python - trying to increment the list value if key exits

Question

for line in open('transactions.dat','r'):
    item=line.rstrip('
').split(',')
    custid=item[2]
    amt=item[4]
    if custid in cust:
        amt1=int(cust[custid])+int(amt)
        cust[custid]=amt1
    else:
        cust[custid]=[amt]

well i am trying to check if customer id already is there in dictionary then simply add the previous amount and new amount in that customer. Otherwise add that amount in a new position in list. But i am getting error:

 Traceback (most recent call last):
File "<pyshell#74>", line 7, in <module>
amt1=int(cust[custid])+int(amt)
TypeError: int() argument must be a string, a bytes-like object or a number, not 'list'                    

some transaction data is like:

101300101,2016-09-03,376248582,1013,10.92

109400132,2016-09-03,391031719,1094,36.72

136100107,2016-09-03,391031719,1361,28.77

Answer 1

Did you try using defaultdict? It would make your job much easier.

from collections import defaultdict

cust = defaultdict(int)
for line in open('transactions.dat','r'):
    item=line.rstrip('
').split(',')
    custid=item[2]
    amt=item[4]

    cust[custid] += float(amt)

and why do you try to cast amt to int? Looks like it's not an integer in the sample lines you posted. If you really want to use integer change float(amt) to int(float(amt)).