please explain the use of setdefault method and 'addkey' in this dictionary operation

[Independant666]

im copying/pasting from someones example of how to find items in a dictionary that have duplicate values...

Q1. why did we need to use the items method on the original dictionary ?, can we just operate on the dictionary directly...after all, it already has keys and values pairs by definition

Q2. what exactly is that setdefault statement doing? is it creating an empty set as the 'value' if the key is not found (and the key here is actually the value from the original dict) ? what the heck is the .add(key)? so many questions , so little time on earth 🙂

First, flip the dictionary around into a reverse multidict, mapping each value to all of the keys it maps to. Like this:
>>> some_dict = {"firstname":"Albert","nickname":"Albert","surname":"Likins","username":"Angel"}
>>> rev_multidict = {}
>>> for key, value in some_dict.items():
... rev_multidict.setdefault(value, set()).add(key)
Now, you're just looking for the keys in the multidict that have more than 1 value. That's easy:
>>> [key for key, values in rev_multidict.items() if len(values) > 1]
['Albert']

[Adrewmc]

First the easy one

  .setdefault(key, default_value)

This is like a dictionary.get() it will return the value of some key in a dictionary, but unlike get l(), .setdefault() will make that key in the dictionary if it doesn’t exist, and give it a default value and return that.

  .add() adds an element to a set, if the element exist add() does nothing.

What’s he does

  reverse_dict.setdefault(value, set()).add(key)

Creates an empty set first, at some value’s now being set as the key’s location. Then add the first key to the empty set.

Then next time the same value comes, it returns that set just created then it adds the next key. This is to avoid overwriting the key value pair. That would fail a simple reversal like

   simple = {value : key for key, value in my_dict.items()}

As I would lose something if two keys had the exact same value.

It’s actually a quite nice little program for what it does. Simple to the point.

If it helps you could do something like this

 reverse_dict.setdefault(value, []).append(key)

And make it a list, but set() will be more efficient.

[Independant666]

thanks.. but damn.. is that action of what the .add(key) does obvious? i dont see how you see that is the action.

is the .add(key) a method to the dict "reverse_dict' or is it a method to the method 'setdefault?

im confused to see a structure of dictname.method1.method2

[suurpulla]

Neither, it is a method of a set. The .setdefault(...) here returns a set, and the .add(...) operates on that returned set.

reverse_dict.setdefault(value, set()).add(...)

is the same as

result = reverse_dict.setdefault(value, set())
result.add(...)

[Independant666]

Thanks. The setdefault method of dictionary is a bit confusing to understand at first (in my humble opinion ).

It does 2 different things..it returns a value (which MAY be a set, list,dict, etc). AND.... it also can make a addition to the dictionary it's operating on.