How can I find the first time of each day in a list of dicts containing timestamps?

I have a list of dicts like this:

data = [
    {"Zeit": datetime(2024, 2, 27, 8, 0), "km": 10},
    {"Zeit": datetime(2024, 2, 27, 13, 30), "km": 20},
    {"Zeit": datetime(2024, 2, 27, 17, 30), "km": 40},
    {"Zeit": datetime(2024, 2, 28, 9, 15), "km": 15},
    {"Zeit": datetime(2024, 2, 28, 14, 45), "km": 25}
]

Now I want to find the first time of each day and assign km = 0. Which should lead to this:

data = [
    {"Zeit": datetime(2024, 2, 27, 8, 0), "km": 0},
    {"Zeit": datetime(2024, 2, 27, 13, 30), "km": 20},
    {"Zeit": datetime(2024, 2, 27, 17, 30), "km": 40},
    {"Zeit": datetime(2024, 2, 28, 9, 15), "km": 0},
    {"Zeit": datetime(2024, 2, 28, 14, 45), "km": 25}
]

How can I do this?

So far I did not even find a good starting point to approach this issue.

>Solution :

Try:

from datetime import datetime
from itertools import groupby

data = [
    {"Zeit": datetime(2024, 2, 27, 8, 0), "km": 10},
    {"Zeit": datetime(2024, 2, 27, 13, 30), "km": 20},
    {"Zeit": datetime(2024, 2, 27, 17, 30), "km": 40},
    {"Zeit": datetime(2024, 2, 28, 9, 15), "km": 15},
    {"Zeit": datetime(2024, 2, 28, 14, 45), "km": 25},
]

# sort if needed:
# data.sort(key=lambda d: d["Zeit"])

for _, g in groupby(data, lambda d: (d["Zeit"].year, d["Zeit"].month, d["Zeit"].day)):
    first = next(g)
    first["km"] = 0

print(data)

Prints:

[
    {"Zeit": datetime.datetime(2024, 2, 27, 8, 0), "km": 0},
    {"Zeit": datetime.datetime(2024, 2, 27, 13, 30), "km": 20},
    {"Zeit": datetime.datetime(2024, 2, 27, 17, 30), "km": 40},
    {"Zeit": datetime.datetime(2024, 2, 28, 9, 15), "km": 0},
    {"Zeit": datetime.datetime(2024, 2, 28, 14, 45), "km": 25},
]