python pandas - display first cell in rows with NULL values in selected column

If I understand you correctly, what you want to do should look like this:

df = pd.DataFrame({'name': ['id1', 'id2', 'id3'], 'a': [20, None, 30], 'b': [10, 40, None]})

df[df.isna().any(axis=1)].iloc[:, 0]

will result in: df2, df3.

Explanation:

The df.isna() will bring us all the nulls. .any(axis=1) will bring where there's at least 1 null (opposite to .all()) from the columns axis.

And finally the .iloc[:, 0] will give us the first column, this is not a must, only if you want the first column (remove if you want all the columns where there's at least one null).

Edit to answer your edits:

In order to select the column by the user, we will add input:

chosen_column = input(f"Please choose one of the following columns: {list(df.columns)}")

# Filter by na and display only the chosen column
df[df.isna().any(axis=1)][chosen_column]

I hope I understood you correctly and this is what you were aiming for.

You can make a custom function for that purpose:

def print_id(col,df=df):
    df=df.copy()
    if isinstance(col,list):
        return df.loc[df[col].isna().any(1),'id_column'].reset_index(drop=True)
    else:
        return df.loc[df[col].isna(),'id_column'].reset_index(drop=True)

Finally call that function(input given by user):

print_id('col_name3')
#OR
print_id('col_name3',df)
#OR
print_id(['col_name3','col_name2'])
#OR
print_id(['col_name3','col_name2'],df)

OR

If you want to enter 2 and it selects col_name2 then use:

def print_id(like,df=df):
    if isinstance(like,list):
        print('like parameter doesn\'t support multiple values')
        return None
    else:
        df=df.copy()
        return df.loc[df.filter(like=str(like)).isna().any(1),'id_column'].reset_index(drop=True)

Finally call that function(input given by user):

print_id(2)
#OR
print_id(3,df)