To merge two DataFrames where the customer ID column has different names (cust_id in one, customer in the other), use:

Why does str.replace usually require regex=True when your pattern uses regex syntax?

Which statement about st.button is correct?

To preserve a value across script reruns in Streamlit (e.g., remembering whether the user clicked a button):

Wide format means:

Trailing/leading whitespace in a string column is most cleanly removed with:

Which join keeps every row from BOTH DataFrames, with NaN for unmatched values?

Which is a quantifier in regex?

You have a column where 30% of values are missing, scattered across many rows. The defensible default strategy is:

To prevent an expensive data-loading function from re-running every time a widget changes, use: