Dictionaries

Looking up a key that is not in a dictionary stops the program with KeyError, but d.get(key, 0) returns 0 instead.

Seoyeon has 12 ballots and three candidates

That table is a dictionary. A dictionary stores pairs: a key, and the value that goes with it. You write it in curly braces, with a colon between each key and its value. To read a value, put the key in square brackets, the way you would put an index after a list. To add a pair or change one, assign to that key.

votes = {"Ari": 0, "Bo": 0, "Cy": 0}
votes["Bo"] = votes["Bo"] + 1
votes["Ari"] += 1
votes["Bo"] += 1
print(votes)
print(votes["Bo"])

{'Ari': 1, 'Bo': 2, 'Cy': 0}
2

Counting the whole box in one loop

Typing one line per ballot is no better than tally marks. Seoyeon puts the ballots in a list and loops over it. There is one snag: what if a ballot names someone she did not set up in advance, or she would rather not list the candidates first? Reading a missing key with square brackets stops the program. The method get(key, default) solves this. It returns the value if the key is there and the default if it is not, so votes.get(name, 0) + 1 works for the first vote and every vote after it.

ballots = ["Bo", "Ari", "Bo", "Cy", "Bo", "Ari",
           "Cy", "Bo", "Ari", "Bo", "Cy", "Bo"]
votes = {}
for name in ballots:
    votes[name] = votes.get(name, 0) + 1
print(votes)
print(len(ballots), sum(votes.values()))

{'Bo': 6, 'Ari': 3, 'Cy': 3}
12 12

The last line is Seoyeon's check. Twelve ballots went in, and the counts add up to twelve, so none were lost. Notice the order of the names in the output: Bo first, because Bo was the first name the loop met. Since Python 3.7, a dictionary keeps its keys in the order they were first added, and the tutorial's section on dictionaries says the same.

Walking through keys and values together

A for loop over a dictionary gives the keys. items() gives key and value together, which is what you want for a results table. in checks whether a key exists, and max() with key=votes.get finds the key with the biggest value. That last one is a pattern worth copying as it is, even before every part of it makes sense.

votes = {"Bo": 6, "Ari": 3, "Cy": 3}
for name, n in votes.items():
    print(name, "#" * n, n)
print("Dan" in votes)
print(max(votes, key=votes.get))

Bo ###### 6
Ari ### 3
Cy ### 3
False
Bo

Dictionaries turn up wherever data comes in name and value pairs: a word and its meaning, a subway station and its line number, a student number and a name. Keys must be unique, and assigning to an existing key replaces its old value rather than adding a second copy.