testbed/astropy__astropy/docs/table/performance.inc.rst

.. note that if this is changed from the default approach of using an *include*
   (in index.rst) to a separate performance page, the header needs to be changed
   from === to ***, the filename extension needs to be changed from .inc.rst to
   .rst, and a link needs to be added in the subpackage toctree

.. _astropy-table-performance:

Performance Tips
================

Constructing |Table| objects row-by-row using
:meth:`~astropy.table.Table.add_row` can be very slow::

    >>> from astropy.table import Table
    >>> t = Table(names=['a', 'b'])
    >>> for i in range(100):
    ...    t.add_row((1, 2))

If you do need to loop in your code to create the rows, a much faster approach
is to construct a list of rows and then create the Table object at the very
end::

  >>> rows = []
  >>> for i in range(100):
  ...    rows.append((1, 2))
  >>> t = Table(rows=rows, names=['a', 'b'])

Writing a |Table| with |MaskedColumn| to ``.ecsv`` using
:meth:`~astropy.table.Table.write` can be very slow::

    >>> from astropy.table import Table
    >>> import numpy as np
    >>> x = np.arange(10000, dtype=float)
    >>> tm = Table([x], masked=True)
    >>> tm.write('tm.ecsv', overwrite=True) # doctest: +SKIP

If you want to write ``.ecsv`` using :meth:`~astropy.table.Table.write`,
then use ``serialize_method='data_mask'``.
It uses the non-masked version of data and it is faster::

    >>> tm.write('tm.ecsv', overwrite=True, serialize_method='data_mask') # doctest: +SKIP