Create a row for each year between two dates - pandas

Question

I have a dataframe with two date columns (format: YYYY-MM-DD). I want to create one row for each year between those two dates. The rows would be identical with a new column which specifies the year. For example, if the dates are 2018-01-01 and 2020-01-01 then there would be three rows with same data and a new column with values 2018, 2019, and 2020.

Answer 1

You can use a custom function to compute the range then explode the column:
# Ensure to have datetime
df['date1'] = pd.to_datetime(df['date1'])
df['date2'] = pd.to_datetime(df['date2'])
# Create the new column
date_range = lambda x: range(x['date1'].year, x['date2'].year+1)
df = df.assign(year=df.apply(date_range, axis=1)).explode('year', ignore_index=True)
Output:
>>> df
date1 date2 year
0 2018-01-01 2020-01-01 2018
1 2018-01-01 2020-01-01 2019
2 2018-01-01 2020-01-01 2020

Answer 2

This should work for you:
import pandas
# some sample data
df = pandas.DataFrame(data={
'foo': ['bar', 'baz'],
'date1':['2018-01-01', '2022-01-01'],
'date2':['2020-01-01', '2017-01-01']
})
# cast date columns to datetime
for col in ['date1', 'date2']:
df[col] = pandas.to_datetime(df[col])
# reset index to ensure that selection by length of index works
df = df.reset_index(drop=True)
# the range of years between the two dates, and iterate through the resulting
# series to unpack the range of years and add a new row with the original data and the year
for i, years in df.apply(
lambda x: range(
min(x.date1, x.date2).year,
max(x.date1, x.date2).year + 1
),
axis='columns'
).iteritems():
for year in years:
new_index = len(df.index)
df.loc[new_index] = df.loc[i].values
df.loc[new_index, 'year'] = int(year)
output:
>>> df
foo date1 date2 year
0 bar 2018-01-01 2020-01-01 NaN
1 baz 2022-01-01 2017-01-01 NaN
2 bar 2018-01-01 2020-01-01 2018.0
3 bar 2018-01-01 2020-01-01 2019.0
4 bar 2018-01-01 2020-01-01 2020.0
5 baz 2022-01-01 2017-01-01 2017.0
6 baz 2022-01-01 2017-01-01 2018.0
7 baz 2022-01-01 2017-01-01 2019.0
8 baz 2022-01-01 2017-01-01 2020.0
9 baz 2022-01-01 2017-01-01 2021.0
10 baz 2022-01-01 2017-01-01 2022.0