how to perform string formatting inside a dictionary?

lets say i have a payload which i am using to hit my API

payload = json.dumps({"from_date":"2019-10-10",
                      "from_date":"2019-10-12",
                     "pg_no": 1,
                     "idx": "fsdf"})

but i wanted to make its pg_no value as dynamic using for loop i.e.

payload = json.dumps({"from_date":"2019-10-10",
                          "from_date":"2019-10-12",
                         "pg_no": {},
                         "idx": "fsdf"})

for i in range(1,5):
   payload.format(i)

getting this error

KeyError                                  Traceback (most recent call last)
<ipython-input-7-5b565a97da5e> in <module>
      1 for i in range(1,5):
----> 2        payload.format(i)

KeyError: '"from_date"'

Answer

first in your dictionary you are using same key from_data which is gonna be only last one present there. second main problem is causing by { bracket format use this place variable if you want to parse single single { then you have to use {{ in string . so simple put value in dictionary then convert to string like this

data = {'rom_date':'2019-10-10',
        'fom_date':'2019-10-12',
                     'pg_no': {},
                     'idx': 'fsdf'}

for i in range(1,5):
    data['pg_no'] = i 
    print(json.dumps(data))