How to create this year_month sales and previous year_month sales in two different columns? - python-3.x

Question

I need to create two different columns, one for this year sales and one column for last year sales from a transactional level data?
Data format:-
Date | bill amount
2019-07-22 | 500
2019-07-25 | 200
2020-11-15 | 100
2020-11-06 | 900
2020-12-09 | 50
2020-12-21 | 600
Required format:-
Year_month |This month Sales | Prev month sales
2019_07 | 700 | -
2020_11 | 1000 | -
2020_12 | 650 | 1000

Answer 1

The relatively tricky bit is to figure out what the previous month is. We do it by figuring out the beginning of the month for each date and then rolling back by 1 month. Note that this will take care of January -> December of previous year issues
We start by creating a sample dataframe and importing some useful modules
from io import StringIO
from datetime import datetime,timedelta
from dateutil.relativedelta import relativedelta
data = StringIO(
"""
date|amount
2019-07-22|500
2019-07-25|200
2020-11-15|100
2020-11-06|900
2020-12-09|50
2020-12-21|600
""")
df = pd.read_csv(data,sep='|')
df['date'] = pd.to_datetime(df['date'])
df
we get
date amount
0 2019-07-22 500
1 2019-07-25 200
2 2020-11-15 100
3 2020-11-06 900
4 2020-12-09 50
5 2020-12-21 600
Then we figure out the month start and the previous month start using datetime utilities
df['month_start'] = df['date'].apply(lambda d:datetime(year = d.year, month = d.month, day = 1))
df['prev_month_start'] = df['month_start'].apply(lambda d:d+relativedelta(months = -1))
Then we summarize monthly sales using groupby on month start
ms_df = df.drop(columns = 'date').groupby('month_start').agg({'prev_month_start':'first','amount':sum}).reset_index()
ms_df
so we get
month_start prev_month_start amount
0 2019-07-01 2019-06-01 700
1 2020-11-01 2020-10-01 1000
2 2020-12-01 2020-11-01 650
Then we join (merge) ms_df on itself by mapping 'prev_month_start' to 'month_start'
ms_df2 = ms_df.merge(ms_df, left_on='prev_month_start', right_on='month_start', how = 'left', suffixes = ('','_prev'))
We are more or less there but now make it pretty by getting rid of superfluous columns, adding labels, etc
ms_df2['label'] = ms_df2['month_start'].dt.strftime('%Y_%m')
ms_df2 = ms_df2.drop(columns = ['month_start','prev_month_start','month_start_prev','prev_month_start_prev'])
columns = ['label','amount','amount_prev']
ms_df2 = ms_df2[columns]
and we get
| | label | amount | amount_prev |
|---:|--------:|---------:|--------------:|
| 0 | 2019_07 | 700 | nan |
| 1 | 2020_11 | 1000 | nan |
| 2 | 2020_12 | 650 | 1000 |

Answer 2

Using #piterbarg's data, we can use resample, combined with shift and concat to get your desired data:
import pandas as pd
from io import StringIO
data = StringIO(
"""
date|amount
2019-07-22|500
2019-07-25|200
2020-11-15|100
2020-11-06|900
2020-12-09|50
2020-12-21|600
"""
)
df = pd.read_csv(data, sep="|", parse_dates=["date"])
df
date amount
0 2019-07-22 500
1 2019-07-25 200
2 2020-11-15 100
3 2020-11-06 900
4 2020-12-09 50
5 2020-12-21 600
Get the sum for current sales:
data = df.resample(on="date", rule="1M").amount.sum().rename("This_month")
data
date
2019-07-31 700
2019-08-31 0
2019-09-30 0
2019-10-31 0
2019-11-30 0
2019-12-31 0
2020-01-31 0
2020-02-29 0
2020-03-31 0
2020-04-30 0
2020-05-31 0
2020-06-30 0
2020-07-31 0
2020-08-31 0
2020-09-30 0
2020-10-31 0
2020-11-30 1000
2020-12-31 650
Freq: M, Name: This_month, dtype: int64
Now, we can shift the month to get values for previous month, and drop rows that have 0 as total sales to get your final output:
(pd.concat([data, data.shift().rename("previous_month")], axis=1)
.query("This_month!=0")
.fillna(0))
This_month previous_month
date
2019-07-31 700 0.0
2020-11-30 1000 0.0
2020-12-31 650 1000.0