Django MultiSelectField how to have disabled=True in the form

Question

I am using django MultiSelectField for storing multiple choice fields in db. In my forms.py I make many fields as read only by setting disabled=True:

xyz = forms.IntergerField(disabled=True) 

This doesnt work with MultiSelectField.

I tried the following but it gives error when saving the updated form :

def __init__(self, *args, **kwargs):
    super().__init__(*args, **kwargs)
    self.fields['working_days'].widget.attrs['disabled'] = True

Error:

Fields required

I would like to show MultiSelectFields as it is in a disabled state, but when saving the edited form in django view, I want django to use old values for this field.