Create query MongoDb with Python2.7

I have the following doubt, I am building a query that performs a massive insertion in mongodb, but part of the data is brought from a function.

The following is the function that I use, my problem maybe is with the return

def formula(df,W, tipo):
    LEQ = []
    pd=data(df,W) 
    for index,info in pd.iterrows():
        bands = info[2:]
        LEQ.append(10**(NivelesPd(bands)/10))
    level = round(10 * math.log10(sum(LEQ)/len(LEQ)),1)
    maxNps = round(10 * math.log10(max(LEQ)),1)
    minNps = round(10 * math.log10(min(LEQ)),1)

    metrica = "'" +tipo +"' : "+ str(level) +", '"+tipo+"_MAX' : " + str(maxNps) + ", '"+tipo+"_MIN' : " + str(minNps)
    return  metrica

Once I get the information from the formula, I perform a "for" cycle

for index, df in grouped_per_time:
   data= { 'ids' : str(ids), 'time' : str(index), 'tzone' : str(tzone) +", "+ formula(df,A, tipo='1') +"," + formula(df,B, tipo='2')+"," + formula(df,C, tipo='3') }
   #realiso el insert en mongo
   pd.save( data)

The insertion is done well, but the concatenation that I do with the function, I put all the data of the formulas as one

{'tzone': "-3, 'a' : 30.5, 'amx' : 30.5, 'amn' : 30.5,'c' : 29.9, 'cmx' : 30.0, 'cmn' : 29.9,'z' : 30.9, 'zmx' : 30.9, 'zmn' : 30.9", 'ids': 'b827', 'time': '1534250153'}

As you can see the following data that are in quotes and should be separated

'tzone': "- 3, 'a': 30.5, 'amx': 30.5, 'amn': 30.5, 'c': 29.9, 'cmx': 30.0, 'cmn': 29.9, 'z': 30.9, 'zmx': 30.9, 'zmn': 30.9 "

The data should look like this:

{'tzone': '-3', 'a' : 30.5, 'amx' : 30.5, 'amn' : 30.5,'c' : 29.9, 'cmx' : 30.0, 'cmn' : 29.9,'z' : 30.9, 'zmx' : 30.9, 'zmn' : 30.9, 'ids': 'b827', 'time': '1534250153'}