How to do a vector of dates in python? [duplicate] - python-3.x

Question

I'm trying to generate a date range of monthly data where the day is always at the beginning of the month:
pd.date_range(start='1/1/1980', end='11/1/1991', freq='M')
This generates 1/31/1980, 2/29/1980, and so on. Instead, I just want 1/1/1980, 2/1/1980,...
I've seen other question ask about generating data that is always on a specific day of the month, with answers saying it wasn't possible, but beginning of month surely must be possible!

Answer 1

You can do this by changing the freq argument from 'M' to 'MS':
d = pandas.date_range(start='1/1/1980', end='11/1/1990', freq='MS')
print(d)
This should now print:
DatetimeIndex(['1980-01-01', '1980-02-01', '1980-03-01', '1980-04-01',
'1980-05-01', '1980-06-01', '1980-07-01', '1980-08-01',
'1980-09-01', '1980-10-01',
...
'1990-02-01', '1990-03-01', '1990-04-01', '1990-05-01',
'1990-06-01', '1990-07-01', '1990-08-01', '1990-09-01',
'1990-10-01', '1990-11-01'],
dtype='datetime64[ns]', length=131, freq='MS', tz=None)
Look into the offset aliases part of the documentation. There it states that 'M' is for the end of the month (month end frequency) while 'MS' for the beginning (month start frequency).

Answer 2

It is worth noting that pandas.date_range() only includes dates within the defined interval, which may not be expected :
start = "2020-03-08"
end = "2021-03-08"
pd.date_range(start, end, freq='MS')
results in
DatetimeIndex(['2020-04-01', '2020-05-01', '2020-06-01', '2020-07-01',
'2020-08-01', '2020-09-01', '2020-10-01', '2020-11-01',
'2020-12-01', '2021-01-01', '2021-02-01', '2021-03-01'],
dtype='datetime64[ns]', freq='MS')
For MS, a workaround to include the first day of the opening month is to work only with the year and month of the start date :
pd.date_range(start[:7], end, freq='MS')
will then give
DatetimeIndex(['2020-03-01', '2020-04-01', '2020-05-01', '2020-06-01',
'2020-07-01', '2020-08-01', '2020-09-01', '2020-10-01',
'2020-11-01', '2020-12-01', '2021-01-01', '2021-02-01',
'2021-03-01'],
dtype='datetime64[ns]', freq='MS')
If you wish to keep the same starting day for each month, you can then add the offset with pd.DateOffset() :
pd.date_range(start[:7], end, freq='MS') + pd.DateOffset(days=7)
will give
DatetimeIndex(['2020-03-08', '2020-04-08', '2020-05-08', '2020-06-08',
'2020-07-08', '2020-08-08', '2020-09-08', '2020-10-08',
'2020-11-08', '2020-12-08', '2021-01-08', '2021-02-08',
'2021-03-08'],
dtype='datetime64[ns]', freq=None)
As mentioned in comments, note that trouble may come with this workaround for offsets higher or equals to 28.