I've recently begun learning Python and I wanted to write a script to extract the day of the month from a CSV column (formatted as YYYY/DD/MM) then compare website users to days of the month (and eventually weeks of the month) as a challenge/learning exercise. The gist is that it extracts the CSV info, formats it/converts to integers, and smushes it back together as a dictionary comparing days 1-31 with the number of site visitors.

My code is below. The error I'm receiving is 'KeyError: 1'on line 29 result[days] = users. I think I understand what is happening (kind of - I'm guessing it's not happy with the way I'm trying to assign values to an empty dictionary? It seems to be looking for the integer 1 as a key but not finding it?) but I can't figure out what to do next. I'm about 2 weeks into learning Python so I hope this isn't too stupid a question. What am I doing wrong? How can I make the columns at index [0] and [1] of users_by_day the key and value in my dictionary?

Note: I am learning and using Python 3.

 import csv

 result = {}

 with open('analytics.csv') as csv_file:

     csv_reader = csv.reader(csv_file, delimiter=',')

     line_count = 0

     users_by_day = list(csv_reader)

     for row in users_by_day: #iterate through data



          day = row[0].split('/') #split date to extract day of month



         try: #skip unsplit cells



             day = day[1]

         except Exception as e:

             pass

         row[0] = day #set list column to extracted day value



 users_by_day = users_by_day[1:-1] #strip headers



 for row in users_by_day:

     days = None

     users = None



         days = int(row[0]) #set values to int for math

         users = int(row[1])



     if days is not None:

         if days in result: #trying to check for days in result

             result[days] = users #where key error occurs

         else:

             result[days] += users



 print(result)

The setdefault() call on dictionaries is great for this kind of thing, and is preferable to the if {thing} in {dict} construct.

So the following code:

if days in result:  # trying to check for days in result

    result[days] = users  # where key error occurs

else:

    result[days] += users

Could become:

result.setdefault(days, 0)

result[days] += users

in the else part , if days not in result, the equation certainly would generate an error, because it using an key that dosent exit:

result[days] =result[days]+ users

but do you really mean like :

 if days is not None:

     if days not in result:     #if result doesn't have that day

         result[days] = users   #get the day and its value into result

     else:                      #if result already has the day value

         result[days] += users  #summary the value

Options aside from dicty.setdefault(key, val) and if key in dicty, include:

Try / Except

try:

    dicty[key] += value

except KeyError:

    dicty[key] = value

collections.defaultdict()

Using collections.defaultdict():

dicty = defaultdict(int)

dicty[key] += value

Last line will execute to the effect of dicty[key] = value or dicty[key] += value as appropriate (really what it does is, if the key is not found, run dicty[key] = int() before running dicty[key] += value ... you'd have to be careful using this for *= therefore).