:orphan:

Memory-efficient I/O

====================

.. NOTE: part of this file is included in doc/overview/implementation.rst.

   Changes here are reflected there. If you want to link to this content, link

   to :ref:`memory` to link to that section of the implementation.rst

   page. The next line is a target for :start-after: so we can omit the title

   from the include:

   memory-begin-content

Preloading continuous (raw) data

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

MNE-Python can read data on-demand using the ``preload`` option provided in

raw reading functions. For example::

    from mne import io

    from mne.datasets import sample

    data_path = sample.data_path()

    raw_fname = data_path / 'MEG' / 'sample' / 'sample_audvis_filt-0-40_raw.fif'

    raw = io.read_raw_fif(raw_fname, preload=False)

.. note:: Filtering, resampling and dropping or selecting channels does not

          work with ``preload=False``.

Preloading epoched data

~~~~~~~~~~~~~~~~~~~~~~~

Similarly, epochs can also be be read from disk on-demand. For example::

    import mne

    events = mne.find_events(raw)

    event_id, tmin, tmax = 1, -0.2, 0.5

    picks = mne.pick_types(raw.info, meg=True, eeg=True, stim=False, eog=True)

    epochs = mne.Epochs(raw, events, event_id, tmin, tmax, picks=picks,

                        baseline=(None, 0), reject=dict(eeg=80e-6, eog=150e-6),

                        preload=False)

When ``preload=False``, the epochs data is loaded from the disk on-demand. Note

that ``preload=False`` for epochs will work even if the ``raw`` object has been

loaded with ``preload=True``. Preloading is also supported for

:func:`mne.read_epochs`.

.. warning:: This comes with a caveat. When ``preload=False``, data rejection

             based on peak-to-peak thresholds is executed when the data is

             loaded from disk, *not* when the ``Epochs`` object is created.

To explicitly reject artifacts with ``preload=False``, use the function :func:`mne.Epochs.drop_bad`.

Loading data explicitly

~~~~~~~~~~~~~~~~~~~~~~~

To load the data if ``preload=False`` was initially selected, use the functions :func:`mne.io.Raw.load_data` and :func:`mne.Epochs.load_data`.

Accessing data as NumPy arrays

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you just want your raw data as a :class:`Numpy array <numpy.ndarray>` to

work with it in a different framework you can use slicing syntax::

    first_channel_data, times = raw[0, :]

    channels_3_and_4, times = raw[3:5, :]