Parenthesis on .upper() and .apply(str.upper)

Question

Why do upper in df.apply(df.str.upper) doesn't require a parenthesis, but upper() method requires them as in df.str.upper()?

Is there some concept I've miss?

Answer 1

The () means "call this function now".

print(str.upper())

Referring to a function without the () does not call the function immediately.

map(str.upper)

The str.upper function is passed to the map function. The map function can now decide what it does with the str.upper function. It might call it once, or multiple times, or store it somewhere for later use.

Answer 2

When you use .apply(function_name), the internals treats function_name as a reference to the function that has been applied. Hence, only the function name is passed and that too without parentheses.
When it is applied to the entity, it will be accessed from the function_name.
Its just like passing a function as an argument to another function.

Example:

def fun(x):
   return x*x

def fun_b(function):
   return function(3)

print(fun_b(fun))