difference between "function()" and "function"
len(s) will return the lenght of the s variable
len will return the function itslelf. So if I do a=len, then I can do a(s). Of course, it is not recommended to do such thing as a=len.
I see

df["col2"] = df["col1"].apply(len)
len(df["col1"])

My question is,

Why use "len" function without parenthesis in 1, but use it with parenthesis in 2?

What is the difference between the two?


I see this kind of occasion a lot, where using a function with and without parenthesis.
Can someone explain to me what exactly is going on?
Thanks.