List of dates with special formatting

Question

I have a code that calls many times a function returning a list of the dates between two dates formatted as

('0001-01-01 00:01:00'),('0001-01-02 00:01:00'), ...

The current solution is:

import numpy as np
import time
from datetime import datetime


beg = datetime.strptime('01 01 0001', '%d %M %Y') #datetime
end = datetime.strptime('01 01 2001', '%d %M %Y')


def get_days(date1, date2):
    day_diff = (date1 - date2).days + 1
    days = [str(start_date + dt.timedelta(d)) for d in range(day_diff)]
    dates = "('" + "'),('".join(days) + "')"
    return dates

Is there a faster way to achieve this?

 #timing
 t0 = time.time()

dates_list = get_days(beg, end) #feed datetime

t1 = time.time()
total_create = t1-t0
print("list comprehension: ", total_create,'s')

Answer 1

With the list comprehension you are filling a whole list with your values, and then you are sending that list to join.

Instead of a generating a list and then sending it, you can send a generator instead: similar to the list comprehension, but generates the values on-demand. With your old approach, if you had 10000 dates you would have them all at a list; with a generator it generates one at a time, so at least you will be consuming less memory.

With a generator, you would directly do:

dates = "('" + "'),('".join(str(start_date + dt.timedelta(d)) for d in range(day_diff)) + "')"

On a side note, the parameter names date1, date2 are not very explicit; it should be clear from the names which is the start and which is the end date.