I recently spent way too long debugging a piece of code, only to realize that the issue was I did not include a () after a command. What is the logic behind which commands require a () and which do not?

For example:

import pandas as pdcol1=['a','b','c','d','e']
col2=[1,2,3,4,5]df=pd.DataFrame(list(zip(col1,col2)),columns=['col1','col2'])df.columns

Returns Index(['col1', 'col2'], dtype='object') as expected. If we use .columns() we get an error.

Other commands it is the opposite:

df.isna()

Returns:

    col1    col2
0   False   False
1   False   False
2   False   False
3   False   False
4   False   False

but df.isna returns:

<bound method DataFrame.isna of   col1  col2
0    a     1
1    b     2
2    c     3
3    d     4
4    e     5>

Which, while not throwing an error, is clearly not what we're looking for.

What's the logic behind which commands use a () and which do not?

I use pandas as an example here, but I think this is relevant to python more generally.

Because functions need parenthesis for their arguments, while variables do not, that's why it's list.append(<item>) but it's list.items.

If you call a function without the parenthesis like list.append what returns is a description of the function, not a description of what the function does, but a description of what it is.

As for classes, a call to a class with parenthesis initiates an object of that class, while a call to a class without the parenthesis point to the class itself, which means that if you were to execute print(SomeClass) you'd get <class '__main__.SomeClass'> which is a description of what it is, the same kind of response you'd get if you were to call a function without parenthesis.