Writing the mean of values to a new column in Pandas dataframe

Question

I need to create a new column in my df that holds the mean of another existing column, but I need it to take into account each individual location over time rather then the mean of the all the values in the existing column.

Based on the sample dataset below, what I am looking for is a new column that contains the Mean for each Site, not the mean of all the values independent of Site.

Sample Dataset

Answer 1

Use groupby :

df.groupby('Site')['TIME_HOUR'].mean().reset_index()

And assign to a column

Answer 2

使用该列的 groupby 和 agg 平均值：

df = df.merge(df.groupby('Site',as_index=False).agg({'TIME_HOUR':'mean'})[['Site','TIME_HOUR']],on='Site',how='left')