The truth value of a Series is ambiguous - Error When when calling a function

I know following error

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

has been asked a long time ago.

However, I am trying to create a basic function and return a new column with df[busy'] with 1 or 0. My function looks like this:

def hour_bus(df):
    if df[(df['hour'] >= '14:00:00') & (df['hour'] <= '23:00:00')&\
             (df['week_day'] != 'Saturday') & (df['week_day'] != 'Sunday')]:
         return df['busy'] == 1
     else:
         return df['busy'] == 0  

I can execute the function, but when I call it with the DataFrame, I get the error mentioned above. I followed the following thread and another thread to create that function. I used & instead of and in my if clause.

Anyhow, when I do the following, I get my desired output:

df['busy'] = np.where((df['hour'] >= '14:00:00') & (df['hour'] <= '23:00:00') & \
                        (df['week_day'] != 'Saturday') & (df['week_day'] != 'Sunday'),'1','0')

Any ideas on what mistake am I making in my hour_bus function?