pandas DataFrame Practice Questions

Aggregations skip NaN by default, so a group still returns a total from its non-missing values.

• sum(), mean() and count() all ignore NaN.
• An all-NaN group sums to 0 unless you pass min_count=1 (shown here).
• Check df.isna().sum() first so you know how much data is being skipped.

loc selects by label, iloc selects by integer position.

• loc uses index values and column names, and slices include the end label.
• iloc uses positions, and slices exclude the end like normal Python.
• loc also takes boolean masks, the usual way to filter rows (shown here).

Use merge, which works like a SQL join. Choose how: inner, left, right or outer.

• Check the row count after merging: a many-to-many match multiplies rows.
• validate="many_to_one" makes pandas raise an error if the right key is not unique (shown here).
• Use pd.concat instead to stack DataFrames when there is no key to match on.