Unchanging Variable in list comprehension

I have a list like this:

dates = [
    datetime.date(2014, 11, 24),
    datetime.date(2014, 11, 25),
    datetime.date(2014, 11, 26),
    # datetime.date(2014, 11, 27), # This one is missing
    datetime.date(2014, 11, 28),
    datetime.date(2014, 11, 29),
    datetime.date(2014, 11, 30),
    datetime.date(2014, 12, 1)]

I'm trying to find the missing dates between the start and end date, with this expr:

date_set = {dates[0] + timedelta(x) for x in range((dates[-1] - dates[0]).days)}

Strangely enough, it throws an error - it can't access the dates variable. But this expression runs fine:

date_set = {date(2015,2,11) + timedelta(x) for x in range((dates[-1] - dates[0]).days)}

I wrote an expression that does what I wanted:

def find_missing_dates(dates: list[date]) -> list[date]:
    """Find the missing dates in a list of dates (that should already be sorted)."""
    date_set = {(first_date + timedelta(x)) for first_date, x in zip([dates[0]] * len(dates), range((dates[-1] - dates[0]).days))}
    missing = sorted(date_set - set(dates))

    return missing

It's an ugly expression and forced me to fill a second list with the same variable. Does anyone have a cleaner expression?

If your dates is sorted, you just need to iterate over it and add dates between into new list. Possible one-line solution I've already provided in this comment.

from datetime import date, timedelta

dates = [
    date(2014, 11, 24), date(2014, 11, 25), date(2014, 11, 26),
    date(2014, 11, 28), date(2014, 11, 29), date(2014, 11, 30),
    date(2014, 12, 1)
]
missing = [d + timedelta(days=j) for i, d in enumerate(dates[:-1], 1) for j in range(1, (dates[i] - d).days)]

You can do it using regular for loops:

from datetime import date, timedelta

dates = [
    date(2014, 11, 24), date(2014, 11, 25), date(2014, 11, 26),
    date(2014, 11, 28), date(2014, 11, 29), date(2014, 11, 30),
    date(2014, 12, 1)
]

missing = []
for next_index, current_date in enumerate(dates[:-1], 1):
    for days_diff in range(1, (dates[next_index] - current_date).days):
        missing.append(current_date + timedelta(days=days_diff))