How do I find missing dates in a list of sorted dates?

using sets

>>> from datetime import date, timedelta
>>> d = [date(2010, 2, 23), date(2010, 2, 24), date(2010, 2, 25),
         date(2010, 2, 26), date(2010, 3, 1), date(2010, 3, 2)]
>>> date_set = set(d[0] + timedelta(x) for x in range((d[-1] - d[0]).days))
>>> missing = sorted(date_set - set(d))
>>> missing
[datetime.date(2010, 2, 27), datetime.date(2010, 2, 28)]
>>> 

Sort the list of dates and iterate over it, remembering the previous entry. If the difference between the previous and current entry is more than one day, you have missing days.

Here's one way to implement it:

from datetime import date, timedelta
from itertools import tee, izip

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    b.next()
    return izip(a, b)

def missing_dates(dates):
    for prev, curr in pairwise(sorted(dates)):
        i = prev
        while i + timedelta(1) < curr:
            i += timedelta(1)
            yield i

dates = [ date(2010, 1, 8),
          date(2010, 1, 2),
          date(2010, 1, 5),
          date(2010, 1, 1),
          date(2010, 1, 7) ]

for missing in missing_dates(dates):
    print missing

Output:

2010-01-03
2010-01-04
2010-01-06

Performance is O(n*log(n)) where n is the number of days in the span when the input is unsorted. As your list is already sorted, it will run in O(n).

>>> from datetime import datetime, timedelta
>>> date_list = [datetime(2010, 2, 23),datetime(2010, 2, 24),datetime(2010, 2, 25),datetime(2010, 2, 26),datetime(2010, 3, 1),datetime(2010, 3, 2)]
>>> 
>>> date_set=set(date_list)         # for faster membership tests than list
>>> one_day = timedelta(days=1)
>>> 
>>> test_date = date_list[0]
>>> missing_dates=[]
>>> while test_date < date_list[-1]:
...     if test_date not in date_set:
...         missing_dates.append(test_date)
...     test_date += one_day
... 
>>> print missing_dates
[datetime.datetime(2010, 2, 27, 0, 0), datetime.datetime(2010, 2, 28, 0, 0)]

This also works for datetime.date objects, but the OP says the list is datetime.datetime objects

USING A FOR LOOP

The imports you'll need:

import datetime
from datetime import date, timedelta

Let's say you have a sorted list called dates with several missing dates in it.

First select the first and last date:

start_date = dates[0]
end_date = dates[len(dates)-1]

Than count the number of days between these two dates:

numdays = (end_date - start_date).days

Than create a new list with all dates between start_date and end_date:

all_dates = []
for x in range (0, (numdays+1)):
all_dates.append(start_date + datetime.timedelta(days = x))

Than check with dates are in all_dates but not in dates by using a for loop with range and adding these dates to dates_missing:

dates_missing = []

for i in range (0, len(all_dates)):
   if (all_dates[i] not in dates):
       dates_missing.append(all_dates[i])
   else:
       pass

Now you'll have a list called dates_missing with all the missing dates.

Put the dates in a set and then iterate from the first date to the last using datetime.timedelta(), checking for containment in the set each time.

Here's an example for a pandas dataframe with a date column. If it's an index, then change df.Date to df.index.

import pandas as pd

df.Date = pd.to_datetime(df.Date) # ensure datetime format of date column
min_dt = df.Date.min() # get lowest date
max_dt = df.Date.max() # get highest date

dt_range = pd.date_range(min_dt, max_dt) # get all requisite dates in range
missing_dts = [d for d in dt_range if d not in df.Date] # list missing
print("There are {n} missing dates".format(n=len(missing_dts)))

import datetime

DAY = datetime.timedelta(days=1)
# missing dates: a list of [start_date, end)
missing = [(d1+DAY, d2) for d1, d2 in zip(dates, dates[1:]) if (d2 - d1) > DAY]

def date_range(start_date, end, step=DAY):
    d = start_date
    while d < end:
        yield d
        d += step

missing_dates = [d for d1, d2 in missing for d in date_range(d1, d2)]

Using a list comprehension

>>> from datetime import date, timedelta
>>> d = [date(2010, 2, 23),date(2010, 2, 24),date(2010, 2, 25),date(2010, 2, 26),date(2010, 3, 1),date(2010, 3, 2)]
>>> date_set=set(d)
>>> missing = [x for x in (d[0]+timedelta(x) for x in range((d[-1]-d[0]).days)) if x not in date_set]

>>> missing
[datetime.date(2010, 2, 27), datetime.date(2010, 2, 28)]