Making a list based off of list of months in Python

Question

I am using Python to make lists. Should be easy! I don't know why I'm struggling so much with this.

I have some data that I am counting up by date. There is a date column like this:

Created on
5/1/2015
5/1/2015
6/1/2015
6/1/2015
7/1/2015
8/1/2015
8/1/2015
8/1/2015

In this case, there would be 2 Units created in May, 2 Units in June, 1 Unit in July, and 3 Units in August.

I want to reflect that in a list that starts in April ([April counts, May counts, June counts, etc...]):

NumberofUnits = [0, 2, 3, 1, 3, 0, 0, 0, 0, 0, 0, 0]  

I have a nice list of months

monthnumbers

Out[69]: [8, 5, 6, 7]

I also have a list with the unitcounts = [2, 3, 1, 3] I got this using value_counts.

So it's a matter of making a list of zeroes and replacing parts with the unitcount list, right?

For some reason all of my tries are either not making a list or making a list with one zero in it.

NumberofUnits = [0]*12

for i in range(0,len(monthnumbers)):
    if  **monthnumbers[i] == (i+4):** **This part is wrong**       
        NumberofUnits.append(unitcounts[i])
        s = slice(0,i+1)

I also tried

NumberofUnits = []
for i in range(0, 12):
    if len(NumberofUnits) > i:
        unitcounts[i:]+unitcounts[:i]
        NumberofUnits.append(unitcounts[i])
        s = slice(0,i+1)
    else:
        unitcounts.append(0)

But this doesn't account for the fact that in this round my data starts with May, so I need a zero in the first slot.

Answer 1

If the data is coming from a file or any iterable you can use an OrderedDict, creating the keys in order starting from 4/april, then increment the count for each month you encounter the finally print the list of values at the end which will be in the required order:

from collections import OrderedDict

od = OrderedDict((i % 12 or 12, 0) for i in range(4, 16))
# -> OrderedDict([(4, 0), (5, 0), (6, 0), (7, 0), (8, 0), (9, 0), (10, 0), (11, 0), (12, 0), (1, 0), (2, 0), (3, 0)])

with open("in.txt") as f:
    for line in f:
        mn = int(line.split("/",1)[0])
        od.setdefault(mn, 0)
        od[mn] += 1

print(list(od.values()))
[0, 2, 2, 1, 3, 0, 0, 0, 0, 0, 0, 0]

Unless you do the logic like above, associating the data when you actually parse it then it is going to be a lot harder figure out what count is for which month. Creating the association straight away is a much simpler approach.

If you have a list, tuple etc.. of values the logic is exactly the same:

 for dte in list_of_dates:
        mn = int(dte.split("/",1)[0])
        od.setdefault(mn, 0)
        od[mn] += 1

Answer 2

You can count entries using collections.counter

from collections import Counter

lines = ['5/1/2015', '5/1/2015', ..., '8/1/2015']
month_numbers = [int(line.split("/")[0]) for line in lines]

cnt = Counter(month_numbers)

If you already have counts you can replace above with

from collections import defaultdict

cnt = defaultdict(int, zip(monthnumbers, unitcounts))

and simply map to entries with (month_number - offset) mod 12:

[x[1] for x in sorted([((i - offset) % 12, cnt[i]) for i in range(1, 13)])]