read_csv`` is slightly faster
than the fast engine for integer and floating-point data; for pure
floating-point data, enabling the fast converter yields a speedup of about
50%. Also note that Pandas uses the exact same method as the fast
converter in Astropy when converting floating-point data.

The difference in performance between the fast engine and Pandas for
text data depends on the extent to which data values are repeated, as
Pandas is almost twice as fast as the fast engine when every value is
identical and the reverse is true when values are randomized. This is
because the fast engine uses fixed-size NumPy string arrays for
text data, while Pandas uses variable-size object arrays and uses an
underlying set to avoid copying repeated values.

Overall, the fast engine tends to be around four or five times faster than
the ordinary ASCII engine. If the input data is very large (generally
about 100,000 rows or greater), and particularly if the data does not
contain primarily integer data or repeated string values.

Another point worth noting is that the fast engine uses memory mapping
if a filename is supplied as input. If you want to avoid this for whatever
reason, supply an open file object instead. However, this will generally
be less efficient from both a time and a memory perspective, as the entire
file input will have to be read at once.