Dictionary Methods¶

Dictionaries have a number of useful built-in methods. The following table provides a summary and more details can be found in the Python Documentation.

The keys method returns what Python 3 calls a view of its underlying keys. We can iterate over the view or turn the view into a list by using the list conversion function.

It is so common to iterate over the keys in a dictionary that you can omit the keys method call in the for loop — iterating over a dictionary implicitly iterates over its keys.

As we saw earlier with strings and lists, dictionary methods use dot notation, which specifies the name of the method to the right of the dot and the name of the object on which to apply the method immediately to the left of the dot. The empty parentheses in the case of keys indicate that this method takes no parameters.

The values and items methods are similar to keys. They return view objects which can be turned into lists or iterated over directly. Note that the items are shown as tuples containing the key and the associated value.

Note that tuples are often useful for getting both the key and the value at the same time while you are looping. The two loops do the same thing.

The in and not in operators can test if a key is in the dictionary:

This operator can be very useful since looking up a non-existent key in a dictionary causes a runtime error.

The get method allows us to access the value associated with a key, similar to the [ ] operator. The important difference is that get will not cause a runtime error if the key is not present. It will instead return None. There exists a variation of get that allows a second parameter that serves as an alternative return value in the case where the key is not present. This can be seen in the final example below. In this case, since “cherries” is not a key, return 0 (instead of None).

Check your understanding

mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
keylist = list(mydict.keys())
keylist.sort()
print(keylist[3])

mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
answer = mydict.get("cat") // mydict.get("dog")
print(answer)

mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
print("dog" in mydict)

mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
print(23 in mydict)

total = 0
mydict = {"cat":12, "dog":6, "elephant":23, "bear":20}
for akey in mydict:
   if len(akey) > 3:
      total = total + mydict[akey]
print(total)