python syntax if in list function

I am trying to find the proper python syntax to create a flag with a value of 'yes' if columnx contains any of the following numbers: 1, 2, 3, 4, 5.

Def create_flag(df):
    if df['columnx'] in (1,2,3,4,5)
    return df['flag']=='yes'

I get the following error.

TypeError: invalid type comparison

Is there an obvious mistake in my syntax?