[python] How to update the value of a key in a dictionary in Python?

I have a dictionary which represents a book shop. The keys represent the book title and values represent the number of copies of the book present. When books are sold from the shop,the number of copies of the book must decrease.

I have written a code for decreasing the number of copies of the sold book, but on printing the dictionary after the update, I get the initial dictionary and not the updated one.

 n=input("Enter number of books in shop:")
 book_shop={} #Creating a dictionary book_shop
 #Entering elements into the dictionary
 for i in range(n):
     book_title=raw_input("Enter book title")
     book_no=input("Enter no of copies")
     book_shop[book_title]=book_no
 ch=raw_input("Do you want to sell")
 if (ch in 'yesYES'):
        for i in range(n):
             print"which book you want to sell??",book_shop
             ch1=raw_input("choice")
             if(book_shop.keys()[i]==ch1):
                    book_shop.keys()[i]=(book_shop.values()[i]-1)
                    break

 print book_shop

I would like to solve the problem in the simplest way possible. Have I missed any logic or any line in the code?

This question is related to python dictionary

The answer is


n = eval(input('Num books: '))
books = {}
for i in range(n):
    titlez = input("Enter Title: ")
    copy = eval(input("Num of copies: "))
    books[titlez] = copy

prob = input('Sell a book; enter YES or NO: ')
if prob == 'YES' or 'yes':
    choice = input('Enter book title: ')
    if choice in books:
        init_num = books[choice]
        init_num -= 1
        books[choice] = init_num
        print(books)

You are modifying the list book_shop.values()[i], which is not getting updated in the dictionary. Whenever you call the values() method, it will give you the values available in dictionary, and here you are not modifying the data of the dictionary.