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Python

Posted by Marc Biggar
Marc Biggar
12,486 Points

Switching Keys and values Python

Hi

How could I go about switching keys and value of a dictionary in Python, I think it would pretty achievable using a for loop, however I am struggling to figure it out with the following requirements

  • Keys are arrays
    • Keys and value could appear twice

Example:

interfaces = {
        ["172.17.110.1", "255.255.255.0"] : "ACCESS",
        ["172.18.165.1", "255.255.255.0"] : "WAN",
        ["172.19.132.1", "255.255.255.0"] : "ACCESS",
        ["172.19.34.1", "255.255.255.0"] : "WAN",
        ["172.12.134.1", "255.255.255.0"] : "APP"
        }

Thanks in advance, really struggled to figure this out.

Marc

alex salman
alex salman
Courses Plus Student 509 Points

Hi!

From what I know, you can't use arrays as keys in a dictionary. There is an article on why here: https://wiki.python.org/moin/DictionaryKeys

If you use tuples instead there should be a way.

Marc Biggar
Marc Biggar
12,486 Points

Ah shame!

The dict I get back is from an API call. Will read the link provided and take a look a tuples.

Thanks

Marc

1 Answer

Jon Mirow
Jon Mirow
9,864 Points

As Alex said, keys must be immutable, and in the case of tuples as keys, all of their contents must be immutable. Tuple's can be a powerful way of using a dictionary, allowing you to look up multiple combinations of values, but of course, most dictionaries probably don't need tuples for keys, so it's worth pointing out that this could be a warning sign to double-check your approach.

Here, the warning sign isn't using tuples - IP addresses and subnet masks (I assume) in tuples would be very good keys. However that "keys and values may not be unique"... Basically that would be a red flag not to do this. If you can't guarantee that each IP and subnet mask pair were unique you couldn't do it. Basically to switch keys with values you have to be sure that the values in the dictionary will always be unique, and then handle any mutable objects.

Moreover you don't control the data creation. If it's an external API, it's perfectly reasonable to assume that a future update to the API may add items to the list, add nested lists, or dictionarys, or custom objects that don't have a .hash method. All of these things could be appended to the list in the dictionary (or JSON) without affecting the "normal" use of the api - dictionary[key][value index]

Anyway to answer the question, Assuming you only had a dictionary of keys and values in the form of a 1D list, you could switch them over like this:

#Assume the Api data looks a bit like this:
api_data =  {
        "ACCESS": ["172.17.110.1", "255.255.255.0"],
        "WAN": ["172.18.165.1", "255.255.255.0"],
        "APP": ["172.19.132.1", "255.255.255.0"],
        "LAN": ["172.19.34.1", "255.255.255.0"],
        "DNS": ["172.12.134.1", "255.255.255.0"]
        }

api_data_formatted = { tuple(value): key for key, value in api_data.items()}

Would do the job.

But like I say, you really want to be quite sure about what's in the dictionary first of all. Hope it helps!