Dictionaries and Identity Operators

Dictionaries

A dictionary is a mutable data type that stores mappings of unique keys to values. Here's a dictionary that stores elements and their atomic numbers.

elements = {"hydrogen": 1, "helium": 2, "carbon": 6}

Dictionaries can have keys of any immutable type, like integers or tuples, not just strings. It's not even necessary for every key to have the same type! We can look up values or insert new values in the dictionary using square brackets that enclose the key.

print(elements["helium"])  # print the value mapped to "helium"
elements["lithium"] = 3  # insert "lithium" with a value of 3 into the dictionary

We can check whether a value is in a dictionary the same way we check whether a value is in a list or set with the in keyword. Dicts have a related method that's also useful, get. get looks up values in a dictionary, but unlike square brackets, get returns None (or a default value of your choice) if the key isn't found.

print("carbon" in elements)
print(elements.get("dilithium"))

This would output:

True
None

Carbon is in the dictionary, so True is printed. Dilithium isn’t in our dictionary so None is returned by get and then printed. If you expect lookups to sometimes fail, get might be a better tool than normal square bracket lookups because errors can crash your program.

Identity Operators

You can check if a key returned None with the is operator. You can check for the opposite using is not.

n = elements.get("dilithium")
print(n is None)
print(n is not None)

This would output:

True
False