Initialize lists with values, ranges & dates

Initialize list with values

In order to initialize a list with a specific value, all you need to do is to use a list comprehension and range() to generate a list of length equal to n, filled with the desired values.

def initialize_list_with_values(n, val = 0):
  return [val for x in range(n)]

initialize_list_with_values(5, 2) # [2, 2, 2, 2, 2]

Initialize list with numeric range

For a numeric range between start and end (inclusive), you can use a list comprehension and range() to generate a list of the appropriate length, filled with the desired values in the given range.

def initialize_list_with_range(end, start = 0, step = 1):
  return list(range(start, end + 1, step))

initialize_list_with_range(5) # [0, 1, 2, 3, 4, 5]
initialize_list_with_range(7, 3) # [3, 4, 5, 6, 7]
initialize_list_with_range(9, 0, 2) # [0, 2, 4, 6, 8]

Initialize list with date range

Similarly, for a list with dates between start (inclusive) and end (not inclusive), you can use a list comprehension and datetime.timedelta to generate a list of datetime.date objects.

from datetime import timedelta, date

def daterange(start, end):
  return [start + timedelta(n) for n in range(int((end - start).days))]

daterange(date(2020, 10, 1), date(2020, 10, 5))
# [date(2020, 10, 1), date(2020, 10, 2), date(2020, 10, 3), date(2020, 10, 4)]

Initialize 2D list

Finally, for a 2D list, you'll need to use a nested list comprehension to generate a list of lists, where each inner list is initialized with the desired value.

def initialize_2d_list(w, h, val = None):
  return [[val for x in range(w)] for y in range(h)]

initialize_2d_list(2, 2, 0) # [[0, 0], [0, 0]]