How to update the value of a key in a dictionary in Python

Question

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

User · Accepted Answer

Well you could directly substract from the value by just referencing the key. Which in my opinion is simpler.

>>> books = {}
>>> books['book'] = 3       
>>> books['book'] -= 1   
>>> books   
{'book': 2}

In your case:

book_shop[ch1] -= 1

User · Answer

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

User · Answer

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

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

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