Dictionary 的值是如何赋值的？

Question

This is a very simple problem where it reads file from a CSV with first column header as "title" and then counts how many times the title appears in side the dictionary. But I am not understanding in which step it is assigning the "title" to "titles" dictionary.

The code is:

import csv

titles = {}

with open("movies.csv", "r") as file:
    reader = csv.DictReader(file)

    for row in reader:
        #title is defined here
        title = row["title"].strip().upper()
        if title in titles:
            titles[title] = titles[title] + 1
        else:
            titles[title] = 1

If it is assigning inside the else block then why is my second code where I just want to assign values to the dictionary named "titles" and not count the number of times it appears, is not Working?:

import csv

titles = {}

with open("movies.csv", "r") as file:
    reader = csv.DictReader(file)

    for row in reader:
        #title is defined here
        title = row["title"].strip().upper()
        if not title in titles:
            titles[title]
            
print(titles[title])

Error: Key Value error

Answer 1

In your second version, you have this line titles[title] , which is not adding the title to your titles dictionary as you do in your first version. Since the title is missing in the dictionary, accessing it will give you a key value error. Why do you have a line titles[title] that does nothing?

But I think there's a bigger problem here with your first version of code. You want to add the title to the dictionary when it's not already in it, and add the count by 1 if otherwise. But your first version is doing the opposite, which will throw you an error.

Answer 2

If titles is a dictionary titles[title] finds the value corresponding to the key title . Your second version does not put anything in the dictionary, so titles[title] raises a key error.

You say you want to "assign values to the dictionary but not count anything". A dictionary is the wrong structure to use for this. If you want unique titles, you could use a set :

import csv

titles = set()

with open("movies.csv", "r") as file:
    reader = csv.DictReader(file)

    for row in reader:
        #title is defined here
        title = row["title"].strip().upper()
        titles.add(title)
            
print(titles)

Notice the add method only adds something if it does not already exist in the set, rather like a mathematical set. You no longer have key, value pairs, just items.