Override a function which is not in class and whitelisted

[NCP]

Possible using monkey patch.

Reference:

Override standard controller methods in erpnext with custom app

In your custom app custom_app/custom_app/__int__.py file For Example from erpnext.controllers.taxes_and_totals import calculate_taxes_and_totals from custom_app...<path_to_file> import calculate_item_values calculate_taxes_and_totals.calculate_item_values = calculate_item_values