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Python Python Collections (2016, retired 2019) Dictionaries Dict Basics

Posted by leonardbode
leonardbode
Courses Plus Student 4,011 Points

Why does test_dict["a"] return 3 if test_dict = {"a": 1, "b": 2, "a": 3} I thought dictionaries were not ordered?

Found in Python Collections course quiz.

1 Answer

Steven Parker
Steven Parker
231,236 Points
Steven Parker
Steven Parker
231,236 Points

Dictionaries are not ordered, but they can be indexed using their keys.

So for example, you could not predict what the "third" item might be in a dictionary. But you can always get the value associated with a particular key (such as "a" in your example above).

This example is unusual since it shows the same key being assigned twice. But since dictionary keys are required to be unique, the value for "a" is simply replaced with 3 instead of creating a new key/value pair.

leonardbode
leonardbode
Courses Plus Student 4,011 Points

Yes, I understand that. However, I don't understand the logic behind test_dict["a"] always returning 3 and never 1?

Edit: Why is the value of "a" replaced with 3?

Jason Anello
Jason Anello
Courses Plus Student 94,610 Points

Because that was the last value assigned to the key 'a'. If there are duplicate keys, then the last value assigned to that key is the one that gets used.

Try printing out your dictionary after you create it.

>>> test_dict = {"a": 1, "b": 2, "a": 3}
>>> test_dict
{'b': 2, 'a': 3}
>>>

You can see that the "a": 1 is dropped and all that you have is the "a": 3

leonardbode
leonardbode
Courses Plus Student 4,011 Points

I understand now, thank you.