django - comparing old and new field value before saving
It is better to do this at ModelForm level. 
There you get all the Data that you need for comparison in save method:

self.data : Actual Data passed to the Form.
self.cleaned_data : Data cleaned after validations, Contains Data eligible to be saved in the Model
self.changed_data : List of Fields which have changed. This will be empty if nothing has changed

If you want to do this at Model level then you can follow the method specified in Odif's answer.
I have a django model, and I need to compare old and new values of field BEFORE saving.
I've tried the save() inheritance, and pre_save signal. It was triggered correctly, but I can't find the list of actually changed fields and can't compare old and new values. Is there a way? I need it for optimization of pre-save actions.
Thank you!