python: Finding min values of subsets of a list - python-2.7

Question

I have a list that looks something like this
(The columns would essentially be acct, subacct, value.):
1,1,3
1,2,-4
1,3,1
2,1,1
3,1,2
3,2,4
4,1,1
4,2,-1
I want update the list to look like this:
(The columns are now acct, subacct, value, min of the value for each account)
1,1,3,-4
1,2,-4,-4
1,3,1,-4
2,1,1,1
3,1,2,2
3,2,4,2
4,1,1,-1
4,2,-1,-1
The fourth value is derived by taking the min(value) for each account. So, for account 1, the min is -4, so col4 would be -4 for the three records tied to account 1.
For account 2, there is only one value.
For account 3, the min of 2 and 4 is 2, so the value for col 4 is 2 where account = 3.
I need to preserve col3, as I will need to use the value in column 3 for other calculations later. I also need to create this additional column for output later.
I have tried the following:
with open(file_name, 'rU') as f: #opens PW file
data = zip(*csv.reader(f, delimiter = '\t'))
# data = list(list(rec) for rec in csv.reader(f, delimiter='\t'))
#reads csv into a list of lists
#print the first row
uniqAcct = []
data[0] not in used and (uniqAcct.append(data[0]) or True)
But short of looping through and matching on each unique count and then going back through and adding a new column, I am stuck. I think there must be a pythonic way of doing this, but I cannot figure it out. Any help would be greatly appreciated!
I cannot use numpy, pandas, etc as they cannot be installed on this server yet. I need to use just basic python2

Answer 1

So the problem here is your data structure, it's not trivial to index.
Ideally you'd change it to something readible and keep it in those containers. However if you insist on changing it back into tuples I'd go with this construction
# dummy values
data = [
(1, 1, 3),
(1, 2,-4),
(1, 3, 1),
(2, 1, 1),
(3, 1, 2),
(3, 2, 4),
(4, 1, 1),
(4, 2,-1),
]
class Account:
def __init__(self, acct):
self.acct = acct
self.subaccts = {} # maps sub account id to it's value
def as_tuples(self):
min_value = min(val for val in self.subaccts.values())
for subacct, val in self.subaccts.items():
yield (self.acct, subacct, val, min_value)
def accounts_as_tuples(accounts):
return [ summary for acct_obj in accounts.values() for summary in acct_obj.as_tuples() ]
accounts = {}
for acct, subacct, val in data:
if acct not in accounts:
accounts[acct] = Account(acct)
accounts[acct].subaccts[subacct] = val
print(accounts_as_tuples(accounts))
But ideally, I'd keep it in the Account objects and just add a method that extracts the minimal value of the account when it's needed.

Answer 2

Here is another way using your initial approach.
Modify the way you import your data, so you can easily handle it in python.
import csv
mylist = []
with open(file_name, 'rU') as f: #opens PW file
data = csv.reader(f, delimiter = '\t')
for row in data:
splitted = row[0].split(',')
# this is in case you need integers
splitted = [int(i) for i in splitted]
mylist += [splitted]
Then, add the fourth column
updated = []
for acc in set(zip(*mylist)[0]):
acclist = [x for x in mylist if x[0] == acc]
m = min(i for sublist in acclist for i in sublist)
[l.append(m) for l in acclist]
updated += acclist