concat

concat stacks tables on top of each other (or side by side).

• pd.concat([jan, feb]) puts February's rows under January's.
• ignore_index=True renumbers the rows 0, 1, 2 ...
• Columns are matched by name; missing ones become NaN.

import pandas as pd
jan = pd.DataFrame({"sales": [10, 20]})
feb = pd.DataFrame({"sales": [30]})
print(pd.concat([jan, feb], ignore_index=True)["sales"].tolist())

[10, 20, 30]

Remember: concat = stack. merge = join on a key.

Common mistake: Forgetting ignore_index=True and ending up with repeated row labels.