Why are dictionary values being overriden at the end of this loop?

Question

I have a dictionary of public transportation stops called stops. I want to duplicate the ones that are transfers (have more than one line) so that there is a duplicate stop in s

Answer 1

Then duplicates[duplicate_stop] and stops[item] both name the same object - and mutating the object, well, changes the object. Objects are not automatically copied/cloned/duplicated on an assignment or when used as function arguments.

The problematic line is most likely

duplicates[new_map_id] = stops[item] # duplicate the stop

.. and the comment is wrong because there is no duplication that occurs.

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The question Understanding dict.copy() - shallow or deep? may be useful; at the very least it shows how to make a real copy.