How to interact with the previous variable in a for loop in Python?

I need a little help with a for loop in Python. At the moment, I am trying to create a random shift generator, but I also need to apply some rules on it. Right now, my function add shifts to a calendar randomly, but what I want is to add day off if the last shift was an evening shift. But I am not really sure how to interact with the previous ‘i’ variable in a for loop. Here is my code:

# Shift types
shifts = {
    "morning": "9:00 - 17:00",
    "evening": "16:00 - 00:00",
}

day_off = {
    "day off": "X"
}


# Schedule
schedule_ll = {
    1: [],
    2: [],
    3: [],
    4: [],
    5: [],
    6: [],
    7: [],
    8: [],
    9: [],
    10: [],
    11: [],
    12: [],
    13: [],
    14: [],
    15: [],
    16: [],
    17: [],
    18: [],
    19: [],
    20: [],
    21: [],
    22: [],
    23: [],
    24: [],
    25: [],
    27: [],
    28: [],
    29: [],
    30: [],
    31: [],
}

# Function to add shifts to schedule randomly
def start_schedule(schedule, shift, day_off):
    for i in schedule:
        schedule[i].append(random.choice(list(shift.values())))
        if schedule[i-1] == ["16:00 - 00:00"]:
            schedule[i] = list(day_off.values())
    return schedule

print(start_schedule(schedule_ll, shifts, day_off))

So when I execute this code, I receive KeyError: 0 error. I know that this error caused due to schedule[i-1], however I am not really sure how to make it work in other way. Will appreciate any help!

>Solution :

Two recommendations:

1. Iterate over the dictionary using .items()
2. Add logic guarding against trying to access a key in schedule that does not exist prior to calling schedule[i - 1]

This makes the code a bit cleaner, prevents the function from crashing on a key error, and checks that the desired condition holds for every day except the first day.

def start_schedule(schedule, shift, day_off):
    for day, list_of_shifts in schedule.items():
        list_of_shifts.append(random.choice(list(shift.values())))
        if (i - 1) in schedule and schedule[i-1] == ["16:00 - 00:00"]:
            schedule[i] = list(day_off.values())
    return schedule