Pandas - Dynamically passing variable to a column name - pandas

Question

I am trying to have one of the columns in the Dataframe to be assigned value of a variable.
For example:
I have a column called : Count of sales done in the past 10 days
I am trying to have the value of 10 in the column name changed dynamically from another variable called date
So each time I update the Dataframe I want the value assigned to the variable date be shown in the column name
For example if date = 4, column name should be Count of sales done in the past 4 days and so on.
I tried to pass df.columns = date but get a KeyError

Answer 1

One idea with position of column is use rename:
df = pd.DataFrame({
'A':list('abcdef'),
'Count of sales done in the past 10 days':[4,5,4,5,5,4],
'C':[7,8,9,4,2,3],
})
date = 4
pos = 1
df = df.rename(columns={df.columns[pos]: f'Count of sales done in the past {date} days'})
print (df)
A Count of sales done in the past 4 days C
0 a 4 7
1 b 5 8
2 c 4 9
3 d 5 4
4 e 5 2
5 f 4 3
Another idea is get column by mask and set new values by Index.where, but if mask return more columns names it rewrite all of them:
m = df.columns.str.startswith('Count of sales done in the past')
df.columns = df.columns.where(~m, f'Count of sales done in the past {date} days')