Error while writing json to file?

Question

Python

read about json , from there I realized that they use

with open("file.json", 'w+') as f :
                             cl = {}
                             json.dump(cl, f)

here is my case:

if not str(message.chat.id) in os.listdir('mat/'):
                        os.mkdir('mat/'+str(message.chat.id))
                        with open("mat/" + str(message.chat.id) + "/users_mat.json", 'w+') as f :
                             cl = {}
                             json.dump(cl, f)
                             print('111111111111')
                    f = open("mat/" + str(message.chat.id) + "/users_mat.json", 'w+')
                    users_mat =json.load(f)
                    if not str(message.from_user.id) in users_mat.keys():
                        users_mat[str(message.from_user.id)]: 0
                        json.dump(users_mat , f)
                    users_mat[str(message.from_user.id)]: users_mat [str(message.from_user.id)]+1
                    json.dump(users_mat ,f)

but the error : raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
swears at my attempt to save the dictionary to a file in the first if.
I do not rule out that I did not correctly understand the meaning of "dump", for the first time in my life I work with json, and all sites say the same thing, and there is no one about working with files.

Answer 1

f = open("mat/" + str(message.chat.id) + "/users_mat.json", 'w+')
users_mat =json.load(f)

Opened the file in write mode -> deleted the contents of the file -> trying to read json from an empty file
Actually, change the file open mode from w+tor