I'm assuming you are referring to
pandas.DataFrame.isnull(). Not to confuse with
pandas.isnull(), which in contrast to the two above isn't a method of the DataFrame class.
These two DataFrame methods do exactly the same thing! Even their docs are identical. You can even confirm this in pandas' code.
But why have two methods with different names do the same thing?
This is because pandas' DataFrames are based on R's DataFrames. In R
null are two separate things. Read this post for more information.
However, in python, pandas is built on top of numpy, which has neither
null values. Instead numpy has
NaN values (which stands for "Not a Number"). Consequently, pandas also uses
NaN values numpy uses
NaN values pandas uses either
NaN values are inherited from the fact that pandas is built on top of numpy, while the two functions' names originate from R's DataFrames, whose structure and functionality pandas tried to mimic.