[Tutor] What's the difference between calling a method with parenthesis vs. without it?

[C W]

Dear Python experts,

I never figured out when to call a method with parenthesis, when is it not?
It seems inconsistent.

For example,
If I do

> data.isnull()

numberair_pressure_9amair_temp_9amavg_wind_direction_9amavg_wind_speed_9am
max_wind_direction_9ammax_wind_speed_9amrain_accumulation_9am
rain_duration_9amrelative_humidity_9amrelative_humidity_3pm
0 False False False False False False False False False False False
1 False False False False False False False False False False False
2 False False False False False False False False False False False

Or I can do without parenthesis,
> data.isnull

<bound method DataFrame.isnull of       number  air_pressure_9am
air_temp_9am  avg_wind_direction_9am  \
0          0        918.060000     74.822000              271.100000
1          1        917.347688     71.403843              101.935179
2          2        923.040000     60.638000               51.000000


Obviously, the second case does not make any senses. But, in data.columns,
it is only correct to do without parenthesis as below:

> data.columns

Index(['number', 'air_pressure_9am', 'air_temp_9am', 'avg_wind_direction_9am',
       'avg_wind_speed_9am', 'max_wind_direction_9am', 'max_wind_speed_9am',
       'rain_accumulation_9am', 'rain_duration_9am', 'relative_humidity_9am',
       'relative_humidity_3pm'],
      dtype='object')


# with parenthesis throws an error
> data.columns()

---------------------------------------------------------------------------TypeError
                                Traceback (most recent call
last)<ipython-input-16-05cf52d0a56e> in <module>()----> 1
data.columns()
TypeError: 'Index' object is not callable


I always thought columns(), isnull() are methods to objects, so, you would
need to pass parameters even when it's empty. But, apparently I am wrong on
this.

What is going on?

Thank you very much!