'Set all field values from a dictionary.
For any key in `dict` that is also a field to store tags the
method retrieves the corresponding value from `dict` and updates
the `MediaFile`. If a key has the value `None`, the
corresponding property is deleted from the `MediaFile`.'

def update(self, dict):

    for field in self.sorted_fields():
        if (field in dict):
            if (dict[field] is None):
                delattr(self, field)
            else:
                setattr(self, field, dict[field])
