How to detect if dates are consecutive in Python?

Question

I have an access table with a 'Date' field. it has random dates for each record. I've built a script to append all the records into a list and then set the list to filter out only the unique values:

dateList = []
# cursor search through each record and append all records in the date 
# field to a python list
for row in rows:
   dateList.append(row.getValue("DATE_OBSERVATION").strftime('%m-%d-%Y'))

# Filter unique values to a set
newList = list(set(dateList))

This returns (on my test table):

['07-06-2010', '06-24-2010', '07-05-2010', '06-25-2010']

Now that I have the unique values for the "DATE_OBSERVATION" field, I want to detect if:

• the dates are single (ie only one unique date is returned because that is the date in every record)
• if the dates are a range of dates (ie all of the dates fall into a consecutive range)
• if the dates are multiple dates, but are not in a range of consecutive dates

Any suggestions would be much appreciated! Mike

Answer 1

Rather than rolling your own consecutive function you can simply convert date objects to integers using the .toordinal() method of datetime objects. The difference between the maximum and minimum value of the set of ordinal dates is one more than the length of the set:

from datetime import datetime

date_strs = ['07-06-2010', '06-24-2010', '07-05-2010', '06-25-2010']
# date_strs = ['02-29-2012', '02-28-2012', '03-01-2012']
# date_strs = ['01-01-2000']
dates = [datetime.strptime(d, "%m-%d-%Y") for d in date_strs]

date_ints = set([d.toordinal() for d in dates])

if len(date_ints) == 1:
    print "unique"
elif max(date_ints) - min(date_ints) == len(date_ints) - 1:
    print "consecutive"
else:
    print "not consecutive"

Answer 2

Another version using the same logic as in my other answer.

from datetime import date, timedelta

# Definition 1: 1/1/14, 1/2/14, 1/2/14, 1/3/14 is consider consecutive
# Definition 2: 1/1/14, 1/2/14, 1/2/14, 1/3/14 is consider not consecutive

# datelist = [date(2014, 1, 1), date(2014, 1, 3),
#             date(2013, 12, 31), date(2013, 12, 30)]

# datelist = [date(2014, 2, 19), date(2014, 2, 19), date(2014, 2, 20),
#             date(2014, 2, 21), date(2014, 2, 22)]

datelist = [date(2014, 2, 19), date(2014, 2, 21),
            date(2014, 2, 22), date(2014, 2, 20)]

datelist.sort()

previousdate = datelist[0]

for i in range(1, len(datelist)):
    #if (datelist[i] - previousdate).days == 1 or (datelist[i] - previousdate).days == 0:  # for Definition 1
    if (datelist[i] - previousdate).days == 1:    # for Definition 2
        previousdate = datelist[i]
    else:
        previousdate = previousdate + timedelta(days=-1)

if datelist[-1] == previousdate:
    print "dates are consecutive"
else:
    print "dates are not consecutive"

Answer 3

Here's my version using the reduce() function.

from datetime import date, timedelta


def checked(d1, d2):
    """
    We assume the date list is sorted.
    If d2 & d1 are different by 1, everything up to d2 is consecutive, so d2
    can advance to the next reduction.
    If d2 & d1 are not different by 1, returning d1 - 1 for the next reduction
    will guarantee the result produced by reduce() to be something other than
    the last date in the sorted date list.

    Definition 1: 1/1/14, 1/2/14, 1/2/14, 1/3/14 is consider consecutive
    Definition 2: 1/1/14, 1/2/14, 1/2/14, 1/3/14 is consider not consecutive

    """
    #if (d2 - d1).days == 1 or (d2 - d1).days == 0:  # for Definition 1
    if (d2 - d1).days == 1:                          # for Definition 2
        return d2
    else:
        return d1 + timedelta(days=-1)

# datelist = [date(2014, 1, 1), date(2014, 1, 3),
#             date(2013, 12, 31), date(2013, 12, 30)]

# datelist = [date(2014, 2, 19), date(2014, 2, 19), date(2014, 2, 20),
#             date(2014, 2, 21), date(2014, 2, 22)]

datelist = [date(2014, 2, 19), date(2014, 2, 21),
            date(2014, 2, 22), date(2014, 2, 20)]

datelist.sort()

if datelist[-1] == reduce(checked, datelist):
    print "dates are consecutive"
else:
    print "dates are not consecutive"

Answer 4

Use your database to select unique dates in the ascending order:

• if the query returns a single date it is your first case

• otherwise find out whether the dates are consecutive:

   import datetime def consecutive(a, b, step=datetime.timedelta(days=1)): return (a + step) == b 

Code layout:

dates = <query database>
if all(consecutive(dates[i], dates[i+1]) for i in xrange(len(dates) - 1)):
   if len(dates) == 1: # unique
      # 1st case: all records have the same date
   else:
      # the dates are a range of dates
else:
   # non-consecutive dates