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Given the following text description, write Python code to implement the functionality described below step by step
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Description:
 The Info data structure
This tutorial describes the 
Step1:  As seen in the introductory tutorial &lt;tut-overview&gt;, when a
Step2:  However, it is not strictly necessary to load the 
Step3:  As you can see, the 
Step4:  Most of the fields contain 
Step5:  Obtaining subsets of channels
It is often useful to convert between channel names and the integer indices
identifying rows of the data array where those channels' measurements are
stored. The 
Step6:  
Step7:  Note that the meg and fnirs parameters of 
Step8:  
Step9:  To obtain several channel types at once, you could embed
Step10:  Alternatively, you can get the indices of all channels of all channel types
present in the data, using 
Step11:  Dropping channels from an Info object
If you want to modify an 
Step12:  We can also get a nice HTML representation in IPython like
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Python Code:
import os
import mne
sample_data_folder = mne.datasets.sample.data_path()
sample_data_raw_file = os.path.join(sample_data_folder, 'MEG', 'sample',
                                    'sample_audvis_filt-0-40_raw.fif')
raw = mne.io.read_raw_fif(sample_data_raw_file)
Explanation: The Info data structure
This tutorial describes the :class:mne.Info data structure, which keeps track
of various recording details, and is attached to :class:~mne.io.Raw,
:class:~mne.Epochs, and :class:~mne.Evoked objects.
We will begin by loading the Python modules we need, and loading the same
example data &lt;sample-dataset&gt; we used in the introductory tutorial
&lt;tut-overview&gt;:
End of explanation
print(raw.info)
Explanation: As seen in the introductory tutorial &lt;tut-overview&gt;, when a
:class:~mne.io.Raw object is loaded, an :class:~mne.Info object is
created automatically, and stored in the raw.info attribute:
End of explanation
info = mne.io.read_info(sample_data_raw_file)
print(info)
Explanation: However, it is not strictly necessary to load the :class:~mne.io.Raw object
in order to view or edit the :class:~mne.Info object; you can extract all
the relevant information into a stand-alone :class:~mne.Info object using
:func:mne.io.read_info:
End of explanation
print(info.keys())
print()  # insert a blank line
print(info['ch_names'])
Explanation: As you can see, the :class:~mne.Info object keeps track of a lot of
information about:
the recording system (gantry angle, HPI details, sensor digitizations,
  channel names, ...)
the experiment (project name and ID, subject information, recording date,
  experimenter name or ID, ...)
the data (sampling frequency, applied filter frequencies, bad channels,
  projectors, ...)
The complete list of fields is given in :class:the API documentation
&lt;mne.Info&gt;.
Querying the Info object
The fields in a :class:~mne.Info object act like Python :class:dictionary
&lt;dict&gt; keys, using square brackets and strings to access the contents of a
field:
End of explanation
print(info['chs'][0].keys())
Explanation: Most of the fields contain :class:int, :class:float, or :class:list
data, but the chs field bears special mention: it contains a list of
dictionaries (one :class:dict per channel) containing everything there is
to know about a channel other than the data it recorded. Normally it is not
necessary to dig into the details of the chs field — various MNE-Python
functions can extract the information more cleanly than iterating over the
list of dicts yourself — but it can be helpful to know what is in there. Here
we show the keys for the first channel's :class:dict:
End of explanation
print(mne.pick_channels(info['ch_names'], include=['MEG 0312', 'EEG 005']))
print(mne.pick_channels(info['ch_names'], include=[],
                        exclude=['MEG 0312', 'EEG 005']))
Explanation: Obtaining subsets of channels
It is often useful to convert between channel names and the integer indices
identifying rows of the data array where those channels' measurements are
stored. The :class:~mne.Info object is useful for this task; two
convenience functions that rely on the :class:mne.Info object for picking
channels are :func:mne.pick_channels and :func:mne.pick_types.
:func:~mne.pick_channels minimally takes a list of all channel names and a
list of channel names to include; it is also possible to provide an empty
list to include and specify which channels to exclude instead:
End of explanation
print(mne.pick_types(info, meg=False, eeg=True, exclude=[]))
Explanation: :func:~mne.pick_types works differently, since channel type cannot always
be reliably determined from channel name alone. Consequently,
:func:~mne.pick_types needs an :class:~mne.Info object instead of just a
list of channel names, and has boolean keyword arguments for each channel
type. Default behavior is to pick only MEG channels (and MEG reference
channels if present) and exclude any channels already marked as "bad" in the
bads field of the :class:~mne.Info object. Therefore, to get all and
only the EEG channel indices (including the "bad" EEG channels) we must
pass meg=False and exclude=[]:
End of explanation
print(mne.pick_channels_regexp(info['ch_names'], '^E.G'))
Explanation: Note that the meg and fnirs parameters of :func:~mne.pick_types
accept strings as well as boolean values, to allow selecting only
magnetometer or gradiometer channels (via meg='mag' or meg='grad') or
to pick only oxyhemoglobin or deoxyhemoglobin channels (via fnirs='hbo'
or fnirs='hbr', respectively).
A third way to pick channels from an :class:~mne.Info object is to apply
regular expression_ matching to the channel names using
:func:mne.pick_channels_regexp. Here the ^ represents the beginning of
the string and . character matches any single character, so both EEG and
EOG channels will be selected:
End of explanation
print(mne.channel_type(info, 25))
Explanation: :func:~mne.pick_channels_regexp can be especially useful for channels named
according to the 10-20 system (e.g., to select all channels
ending in "z" to get the midline, or all channels beginning with "O" to get
the occipital channels). Note that :func:~mne.pick_channels_regexp uses the
Python standard module :mod:re to perform regular expression matching; see
the documentation of the :mod:re module for implementation details.
<div class="alert alert-danger"><h4>Warning</h4><p>Both :func:`~mne.pick_channels` and :func:`~mne.pick_channels_regexp`
   operate on lists of channel names, so they are unaware of which channels
   (if any) have been marked as "bad" in ``info['bads']``. Use caution to
   avoid accidentally selecting bad channels.</p></div>
Obtaining channel type information
Sometimes it can be useful to know channel type based on its index in the
data array. For this case, use :func:mne.channel_type, which takes
an :class:~mne.Info object and a single integer channel index:
End of explanation
picks = (25, 76, 77, 319)
print([mne.channel_type(info, x) for x in picks])
print(raw.get_channel_types(picks=picks))
Explanation: To obtain several channel types at once, you could embed
:func:~mne.channel_type in a :term:list comprehension, or use the
:meth:~mne.io.Raw.get_channel_types method of a :class:~mne.io.Raw,
:class:~mne.Epochs, or :class:~mne.Evoked instance:
End of explanation
ch_idx_by_type = mne.channel_indices_by_type(info)
print(ch_idx_by_type.keys())
print(ch_idx_by_type['eog'])
Explanation: Alternatively, you can get the indices of all channels of all channel types
present in the data, using :func:~mne.channel_indices_by_type,
which returns a :class:dict with channel types as keys, and lists of
channel indices as values:
End of explanation
print(info['nchan'])
eeg_indices = mne.pick_types(info, meg=False, eeg=True)
print(mne.pick_info(info, eeg_indices)['nchan'])
Explanation: Dropping channels from an Info object
If you want to modify an :class:~mne.Info object by eliminating some of the
channels in it, you can use the :func:mne.pick_info function to pick the
channels you want to keep and omit the rest:
End of explanation
info
Explanation: We can also get a nice HTML representation in IPython like:
End of explanation
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