How to append a new value to a list in a dictionary and write it to JSON file

Question

I'm very new to python and I'm still trying to learn dictionaries. I made a dictionary of lists but I'm trying to append a value that I added to an existing list within the dictionary. However when I try to append that new value to that list, it doesn't write to the JSON file, it just keeps the file as it is.

I have this code

with open('students.json') as j:
    students = json.load(j)
for i in students:
    #print "entered here4"
    if(name == i):
            print "entered here5"
            for k in students[i]:
                print "entered here6"
                print k
                if(classes == k):
                    print "classes exists Success"
                     sys.exit()
                else:
                        students[i].append(classes)
                                #x = students[i].append(classes) 
                                #with open ('students.json', 'w') as f:
                                    #json.dump(x,f)
                                #this didn't work for me, it wrote "null" to my JSON file
                                print "user added"
                                sys.exit()

(Ignore the print statements, they were just for when I was trying to debug)

Currently my JSON file looks like this with only one student and his class

 {"student1": ["math"]}

I eventually want my dictionary to look like this

{'student1': ["math", "biology"], "student2": ["chemistry"], "student3": ["History", "biology", "physics"]}

How do I append new values to an existing list and how do I add new keys with its corresponding lists?

Answer 1

In your code, you are updating the local variable but not writing the updates to the file. What you need is to dump the final dictionary.

with open("students.json", "w") as jsonFile:
    json.dump(students, jsonFile)