: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, :]
