Pandas column arguments: Where one name works, a list does too

A Python Pandas rule of thumb: Wherever you can use a column name (string), you can use multiple column names (a list of strings):

df.set_index('x')
df.set_index(['x', 'y'])

df.groupby('last')
df.groupby(['last', 'first'])

df['x']
df[['x', 'y']]  # Notice nested []