Pandas cards
`agg` reduces groups, `transform` returns group-shaped output, and `apply` handles custom logic but is usually slower.
groupby with agg vs transform vs apply
Inspect null patterns first, then choose to drop, fill, or flag them based on meaning.
How do you handle missing data in pandas?
`loc` selects by labels and `iloc` selects by integer position.
loc vs iloc in pandas
`merge` combines rows by key columns, `join` is index-oriented sugar, and `concat` stacks objects along an axis.
merge vs join vs concat in pandas
`pivot` reshapes unique values only, while `pivot_table` can aggregate duplicates.
pivot vs pivot_table in pandas
Limit parsed columns, set dtypes intentionally, and stream large files in chunks when needed.
read_csv performance tips in pandas
A Series is one labeled column, while a DataFrame is a 2D labeled table of columns.
Series vs DataFrame in pandas
It happens when pandas is unsure whether you are writing to a view or a detached copy.
What causes SettingWithCopyWarning in pandas?
`explode` turns each item in a list-like cell into its own row while repeating the other column values.
What does explode do in pandas?
Vectorization means using array-style operations on whole columns instead of Python loops over rows.
What does vectorization mean in pandas?