CODEWITHSUNDEEP
pythondictionary
JuanD
JuanD

Reputation: 35

Dictionary inside function not updating (Python)

I'm new in stackoverflow and also in Python. I been trying for a few days to get this to work but I don't quite get what I'm doing wrong, also I have searched a lot for a common problem but with no luck.

The following code reads some sales data from a csv file, then returns a dictionary with two keys (a unique code and the year value) and the sum of the sales of an specific month. I pass two arguments to the function, the code and the month. That part is doing what I want, the problem comes when I want to iterate trough different months, no matter what value (month) I assign to the function it will always return me the first value analyzed

d = defaultdict(int)
spamReader = csv.reader(open('C:\Sales\Sales_CO05.csv', 'rU'))

#Return the sales sum for a given month and code
def sum_data_by_month(ean, m):
    for line in spamReader:
        tokens = [t for t in line]
        if tokens[5] == str(ean):
            try:
                sid = tokens[5]
                dusid = tokens[18]
                value = int(str(tokens[m]).replace(',',''))
            except ValueError:
                continue
            d[sid,dusid] += value
    return d

#Try to iterate over different month values
j = sum_data_by_month('7702010381089', 6)
f = sum_data_by_month('7702010381089', 7)
m = sum_data_by_month('7702010381089', 8)
a = sum_data_by_month('7702010381089', 9)

This is the result that I'm getting:

defaultdict(<type 'int'>, {('7702010381089', '2013'): 80, ('7702010381089', '2014'): 363})
defaultdict(<type 'int'>, {('7702010381089', '2013'): 80, ('7702010381089', '2014'): 363})
defaultdict(<type 'int'>, {('7702010381089', '2013'): 80, ('7702010381089', '2014'): 363})
defaultdict(<type 'int'>, {('7702010381089', '2013'): 80, ('7702010381089', '2014'): 363})

And this is what I’m expecting:

defaultdict(<type 'int'>, {('7702010381089', '2013'): 80, ('7702010381089', '2014'): 363})
defaultdict(<type 'int'>, {('7702010381089', '2013'): 229, ('7702010381089', '2014'): 299})
etc..

It seems like if the dict is stuck in some kind of memory state that's is not allowing it to get updated, if I run a single instance of the function (i.e j = sum_data_by_month('7702010381089', 8)I I get de desired value.

Any help will be really apreciated.

Thanks!

Upvotes: 2

Views: 207

Answers (1)

Joran Beasley
Joran Beasley

Reputation: 114098

dictionaries are mutable

j = sum_data_by_month('7702010381089', 6) 
j is d # true

does not create a new dictionary ... it just points to the existing dictionary

f = sum_data_by_month('7702010381089', 7) #the dictionary has changed
f is j # true  , both point to the same dictionary
f is d # true  , both point to d to be specific

you can fix it by

from copy import deepcopy

...
def sum_data_by_month(ean, m):
    ...
    return deepcopy(d) # a new dict no longer just a pointer to the same d
    #or maybe even better
    return dict(d)

now 

j = sum_data_by_month('7702010381089', 6) 
j is d # false

Upvotes: 1

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