Python – If the same consecutive value, grouped by the remainder of the day to show True

Given the following DataFrame. How to add a new val column that shows True for the rest of the day when two consecutive “y" appear in the day val column (otherwise False).

• The logic is reset every day.

This is close to but True should be seen on every line of the day after the condition.

Code

df_so = pd. DataFrame(
    {
        "val": list("yynnnyyynn")
    },
    index=pd.date_range(start="1/1/2018", periods=10, freq="6h"),
)

val
2018-01-01 00:00:00 y
2018-01-01 06:00:00 y
2018-01-01 12:00:00 n
2018-01-01 18:00:00 n
2018-01-02 00:00:00 n
2018-01-02 06:00:00 y
2018-01-02 12:00:00 y
2018-01-02 18:00:00 y
2018-01-03 00:00:00 n
2018-01-03 06:00:00 n

Expected output

                    val  out
2018-01-01 00:00:00  y   False
2018-01-01 06:00:00  y   False
2018-01-01 12:00:00  n   True
2018-01-01 18:00:00  n   True
2018-01-02 00:00:00  n   False
2018-01-02 06:00:00  y   False
2018-01-02 12:00:00  y   False
2018-01-02 18:00:00  y   True
2018-01-03 00:00:00  n   False
2018-01-03 06:00:00  n   False

You can use cummax to check if the condition holds at some point in the past:

target = 2
df_so['out'] = (df_so['val'].eq('y')
                    .groupby(df_so.index.normalize())
                    .transform(lambda x: x.rolling(target).sum().shift().eq(target).cummax())
               )

Output:

                    val    out
2018-01-01 00:00:00   y  False
2018-01-01 06:00:00   y  False
2018-01-01 12:00:00   n   True
2018-01-01 18:00:00   n   True
2018-01-02 00:00:00   n  False
2018-01-02 06:00:00   y  False
2018-01-02 12:00:00   y  False
2018-01-02 18:00:00   y   True
2018-01-03 00:00:00   n  False
2018-01-03 06:00:00   n  False